Seems like I made a rather big mistake in the video. My transformer should not be switched on during the zero crossing point but at the voltage peak in order to decrease the current flow. While recording the current waveforms on the oscilloscope, everything appeared good to me and I am not sure why my circuit breaker stopped tripping when I switched at the zero crossing point. But many viewers pointed me in this direction and after doing research, it seems to be correct. Sorry about this. But if you want to build the softstarter you can still try it out whether it works for you and if not then you can always change the Arduino code in order to switch at a different time point. Sorry again for the inconvenience. I do make mistakes from time to time since this is a one man production. Stay creative :-)
@@aks8403 You have to change the timer value OCR1B. In my case I firstly used 590 which is around 9.44ms after the zero crossing point. 625 would be 10ms. So simply use half of 625 so around 313 in order to get a 5ms value at which point a voltage peak should appear after the zero crossing point.
Tripping circuit breaker? Hah, I don't have such weakness with my 30-years-old house wiring. The circuit resistance is high enough to limit any inrush current.
The current into an inductor equals the integral of the voltage accross it, in formula: i = 1/L.integral(Vdt). If you switch it on at the (rising) zero crossing the voltage remains positive for 180 degrees. This causes the magnetizing current to rise to approx. 2x its normal value. However the inductor core is not made for this 2x magnetizing current and thus the core will saturate. This causes the coil inductance to drop, which increases the current even (a lot) more. If you switch at the maximum voltage, the current also starts to rise, following the integral relation, but after 90 degrees the voltage polarity reverses and the current starts to go down again. In this way the current starts of nicely lagging the voltage by 90 degrees, as it should be in the stationary situation, with peaking and tripping the breaker. Best regards, Ben.
"If you switch it on at the (rising) zero crossing the voltage remains positive for 180 degrees." ....... But in normal operation, the voltage remains positive for 180 degrees and then becomes negative again for 180 degrees. What's the difference now?
@@Thomas72B The difference is exactly what you say: in normal operation a positive periode is always preceded by a negative period, and visa versa. An inductor stores current in a similar way as a capacitor stores voltage. Therefore the history is important. But at startup there is no history (current) stored in the inductor. When you then start with a full positive (or negative) period, the current becomes too high. This is only a temporary problem, after a number of periods the current normalizes, due to the series resistance of the inductor. Regards, Ben.
@@beneindhoven3424 Not always ! If you put the plug in the socket you cannot know where the sine curve is. 0 degrees, 130 degrees or 174 degrees you don't know .... That's why I still don't quite understand where the problem is supposed to be.
@@Thomas72B Yes, that’s correct, you never know where you start when you connect the plug. That’s why the circuit breaker not always trips, but only sometimes. Switching in near the zero crossing is the worst case situation. Having said that, I have to admit that I don’t understand why Great Scott’s tests at zero cross switching didn’t trip the breaker. I tried this same experiment a few months ago, and could clearly see the current peaks are maximum at the zero crossing. And minimum when switching at the voltage peaks. Regards, Ben.
Ohhhh.... Brilliant comment! I think it's also just answered a question I've had about adding energy into a LC tank. Thanks for sharing the knowledge. Edit: Yes! I've been trying to get my head around this for days. You're a catylist for understanding good sir. Double thanks!
not gonna lie but this is exactly something I need to make for about 10-15 past days and here you go, guess a trip to DIY electronic store is required!
Sorry @GreatScott!, your theory behind switching inductive loads at voltage zero crossing is wrong, it should be switched at current zero crossing (that happens at peak voltage), look around for documentations, the reason behind breaker tripping should be elsewhere. (Maybe Arduino and/or SSR are taking up time and they are switching correctly? Or is it simply because of the soft-start that it works?) Try to capture also the mains voltage with the oscilloscope in sync with the surge current and we will see. Thanks!
On the large DC power supplies we design at work, we just use a 20 ohm NTC inrush current limiting resistor, and a contactor that bypasses it after the main bulk caps are charged. Which could be done with an RC timer + Comparator
Another much easier and cheaper way which I used succesfuly in the past was a 1s timed relay in parallel with a high power resistor with just enough ohms to limit the current to under 16A. Both of that in series to the load will limit the current for 1s and then the relay shorts the resistor. Worked like a charm :-)
good idea, but using an ntc instead of a power resistor is better, since it can prevent from burning down your house in case of failure of the relay/connection..
@@nicolaswannen1743 can you get ntc with lots of watt dissipating ? Better to use onetime thermal fuse on the resistors.. If relay is dead resistors heats and kills it series connection to the load by melting thermofuse once... Before the relay and resistor an inline glas tube as slow 16A is needed as well... No ntc/ptc disc that ages and sets it self on fire, just metal case grounded and fuse will do better job maybe?
In most cases you can also use a normal relay and connect it's coil parallel to your load. Once the current draw drops the voltage rises and the relay closes. You may have to match your resistor and relay a little. But that's probably the easiest way to limit inrush current fairly efficiently (the only power loss being the holding current of the relay).
Thanks for admitting the error. Thats news to me, since I don't normally work with inductors, so zero point made sense originally. Thanks to all the Commenters for the additional explanations!
That was my first thought, but i guess it isn't a viable solution if transformer shares breaker with other devices. Type C is really slow and if wires aren't properly sized it can cause all sorts of problems.
As you mentioned, even a zero volt point initial turn-on is not always curing the problem. The reason is the remanent magnetism in the transformer core. If the magnetism left in the core at previous turn-off matches the direction of the start time voltage growth slope, there still is a big inrush. The theoretical analysis considers the fact that there is a 90 degree phase shift between the voltage and the current (on the primary). So, at the positive going zero crossing of the voltage, the usual transformer operation condition would have actually the negative peak current at this moment. You don't have that, if you make the primary connection at the voltage zero crossing moment. Your gradual increase of the voltage is indeed the way to prevent the breaker popping inrush. As funny as it may sound, if the transformer was turned OFF gradually, you would get near zero remanence and then the optimum time to turn it again on would be near the voltage peak (either positive or negative) !!! Too bad, you generally cannot know the state of the remanence, nor force it to any known state. I hope this explanation can serve other people who may suffer from the same issue and are wondering why.
I had a similar issue with a 3000 watt transformer for reducing power from 220 to 120 volts. I later learned that the type of circuit breaker used makes a difference. There are some breakers rated for inductive loads and some for resistive loads. Most home breakers are of the resistive loads found in most homes. Replace breaker with an inductive rated breaker. Did this and no problems now.
Great project! For softstart I used a "Finder DPDT Multi Function Timer Relay" set to 1 second with a thermistor over the "off" position. The first second you power up the thermistor heats up then the relay clicks to the "on" position so the attached device is powered directly. The switching time seems fast enough so far.
@@gblargg yes but they will become very hot, not ideal for long-term use. Also when you switch off and on before the ntc had time to cool off the resistance is still too low. This is why I used a time relay in combination with an ntc.
@@o0arend0o this solution still has the lingering heat from previous on time. Unless chosing time delay longer allowing ntc to cool down enough to make a difference.
@@muthumanikya1847 the ntc only heats up for less then a second when power is switched on for first time. The rest of the time the ntc cools off while the device is on.
May I suggest that you add a relay to bypass the triac after completion of the load soft starting in order to limit the power loss in triac and saving it from overheating. And you might want to add a variable resistor to enable the user to set the soft starting phase duration. Great job
Your programming has become much better over the years. You started off writing terrible code and now your code looks quite nice. I'm glad to see you improving that skill!
Only feedback I'd have for you would be to move those LEDs, put the red one near the back, as it indicates power in, move the other one to out outlet side and make it light up red when the button is pushed and go green when the sequence is done (those looked to be three or 4 legged LEDs, so either dual color or maybe RGB?). This way it is much easy to know what the state of the thing is by a quick visual glance, and therefore a bit safer to use.
Thermistor with relay latching. Monitor voltage drop across thermistor and latch when it falls low enough. Your solution of course works and is very creative.
I have a 15A variac behind a 15A capable isolation transformer (isolated and adjustable voltage is sometimes handy). All in parallel with a second lower power isolation transformer. I use a high power resistor and a timer relay. The inrush current is limited by a simple resistor in series on the mains side and after a second or two, the resistor is bypassed by a timer relay. Works a treat.
Thanks so much for posting this! I've needed an AC soft starter for quite some time but was hesitant to spend the $300+ that most places want for a decent one. Thought for sure the circuit would be much more complicated than it is! I'll be using it for an air conditioner instead of an autotransformer.
Getting better and better. It's explained well, and the whole lesson is demonstrated by a working device. I also like how this guy goes to great lengths to show the flow between the problem and the solution. I mean he hauled a big and heavy electric motor just to show that it applies to them as well. Well worth the subscription. P.S. What I don't like is the fact that he doesn't want to admit that he took the "Great Scott" name from the movie "Back to the future" (the mad scientist). Yet, I figured it out! Got him!
I rly wanna thank you for everything!!! I learned so much from your Videos! I started watching your Videos like back in 2017 and learned almost everything about Electronics including English. Please never stop making Videos☺️☺️
Large transformers are always a problem for standard house breakers, if permanently installed the type of breaker can be changed to a C or even D, but then the tripping time has to be correlated with local disconnection times as dictated by local law. If the appliance is portable, then a soft start (cheaper than a full inverted) can be used to reduce the inrush, many are available commercially and are mainly used for motors. I am a little concerned that the motor jumps into life at the button press then returns to slowly accelerating, not something one would expect from an off the shelf solution!
Note that an autotransformer is just one with a single shared winding, yours is a variable autotransformer. Also, the input voltage is only across part of the winding, which is what gives it the ability to output a higher voltage than the input.
Basic AC theory suggests that with a purely inductive load the voltage zero crossing point is the worst choice for switching on as the steady-state current would be at its maximum; one really needs to switch on at or near to a voltage maximum. However my preferred method would be the use of a series resistor to limit the inrush current which is then short-circuited by a relay with a small time delay. Simple, cheap and easy to implement.
Hi G.S., I had the same problem with a 30KVA autotransformer and the simplest solution was to turn on the circuit with a load applied to the output of the secondary: the large initial current is due to the magnetization current of the primary which is not reabsorbed by the current magnetization of the secondary (output circuit) due to the absence of load. This suggestion was given to me by my service manager (I worked in ENEL) who had worked in a company with a large transformer cabin and who had the same problem, when the softstarters could not be used due to the load (> 4 MVA at 15 KV) and possibly triac, mosfet and arduino did not exist. Try to apply a load to the output (one ohm resistance is fine too: you adjust the output voltage so as not to let it exceed its power) and then power it up: you should no longer fail a ignition P.S. of course the resistor is used only for switching on and must not remain active for all use of the variac. Thanks anyway for sharing the design of a softstarter. P.S. there are modules for universal electric motors, up to 2KWp, very compact (with three wires and automatic, to be inserted in the casings of power tools) on the market, but you would not have made the video :-)
Nice video! One recommendation, make sure your diy pcb can handle up to 3000 watt or preferred 3600 watt. The small screw connectors are not designed for this loads.
I like your design. I had similar problem for putting an AC capacitor in parallel with load to correct for power factor. When you want to connect the capacitor to AC line you might get a huge "inrush current", which is similar to “locked rotor current” in motors. -- By the way you can measure inrush current with many types good multimeters these days, and you do not need to use oscilloscope. -- I solved the problem by putting a small inductor in series with the capacitor, so the total impedance is still capacitive at the frequency of operation. To find out how much inductor you need you should following laws: Current in the inductor is continues and does not change instantaneously. Voltage in a capacitor is continues and does not change instantaneously. Rate of change of inductor current is d(I)/d(t) = V/L So at worse time is when you connect at the peak of the cycle: Vp/L is the maximum rate of change . To find out what is maximum current you have to integrate d(I)/d(t) for 1/4 of frequency cycle. so I=Integral of Vp/L Cos(2 pi f t) d(t) where t from 0 to 1/(4f) where f is frequency of the AC line, Vp = SQRT(2) Vrms of the line. In your case you could have put a huge AC capacitor in series with your auto transformer. Other solutions I found out in the web: Using NTC thermistors: When power is first applied, the Negative Temperature Coefficient thermistor is cold, so it has a high impedance and effectively limits the current. The NTC thermistor heats up very rapidly due to its own losses when it used in series with the load, and as result its resistance decreases. *** Problem ambient temperature dependent *** Using fixed resistors After the electrolytic capacitors on the input of the power supply are charged, the resistor is bypassed. Trailing edge phase dimming:
You know what you need to make? An infinite-loop, programmable, loss-suppressing contest repeater tuned for an MSO ScopeNmultimeter that's compatible with a pre-century reserved shelf-gap filled with transient notta-waves that keeps inducing negative optical feedback?
I had the same problem and I built a "dim bulb tester", basically I start the transformer with 2 40 watts lamps in series with the load and I added a switch to bypass the lamps once the transformer is powered on. Maybe this helps somebody that wants to make this really cheap and easy. Great idea and video as always btw
ah, the old "dim bulb" great for slowly powering up something like an old radio or TV that has wax caps in them. light bulb lights up brightly you shut it the hell off.
Great Scott, I love your videos a lot. Your content is on the point with no detours, but you still add all the background information. And I like your visualization style, with pen and paper. It gives me the feeling that I witness you developing the idea in the very moment. Please don't change too much in the future 👍
Many Auto transformers don't have the primary not across the entire coil, but tapping at a 90% or so. The wiper however does extend across the entire coil, permitting the output to actually be higher than the input voltage. Eddie Van Halen used to use an auto transformer to lower the line voltage to his guitar tube amplifiers, for what guitarists call the brown sound. Sorry if this is a duplicate comment. Thanks for Colorado.
This is absolutely a project that I am going to build. I have a table saw that shakes the whole thing and makes this horrible sound that makes me cringe every time I use it. This project is perfect, and add always, ein weiteres herausragendes Video
You probably have a core saturation problem during the initial transient, this most likely to occur when you start close to a zero crossing, if you start close to a voltage maximum you flux peak will be lower.
Not real sure of how to fix it, but from what I understand is that the core material has memory. When turned off all the lines get lined up in one direction, and if you turn the power back on, and the phase is 90 deg. from the lines of force, it acts like a short circuit. It is kind of hard to explain, so I hope it is a little bit clearer. Michael from Washington State, near Seattle.
One super simple solution I seen used in an 40amp Isolation transformer was just a contactor that had a power resistor over it, the contactor was wired in wit the feed power. Now the time that takes for the contactor to pull, the transformer will start true the resistor. It was actually working great, with no output loud the transformer started even with 10A fuse!!! A super simple solution that just works! I got my hands on something like 4 or 5 off those transformers. Back then I gave them all away for other hobbies, but played around with one off them for a while. I ewen had small plans to make a 700A 12v charger / start jumper. But never got around it and just decided it's better served with someone who has "reall needs" for one phase 40A isolators. I think some of them went to a both harbor for feeding aluminum bouts
GREAT video!!! I think for a normal house.. all circuit breaker are of for normal devices..!!! you had this problem but... not every person has a autotransformer at home!!! ajaja
I think a SSR may be a good idea for my well pump, I just want something to take the brunt of the starting current off my inverter! A NTC, with SSR should do fine, then I can use 12-24vdc on the pressure switch, maybe at low voltage/current it will nearly last forever! The pump is 120vac, abd is about a 7 amp peak load, it's in a shallow well, and has a easy job! It isn't worked hard! I just don't want it to possibly blow gets in my inverter if it's bear full output, and the pump comes on at the peak of the sinewave! Great video!
You may want to add a reverse diode in parallel to the zero crossing detection optocoupler. Otherwise, the LED inside might get destroyed by overvoltage while in reverse. Since practically no current is flowing no voltage is dropped over the current limiting resistor and the optocoupler LED sees the full reverse voltage.
I solved this very same issue with my variac with a beefy enough inrush current limiter (NTC thermistor) in series with mains input. Much simpler and immediate than a whole softstarter
Yet another great video. Most circuit breakers have a time delay trip to allow for inrush from motor starting. Maybe there is one with a longer delay available? When transformers are turned off the core steel will stay magnetized in the direction of the polarity at that exact moment north and south. When you energize it again, if the polarity is opposite at that instant, there will be a longer inrush time to correct it. Using the SCR relay seemed to be a great solution switching at current zero. The soft start would be great for tungsten light bulb filaments too.
Schöne Schaltung! Ich hatte mir vor Jahren einen Sanftstart mit einem 5Ohm Lastwiderstand gebaut. Dieser wird nach 1s durch ein Solid State Relais überbrückt. Als Zeitgeber dient ein Ne555. Ich benötige die Schaltung für leistungsstarke PA Endstufen. Der reine Nulldurchgang hat alleine nicht gereicht, hatte immer noch Peakströme größer 90A. Du hast echt klasse Ideen, weiter so!
I've seen this problem several times. Most recently at a medical company, we used a 400VA toroidal transformer to feed the various AC-DC components. It popped the 5-6A slow-blow AC line fuse at 120V, but only when the unit was plugged into an AC outlet close to the electrical panel. Adding a 20' extension cord or moving to an outlet 50' away was enough line resistance that the problem never occurred. I measured the current spikes and they were >100A near the electrical panel. We used two NTC surge limiters (EPCOS-TDK, 5 ohm, 5A, I think). One in series with each 120V primary winding. So for 240V the 2 NTCs were in series. Problem solved forever. BTW I looked inside a commercial 1000VA medical isolation transformer and they did the same thing.
you can use a timer relay and a 200w bulb lamp the lamp connected at the normally closed switch it's other terminal to the load and to the normally open switch . when power up the relay turns on (set to 1 second ) after 1 second delay the lamp is connected in series with the load through the time relay normally closed switch the load powers up slowly (motor , transformer , bulb but no electronic circuit ) after 1 second the the normally closed switch opens and the load directly connected to the power source.
If you decide to use your small variac again you might want to use ring terminals and nuts instead of directly soldering wires. It should be safer and more durable.
Great job! Only question is what is the motor jump at 11:17. Just at the button push there is a jump and than slow speed up. Looks like a short maximum peak at the initial. It shoudn't be. Am I right? Did you fix it?
If you need to start a high inrush transformer -for example the type used in industry for 240 v to 120 v centre tap for use for industrial site safety. the simple way to "soft start" it is pulg it into a standard 15 to 25 metre 13 to 16 amp extension lead, which slows down the initial rush . The alternative is a C type rather than B type circuit breaker but in some -particularly domestic sytems it is not allowed - but would be on most PME systems.
If you add a potentiometer and a little bit code for the potentiometer, you can control lights to dim and be brighter depending on where you leave the pot knob. Big fan of your work.
Sorry to be the one to tell you that, but your calculations about the inrush current are not really correct. The inrush current in transformers is caused by the magnetic remanence (magnetization of the core). When you turn off a transformer it usually has a magnetization in the core afterwards. If you then connect the transformer again, this "offset" can drive the transformer into saturation, which creates losses in the core. This offset only lasts after a couple of cycles, until its gone. This explains the unreliable tripping of the circuit breaker, since the stored magnatization is very much dependent on the timing when it was shut off during a cycle. The DC resistance only limits the inrush current into the parasitic capacitance of the transformer (brings down the quality factor Q of the whole system). So technically your not wrong, just not the whole story. It's probably rather hard to find a balance between entertainment and bone dry mathematical theory. But all in all a great well explained video as usual. :) Keep it up.
Finally somebody who knows what they are talking about! My simple solution is a resistor and a timer relay. The resistor needs to be small enough to drive current into the primary inductance but large enough to limit the current if it saturates. The time delay need only be a few cycles, so 100-200ms is sufficient.
Your circuit needs one more thing - in case you haven't thought of it yet. You need to drive a relay to close the AC circuit across the triac when the ramp-up is finished - and keep it closed as long as the circuit is on. This will keep the triac happier in the log run. Other than this, good job!
Will this soft starter work for a solar setup? Starting a motor running off an inverter and solar panels, the startup current is too high and trips the inverter.
i solved this problem (popping breaker) by adding two NTC-thermistors to the input (L and N), so as you plug it in, they "puffer" the current for just a few milliseconds and prevent the breaker from popping. but for this a) i cannot use loads over 10A anymore and b) they only works, when they are not hot. So plug the transformer in and instantly pulll it out and plug it in again will make the breaker pop again. also another super simple solution, which i dont like, is to use a 20m extension cable. this also works like a puffer and will prevent the breaker from popping. (tested ;) )
I like your video style. Doing the math and explanation while writing it out. Pretty cool. Trying to understand this and the math/diagrams help. You got a new subscriber from me!!
by using a primary to secodary winding in a toroidal transformer and make the output winding voltage divided chould prevent the spike all together making the output winding like a auto transformer. the primary winding would have nearly zero core loss in a toroidal transformer so there woudl be no initial overload by powerup.
Did a similar thing for the mystrey rating variable voltage transformer I got. Unknown what the ratings are, but it seems fine to handle 15A output current. Just used a high power resistor in the input line, 4 10R 10W in parallel, and with a 100C thermal fuse between them, with a relay fed from a simple resistor, bridge rectifier and capacitor smoothing, so the relay turns on around 5 cycles after switch on and shorts out the resistors. Also used a winding tap that gave 60VAC (lowest one on it) that fed a half wave rectifier, smoothing capacitor, and then a 47V zener regulator, that in turn feeds a 33V zener regulator, providing power to a LM723 set to provide an output of 6V, that can sink 10mA if needed. This is the one side of a simple voltmeter, so that the scale starts at 60VAC, and goes all the way to 360VAC, which is the range of the output at maximum. All made using parts either repurposed or lying around, including the box, which was the case the variable voltage transformer came in, just with a coat of paint, and the assorted outlet sockets attached. 60VAC to 360VAC because the surplus meter I had came with a 1mA FSD coil, and was marked with 0 to 30 already, so quick to simply use a black drawing pen to relabel the numbers, after a painting over of the originals.
Howdy. With all due respect. Bear in mind that the phase shift lags 90 degrees for inductors. This means that switch on at voltage peaks actually produces the lowest inrush. Voltage and current are "already in phase". Switch on at voltage null crossing will produce the largest inrush current until the current shifts out of phase. Depending on the rated VA it may take from about a few cycles to tens of cycles for Your 3000 VA Variac. Please, feel free to verify this with an oscilloscope with recording capability. Best Regards.
If you read the data sheet of the SSR, they state with inductive loads, the face value should be divided by 10... So yes, the inrush is the smallest when switching at max voltage.... anyway, energizing through a resistor is pretty effective. Great job. I've a similar problem. A colab maybe?
Newton's Third Law. The torque that turns the rotor has to push against something, which in this case was the frame of the motor, causing it to move as well.
@@thereynolds2725 Agree. But it looked like the same jolt as an across-the-line start of a motor that's not bolted down. Didn't expect it would be noticeable under soft start especially because the armature would have only had turned a few degrees to align itself. Cant make out and shaft movement from the video.
I have seen a simular control in high end, hi power stereo receivers..soft start off the mains before the amplifier comes to full power reducing in- rush current to other components..on a failure to soft start, I experience a corona in the cabinet, finding the regulators themselves burned along with local components damaged beyond recognition. Thank you friend for sharing .. Oh and see my suggestion on your generator project..👍
Another funny thing is capacitors in switch mode power supplies. I have a lot of SMPS devices, computers and servers in the house and I came across a terrible annoying thing with power failures. When the power comes back on the SMPS devices all charge up their capacitors and almost always trips the inrush breakers. This has happened to two different houses with modern wiring, which means it is more likely my devices causing the issue. Petition for requiring (adequate) NTC thermistors in commercial SMPS to limit inrush current to capacitors, some of the computer PSU's have them but I don't think they limit enough when you have several SMPS devices. Alternatively a precharge circuit, I don't mind that the power supply is charging up over 5-10 seconds after applying power and then it just keeps that power in capacitors. There is no need for instant charging as far as I can think. Also, with adequate NTC thermistors I also imagine there should never be soundable sparks when connecting power plug to the power supply, but most often there is quite an audible spark (with computer PSU's mainly), and sometime even a flash. With all that said, I've had an interest of soft-starting groups of devices in my home after potential power failures (doesn't happen that often in new house though). Basically all I would need is to have a timed relays clicking on at different set times when power comes back which should not overload the breaker(s).
Since you have a 3-phase induction motor lying around, why don't you make a soft-starter for that as well? Or do a DIY or BUY video for a 3-phase induction motor soft-starter?
You should have used a mechanical relay to bypass the TRIAC after softstart. The would have allowed you to use a smaller heatsink. And at 3000W, the amount of current flowing through the TRIAC might heat it up a lot, so having the circuit inside a closed housing with no airflow might cause it to overheat.
I had a similar problem 20 years ago. Solved with a 2kW heater in series with the transformer that was shorted by a relay. It takes some tens of ms for the relay to close the contacts and this time is enough to limit current surge
Informative video. I didn't realize that Solid State Relays have a zero crossing circuit. This should fix my Siglent Bench PS from tripping the UPS on my router.
Just a tiny note, and I hope it's not rude, but "Such a" refers to something you've already described. Like "A resistor is a device that resists current flow, such a device can be used to..." Not something like "you can use such a resistor". Instead "a device such as this resistor" or "we can use something to limit the current such as this resistor" etc
In industrial applications we would replace the breaker with a C type breaker. those breakers can handle a higher inrush current! A Hager MCN116 would be the replacement for that. That costs only 15 euro's. That display on the auto transformer might break when using a soft starter on the main's voltage. Maybe do a video about the karakteristics on circuit breakers?
An autotransformer isn't really a transformer. It's really a potential divider, with inductors instead of resistors. The inductors have reactive impedance, so they don't dissipate power, but that's really how it works.
How would I go to... instead of a push button, use the thermostat of the ac unit make on and off... So instead of a momentary switch have it for a toggle switch at line voltage ? Great work thanks !!!
Why make it so complicated? I made softstarter to my 10kVA transformer by pre magnetizing the coil through power resistors for some milliseconds before directly powering the coils. Very simple and works.
Nice project I have a couple questions. Does this work for 120V? I've seen a couple other soft start projects which claim that a relay is a more reliable long term solution than a triac? Also would like to see a soft start which starts up automatically when the load device is powered on. Thanks.
Err.. we used to do this for big ham amp power supplies with a resistor and a time delay relay. Cheap simple potter and brumfield socket relays set to 3 sec or something like that. Those were supplies with big caps to charge though.
@@legominimovieproductions thank you for correction. Im not fsmiliar with German standards. Pretty sure its recommended to use B for home appliances tho. Is it easy to justify using C's with electrical regulators?
Your explanation of the inrush current on an inductive load isn't quite correct; what you describe is more like how a capacitive load acts than anything. For motors, the inrush current is due to the mechanical inertia; when first turned on, the motor isn't spinning and so there's no back EMF to limit current flow, but once it gets up to speed the back EMF lowers the effective voltage across the coils. For transformers, it's actually caused by connecting *at the zero crossing*, which can even saturate the core and cause its inductance to fall dramatically, further spiking the current upward. I'm not sure how your zero-crossing switch fixed the problem, actually... The ideal moment to switch a purely inductive load on is actually at the peak of the AC supply voltage. Your transformer may have a significant capacitive component to it, perhaps? Could be a parallel capacitor added for power factor reasons; I doubt it's parasitic winding capacitance.
Great video, although using a microcontroller, a triac and a lot of other components seems like a massive overkill. A NTC thermistor with a relay that bypasses the thermistor after around 1-2 seconds would do the job as well, but with way less components. But of course, with the microcontroller route you learn a lot in the process.
The core of the transfer is probably saturated when turned off in a certain point in the AC cycle. The next time it is powered and the current is in the same direction as the created saturation, a huge current flows through the transformer, that trips the breaker.
I've got a 4kVA variac, if I switch it on it's 50:50 whether it'll trip the breaker or not. I use a big 7kVA mains conditioner which soft-starts it and prevents tripping.
Seems like I made a rather big mistake in the video. My transformer should not be switched on during the zero crossing point but at the voltage peak in order to decrease the current flow. While recording the current waveforms on the oscilloscope, everything appeared good to me and I am not sure why my circuit breaker stopped tripping when I switched at the zero crossing point. But many viewers pointed me in this direction and after doing research, it seems to be correct. Sorry about this. But if you want to build the softstarter you can still try it out whether it works for you and if not then you can always change the Arduino code in order to switch at a different time point. Sorry again for the inconvenience. I do make mistakes from time to time since this is a one man production. Stay creative :-)
Can you sir please explain me why to turn it on at the peak and not at zero crossing point?
@@aks8403 ELI the ICE man
@@aks8403 You have to change the timer value OCR1B. In my case I firstly used 590 which is around 9.44ms after the zero crossing point. 625 would be 10ms. So simply use half of 625 so around 313 in order to get a 5ms value at which point a voltage peak should appear after the zero crossing point.
it's ok you are a legend of electronics for me
Das war wieder ein sehr priffiges Video! Gruß aus Karlsruhe in Baden! :)
Tripping circuit breaker? Hah, I don't have such weakness with my 30-years-old house wiring. The circuit resistance is high enough to limit any inrush current.
Well.....that would also be a solution
@@403_Tuna aluminium wire and ceramic conectors and large swich board for a hause ?
My 80 year old cloth wiring has the same effect. Built in protection :)
@@403_Tuna the is thousands more over the world or even millions
@@alangolab6657 Same with our house
The current into an inductor equals the integral of the voltage accross it, in formula: i = 1/L.integral(Vdt). If you switch it on at the (rising) zero crossing the voltage remains positive for 180 degrees. This causes the magnetizing current to rise to approx. 2x its normal value. However the inductor core is not made for this 2x magnetizing current and thus the core will saturate. This causes the coil inductance to drop, which increases the current even (a lot) more. If you switch at the maximum voltage, the current also starts to rise, following the integral relation, but after 90 degrees the voltage polarity reverses and the current starts to go down again. In this way the current starts of nicely lagging the voltage by 90 degrees, as it should be in the stationary situation, with peaking and tripping the breaker.
Best regards, Ben.
"If you switch it on at the (rising) zero crossing the voltage remains positive for 180 degrees." .......
But in normal operation, the voltage remains positive for 180 degrees and then becomes negative again for 180 degrees. What's the difference now?
@@Thomas72B The difference is exactly what you say: in normal operation a positive periode is always preceded by a negative period, and visa versa. An inductor stores current in a similar way as a capacitor stores voltage. Therefore the history is important. But at startup there is no history (current) stored in the inductor. When you then start with a full positive (or negative) period, the current becomes too high. This is only a temporary problem, after a number of periods the current normalizes, due to the series resistance of the inductor.
Regards, Ben.
@@beneindhoven3424 Not always ! If you put the plug in the socket you cannot know where the sine curve is. 0 degrees, 130 degrees or 174 degrees you don't know .... That's why I still don't quite understand where the problem is supposed to be.
@@Thomas72B Yes, that’s correct, you never know where you start when you connect the plug. That’s why the circuit breaker not always trips, but only sometimes. Switching in near the zero crossing is the worst case situation. Having said that, I have to admit that I don’t understand why Great Scott’s tests at zero cross switching didn’t trip the breaker. I tried this same experiment a few months ago, and could clearly see the current peaks are maximum at the zero crossing. And minimum when switching at the voltage peaks. Regards, Ben.
Ohhhh.... Brilliant comment! I think it's also just answered a question I've had about adding energy into a LC tank. Thanks for sharing the knowledge.
Edit: Yes! I've been trying to get my head around this for days. You're a catylist for understanding good sir. Double thanks!
not gonna lie but this is exactly something I need to make for about 10-15 past days and here you go, guess a trip to DIY electronic store is required!
I hope it works for you :-) But be careful when working with mains voltage. Thanks :-)
don't forget to check that you have adequate mains voltage by doing the good old tongue test
Sorry @GreatScott!, your theory behind switching inductive loads at voltage zero crossing is wrong,
it should be switched at current zero crossing (that happens at peak voltage),
look around for documentations, the reason behind breaker tripping should be elsewhere.
(Maybe Arduino and/or SSR are taking up time and they are switching correctly? Or is it simply because of the soft-start that it works?)
Try to capture also the mains voltage with the oscilloscope in sync with the surge current and we will see. Thanks!
Thanks for the feedback. I just did lots of research and it seems you are right. I added a pinned comment that explains it. Sorry.
On the large DC power supplies we design at work, we just use a 20 ohm NTC inrush current limiting resistor, and a contactor that bypasses it after the main bulk caps are charged. Which could be done with an RC timer + Comparator
Another much easier and cheaper way which I used succesfuly in the past was a 1s timed relay in parallel with a high power resistor with just enough ohms to limit the current to under 16A.
Both of that in series to the load will limit the current for 1s and then the relay shorts the resistor.
Worked like a charm :-)
good idea, but using an ntc instead of a power resistor is better, since it can prevent from burning down your house in case of failure of the relay/connection..
@@nicolaswannen1743 can you get ntc with lots of watt dissipating ? Better to use onetime thermal fuse on the resistors.. If relay is dead resistors heats and kills it series connection to the load by melting thermofuse once...
Before the relay and resistor an inline glas tube as slow 16A is needed as well...
No ntc/ptc disc that ages and sets it self on fire, just metal case grounded and fuse will do better job maybe?
In most cases you can also use a normal relay and connect it's coil parallel to your load. Once the current draw drops the voltage rises and the relay closes. You may have to match your resistor and relay a little. But that's probably the easiest way to limit inrush current fairly efficiently (the only power loss being the holding current of the relay).
Thanks for admitting the error. Thats news to me, since I don't normally work with inductors, so zero point made sense originally. Thanks to all the Commenters for the additional explanations!
Easier and cheaper to change the breaker to type C time curve which is for high startup loads.
Also possible. But not always the best solution.
That was my first thought, but i guess it isn't a viable solution if transformer shares breaker with other devices. Type C is really slow and if wires aren't properly sized it can cause all sorts of problems.
@@cekpi7 true in some cases, but if IK is in the right range, it shoul‘d be ok
Might be true. But less fun😜
@@greatscottlab Yes, why do it simple when you can make it complicated 🤔
As you mentioned, even a zero volt point initial turn-on is not always curing the problem. The reason is the remanent magnetism in the transformer core. If the magnetism left in the core at previous turn-off matches the direction of the start time voltage growth slope, there still is a big inrush. The theoretical analysis considers the fact that there is a 90 degree phase shift between the voltage and the current (on the primary). So, at the positive going zero crossing of the voltage, the usual transformer operation condition would have actually the negative peak current at this moment. You don't have that, if you make the primary connection at the voltage zero crossing moment. Your gradual increase of the voltage is indeed the way to prevent the breaker popping inrush. As funny as it may sound, if the transformer was turned OFF gradually, you would get near zero remanence and then the optimum time to turn it again on would be near the voltage peak (either positive or negative) !!! Too bad, you generally cannot know the state of the remanence, nor force it to any known state. I hope this explanation can serve other people who may suffer from the same issue and are wondering why.
I had a similar issue with a 3000 watt transformer for reducing power from 220 to 120 volts. I later learned that the type of circuit breaker used makes a difference. There are some breakers rated for inductive loads and some for resistive loads. Most home breakers are of the resistive loads found in most homes. Replace breaker with an inductive rated breaker. Did this and no problems now.
Great project! For softstart I used a "Finder DPDT Multi Function Timer Relay" set to 1 second with a thermistor over the "off" position. The first second you power up the thermistor heats up then the relay clicks to the "on" position so the attached device is powered directly. The switching time seems fast enough so far.
There are NTC thermistors that have an initial resistance then heat up quickly and lower the resistance, made just for this task of soft-starting.
@@gblargg yes but they will become very hot, not ideal for long-term use. Also when you switch off and on before the ntc had time to cool off the resistance is still too low. This is why I used a time relay in combination with an ntc.
@@o0arend0o Good points. They're probably used for lower-current devices like switching supplies.
@@o0arend0o this solution still has the lingering heat from previous on time. Unless chosing time delay longer allowing ntc to cool down enough to make a difference.
@@muthumanikya1847 the ntc only heats up for less then a second when power is switched on for first time. The rest of the time the ntc cools off while the device is on.
May I suggest that you add a relay to bypass the triac after completion of the load soft starting in order to limit the power loss in triac and saving it from overheating. And you might want to add a variable resistor to enable the user to set the soft starting phase duration.
Great job
You know the day is gonna be great when GreatScott uploads
Ohhh stop it you ;-)
Your programming has become much better over the years. You started off writing terrible code and now your code looks quite nice.
I'm glad to see you improving that skill!
Only feedback I'd have for you would be to move those LEDs, put the red one near the back, as it indicates power in, move the other one to out outlet side and make it light up red when the button is pushed and go green when the sequence is done (those looked to be three or 4 legged LEDs, so either dual color or maybe RGB?). This way it is much easy to know what the state of the thing is by a quick visual glance, and therefore a bit safer to use.
Thermistor with relay latching. Monitor voltage drop across thermistor and latch when it falls low enough. Your solution of course works and is very creative.
I have a 15A variac behind a 15A capable isolation transformer (isolated and adjustable voltage is sometimes handy). All in parallel with a second lower power isolation transformer.
I use a high power resistor and a timer relay. The inrush current is limited by a simple resistor in series on the mains side and after a second or two, the resistor is bypassed by a timer relay. Works a treat.
Thanks so much for posting this! I've needed an AC soft starter for quite some time but was hesitant to spend the $300+ that most places want for a decent one. Thought for sure the circuit would be much more complicated than it is! I'll be using it for an air conditioner instead of an autotransformer.
Getting better and better. It's explained well, and the whole lesson is demonstrated by a working device. I also like how this guy goes to great lengths to show the flow between the problem and the solution. I mean he hauled a big and heavy electric motor just to show that it applies to them as well. Well worth the subscription.
P.S.
What I don't like is the fact that he doesn't want to admit that he took the "Great Scott" name from the movie "Back to the future" (the mad scientist). Yet, I figured it out! Got him!
Haha thanks for the feedback :-) And yes, my name is from Back to the future :-) My favourite movie
@@greatscottlab You should also get into AI (i.e Jetson Nano, Jetson Xavier). It's the future, herr professor !!! ;-)
I rly wanna thank you for everything!!!
I learned so much from your Videos! I started watching your Videos like back in 2017 and learned almost everything about Electronics including English. Please never stop making Videos☺️☺️
Thanks for the feedback :-) I am glad that I could help you :-)
Large transformers are always a problem for standard house breakers, if permanently installed the type of breaker can be changed to a C or even D, but then the tripping time has to be correlated with local disconnection times as dictated by local law. If the appliance is portable, then a soft start (cheaper than a full inverted) can be used to reduce the inrush, many are available commercially and are mainly used for motors. I am a little concerned that the motor jumps into life at the button press then returns to slowly accelerating, not something one would expect from an off the shelf solution!
Note that an autotransformer is just one with a single shared winding, yours is a variable autotransformer. Also, the input voltage is only across part of the winding, which is what gives it the ability to output a higher voltage than the input.
Basic AC theory suggests that with a purely inductive load the voltage zero crossing point is the worst choice for switching on as the steady-state current would be at its maximum; one really needs to switch on at or near to a voltage maximum. However my preferred method would be the use of a series resistor to limit the inrush current which is then short-circuited by a relay with a small time delay. Simple, cheap and easy to implement.
Hi G.S.,
I had the same problem with a 30KVA autotransformer and the simplest solution was to turn on the circuit with a load applied to the output of the secondary: the large initial current is due to the magnetization current of the primary which is not reabsorbed by the current magnetization of the secondary (output circuit) due to the absence of load.
This suggestion was given to me by my service manager (I worked in ENEL) who had worked in a company with a large transformer cabin and who had the same problem, when the softstarters could not be used due to the load (> 4 MVA at 15 KV) and possibly triac, mosfet and arduino did not exist.
Try to apply a load to the output (one ohm resistance is fine too: you adjust the output voltage so as not to let it exceed its power) and then power it up: you should no longer fail a ignition
P.S. of course the resistor is used only for switching on and must not remain active for all use of the variac.
Thanks anyway for sharing the design of a softstarter.
P.S. there are modules for universal electric motors, up to 2KWp, very compact (with three wires and automatic, to be inserted in the casings of power tools) on the market, but you would not have made the video :-)
i'm always amazed by your penmanship
Nice video! One recommendation, make sure your diy pcb can handle up to 3000 watt or preferred 3600 watt.
The small screw connectors are not designed for this loads.
I like your design.
I had similar problem for putting an AC capacitor in parallel with load to correct for power factor. When you want to connect the capacitor to AC line you might get a huge "inrush current", which is similar to “locked rotor current” in motors. -- By the way you can measure inrush current with many types good multimeters these days, and you do not need to use oscilloscope. --
I solved the problem by putting a small inductor in series with the capacitor, so the total impedance is still capacitive at the frequency of operation.
To find out how much inductor you need you should following laws:
Current in the inductor is continues and does not change instantaneously.
Voltage in a capacitor is continues and does not change instantaneously.
Rate of change of inductor current is d(I)/d(t) = V/L
So at worse time is when you connect at the peak of the cycle:
Vp/L is the maximum rate of change .
To find out what is maximum current you have to integrate d(I)/d(t) for 1/4 of frequency cycle. so I=Integral of Vp/L Cos(2 pi f t) d(t) where t from 0 to 1/(4f) where f is frequency of the AC line, Vp = SQRT(2) Vrms of the line.
In your case you could have put a huge AC capacitor in series with your auto transformer.
Other solutions I found out in the web:
Using NTC thermistors: When power is first applied, the Negative Temperature Coefficient thermistor is cold, so it has a high impedance and effectively limits the current. The NTC thermistor heats up very rapidly due to its own losses when it used in series with the load, and as result its resistance decreases. *** Problem ambient temperature dependent ***
Using fixed resistors After the electrolytic capacitors on the input of the power supply are charged, the resistor is bypassed.
Trailing edge phase dimming:
You know what you need to make? An infinite-loop, programmable, loss-suppressing contest repeater tuned for an MSO ScopeNmultimeter that's compatible with a pre-century reserved shelf-gap filled with transient notta-waves that keeps inducing negative optical feedback?
I love it :-)
I am on it ;-)
Cool
Bazinga!
Brain hurty
I had the same problem and I built a "dim bulb tester", basically I start the transformer with 2 40 watts lamps in series with the load and I added a switch to bypass the lamps once the transformer is powered on. Maybe this helps somebody that wants to make this really cheap and easy. Great idea and video as always btw
An old TV mechanic I knew had a soft start circuit made with 200W light bulb and a switch 😀
ah, the old "dim bulb" great for slowly powering up something like an old radio or TV that has wax caps in them. light bulb lights up brightly you shut it the hell off.
Great Scott, I love your videos a lot. Your content is on the point with no detours, but you still add all the background information. And I like your visualization style, with pen and paper. It gives me the feeling that I witness you developing the idea in the very moment. Please don't change too much in the future 👍
Many Auto transformers don't have the primary not across the entire coil, but tapping at a 90% or so. The wiper however does extend across the entire coil, permitting the output to actually be higher than the input voltage. Eddie Van Halen used to use an auto transformer to lower the line voltage to his guitar tube amplifiers, for what guitarists call the brown sound. Sorry if this is a duplicate comment. Thanks for Colorado.
Greatscott " warning do not touch its high voltage "
Electroboom " Hmm this is high voltage wire 🎆 yeeeeaaawoooouch fu** fu** fu** I am an idiot "
I saw he made use of a FOOL BRIDGEE RECITFYERRRR >:U
Electroboom would also try to lick the transformer xD
This is absolutely a project that I am going to build. I have a table saw that shakes the whole thing and makes this horrible sound that makes me cringe every time I use it. This project is perfect, and add always, ein weiteres herausragendes Video
You probably have a core saturation problem during the initial transient, this most likely to occur when you start close to a zero crossing, if you start close to a voltage maximum you flux peak will be lower.
Any more information on this? Would it work to have an SCR or something wait for a peak before turning on?
Not real sure of how to fix it, but from what I understand is that the core material has memory. When turned off all the lines get lined up in one direction, and if you turn the power back on, and the phase is 90 deg. from the lines of force, it acts like a short circuit. It is kind of hard to explain, so I hope it is a little bit clearer.
Michael from Washington State, near Seattle.
Just saw this...very useful. I am going to try on a few things. Gemutlichkeit!
One super simple solution I seen used in an 40amp Isolation transformer was just a contactor that had a power resistor over it, the contactor was wired in wit the feed power. Now the time that takes for the contactor to pull, the transformer will start true the resistor. It was actually working great, with no output loud the transformer started even with 10A fuse!!! A super simple solution that just works! I got my hands on something like 4 or 5 off those transformers. Back then I gave them all away for other hobbies, but played around with one off them for a while. I ewen had small plans to make a 700A 12v charger / start jumper. But never got around it and just decided it's better served with someone who has "reall needs" for one phase 40A isolators. I think some of them went to a both harbor for feeding aluminum bouts
GREAT video!!! I think for a normal house.. all circuit breaker are of for normal devices..!!! you had this problem but... not every person has a autotransformer at home!!! ajaja
I think a SSR may be a good idea for my well pump, I just want something to take the brunt of the starting current off my inverter! A NTC, with SSR should do fine, then I can use 12-24vdc on the pressure switch, maybe at low voltage/current it will nearly last forever! The pump is 120vac, abd is about a 7 amp peak load, it's in a shallow well, and has a easy job! It isn't worked hard! I just don't want it to possibly blow gets in my inverter if it's bear full output, and the pump comes on at the peak of the sinewave! Great video!
You may want to add a reverse diode in parallel to the zero crossing detection optocoupler. Otherwise, the LED inside might get destroyed by overvoltage while in reverse.
Since practically no current is flowing no voltage is dropped over the current limiting resistor and the optocoupler LED sees the full reverse voltage.
That was very cool that the SSR has a zero crossing bult-in, however make sense when you think about what it does. Very cool solution.
I'd been pondering a soft start circuit for DIY audio amp builds and I think you've nailed it, really nicely done! Thanks!
most of those just use a large current limiting resistor in series with the transformer and a relay on a short timer that shorts the resistor.
I solved this very same issue with my variac with a beefy enough inrush current limiter (NTC thermistor) in series with mains input. Much simpler and immediate than a whole softstarter
Also a solution :-)
Yet another great video. Most circuit breakers have a time delay trip to allow for inrush from motor starting. Maybe there is one with a longer delay available? When transformers are turned off the core steel will stay magnetized in the direction of the polarity at that exact moment north and south. When you energize it again, if the polarity is opposite at that instant, there will be a longer inrush time to correct it. Using the SCR relay seemed to be a great solution switching at current zero. The soft start would be great for tungsten light bulb filaments too.
I've lost count of how many times I got scared with induction motors jumping when they are switched on in practical college classes 11:16 haha
Aha. So it's a motor characteristic, not something caused by the slow-start circuit?
@@RandomNullpointer yes, it is very common if they are not attached to a surface
Schöne Schaltung! Ich hatte mir vor Jahren einen Sanftstart mit einem 5Ohm Lastwiderstand gebaut. Dieser wird nach 1s durch ein Solid State Relais überbrückt. Als Zeitgeber dient ein Ne555. Ich benötige die Schaltung für leistungsstarke PA Endstufen. Der reine Nulldurchgang hat alleine nicht gereicht, hatte immer noch Peakströme größer 90A. Du hast echt klasse Ideen, weiter so!
I've seen this problem several times. Most recently at a medical company, we used a 400VA toroidal transformer to feed the various AC-DC components. It popped the 5-6A slow-blow AC line fuse at 120V, but only when the unit was plugged into an AC outlet close to the electrical panel. Adding a 20' extension cord or moving to an outlet 50' away was enough line resistance that the problem never occurred. I measured the current spikes and they were >100A near the electrical panel. We used two NTC surge limiters (EPCOS-TDK, 5 ohm, 5A, I think). One in series with each 120V primary winding. So for 240V the 2 NTCs were in series. Problem solved forever.
BTW I looked inside a commercial 1000VA medical isolation transformer and they did the same thing.
you can use a timer relay and a 200w bulb lamp
the lamp connected at the normally closed switch it's other terminal to the load and to the normally open switch .
when power up the relay turns on (set to 1 second ) after 1 second delay
the lamp is connected in series with the load through the time relay normally closed switch
the load powers up slowly
(motor , transformer , bulb but no electronic circuit )
after 1 second the the normally closed switch opens and the load directly connected to the power source.
If you decide to use your small variac again you might want to use ring terminals and nuts instead of directly soldering wires. It should be safer and more durable.
Great job! Only question is what is the motor jump at 11:17. Just at the button push there is a jump and than slow speed up. Looks like a short maximum peak at the initial. It shoudn't be. Am I right? Did you fix it?
If you need to start a high inrush transformer -for example the type used in industry for 240 v to 120 v centre tap for use for industrial site safety. the simple way to "soft start" it is pulg it into a standard 15 to 25 metre 13 to 16 amp extension lead, which slows down the initial rush . The alternative is a C type rather than B type circuit breaker but in some -particularly domestic sytems it is not allowed - but would be on most PME systems.
If you add a potentiometer and a little bit code for the potentiometer, you can control lights to dim and be brighter depending on where you leave the pot knob.
Big fan of your work.
he did a project like this
Sorry to be the one to tell you that, but your calculations about the inrush current are not really correct.
The inrush current in transformers is caused by the magnetic remanence (magnetization of the core).
When you turn off a transformer it usually has a magnetization in the core afterwards. If you then connect the transformer again, this "offset" can drive the transformer into saturation, which creates losses in the core. This offset only lasts after a couple of cycles, until its gone.
This explains the unreliable tripping of the circuit breaker, since the stored magnatization is very much dependent on the timing when it was shut off during a cycle.
The DC resistance only limits the inrush current into the parasitic capacitance of the transformer (brings down the quality factor Q of the whole system).
So technically your not wrong, just not the whole story. It's probably rather hard to find a balance between entertainment and bone dry mathematical theory.
But all in all a great well explained video as usual. :) Keep it up.
Finally somebody who knows what they are talking about!
My simple solution is a resistor and a timer relay. The resistor needs to be small enough to drive current into the primary inductance but large enough to limit the current if it saturates. The time delay need only be a few cycles, so 100-200ms is sufficient.
Your circuit needs one more thing - in case you haven't thought of it yet. You need to drive a relay to close the AC circuit across the triac when the ramp-up is finished - and keep it closed as long as the circuit is on. This will keep the triac happier in the log run. Other than this, good job!
Will this soft starter work for a solar setup? Starting a motor running off an inverter and solar panels, the startup current is too high and trips the inverter.
i solved this problem (popping breaker) by adding two NTC-thermistors to the input (L and N), so as you plug it in, they "puffer" the current for just a few milliseconds and prevent the breaker from popping.
but for this a) i cannot use loads over 10A anymore and
b) they only works, when they are not hot. So plug the transformer in and instantly pulll it out and plug it in again will make the breaker pop again.
also another super simple solution, which i dont like, is to use a 20m extension cable. this also works like a puffer and will prevent the breaker from popping. (tested ;) )
Always add extra bypass relay for post soft start bypassing to prevent lose while system is running at full Walt.
I like your video style. Doing the math and explanation while writing it out. Pretty cool. Trying to understand this and the math/diagrams help. You got a new subscriber from me!!
by using a primary to secodary winding in a toroidal transformer and make the output winding voltage divided chould prevent the spike all together making the output winding like a auto transformer. the primary winding would have nearly zero core loss in a toroidal transformer so there woudl be no initial overload by powerup.
Did a similar thing for the mystrey rating variable voltage transformer I got. Unknown what the ratings are, but it seems fine to handle 15A output current. Just used a high power resistor in the input line, 4 10R 10W in parallel, and with a 100C thermal fuse between them, with a relay fed from a simple resistor, bridge rectifier and capacitor smoothing, so the relay turns on around 5 cycles after switch on and shorts out the resistors.
Also used a winding tap that gave 60VAC (lowest one on it) that fed a half wave rectifier, smoothing capacitor, and then a 47V zener regulator, that in turn feeds a 33V zener regulator, providing power to a LM723 set to provide an output of 6V, that can sink 10mA if needed. This is the one side of a simple voltmeter, so that the scale starts at 60VAC, and goes all the way to 360VAC, which is the range of the output at maximum. All made using parts either repurposed or lying around, including the box, which was the case the variable voltage transformer came in, just with a coat of paint, and the assorted outlet sockets attached. 60VAC to 360VAC because the surplus meter I had came with a 1mA FSD coil, and was marked with 0 to 30 already, so quick to simply use a black drawing pen to relabel the numbers, after a painting over of the originals.
Howdy.
With all due respect.
Bear in mind that the phase shift lags 90 degrees for inductors.
This means that switch on at voltage peaks actually produces the lowest inrush. Voltage and current are "already in phase".
Switch on at voltage null crossing will produce the largest inrush current until the current shifts out of phase. Depending on the rated VA it may take from about a few cycles to tens of cycles for Your 3000 VA Variac.
Please, feel free to verify this with an oscilloscope with recording capability.
Best Regards.
If you read the data sheet of the SSR, they state with inductive loads, the face value should be divided by 10... So yes, the inrush is the smallest when switching at max voltage....
anyway, energizing through a resistor is pretty effective. Great job. I've a similar problem. A colab maybe?
Literally the best videos on youtube
I’ve faced the same problem with my 10Kw Variac, I’ll try your solutions ASAP, Thanks 🙏
brillianT - just one question... why does the motor jump at the end before the shaft even starts turning?????
I think that was just the initial moment when the coils were magnetized and the rotor aligned with the field.
Newton's Third Law. The torque that turns the rotor has to push against something, which in this case was the frame of the motor, causing it to move as well.
@@thereynolds2725 Agree. But it looked like the same jolt as an across-the-line start of a motor that's not bolted down. Didn't expect it would be noticeable under soft start especially because the armature would have only had turned a few degrees to align itself. Cant make out and shaft movement from the video.
I'm curious why you didn't just use a thermistor? I'm assuming it's because they wast energy as heat, but Im curious if that's the reason or not
soft start circuits a class to have for loads of projects. power tools especially.
I have seen a simular control in high end, hi power stereo receivers..soft start off the mains before the amplifier comes to full power reducing in- rush current to other components..on a failure to soft start, I experience a corona in the cabinet, finding the regulators themselves burned along with local components damaged beyond recognition. Thank you friend for sharing ..
Oh and see my suggestion on your generator project..👍
Outstanding project Scott! You are a very talented!
I put an NTC in my Variac to prevent the circuit breaker issue. But this is cool for other reasons!
Another funny thing is capacitors in switch mode power supplies. I have a lot of SMPS devices, computers and servers in the house and I came across a terrible annoying thing with power failures. When the power comes back on the SMPS devices all charge up their capacitors and almost always trips the inrush breakers. This has happened to two different houses with modern wiring, which means it is more likely my devices causing the issue.
Petition for requiring (adequate) NTC thermistors in commercial SMPS to limit inrush current to capacitors, some of the computer PSU's have them but I don't think they limit enough when you have several SMPS devices. Alternatively a precharge circuit, I don't mind that the power supply is charging up over 5-10 seconds after applying power and then it just keeps that power in capacitors. There is no need for instant charging as far as I can think.
Also, with adequate NTC thermistors I also imagine there should never be soundable sparks when connecting power plug to the power supply, but most often there is quite an audible spark (with computer PSU's mainly), and sometime even a flash.
With all that said, I've had an interest of soft-starting groups of devices in my home after potential power failures (doesn't happen that often in new house though). Basically all I would need is to have a timed relays clicking on at different set times when power comes back which should not overload the breaker(s).
WOW WOW. You’re so good with electronics.
You also could change your old B16 Brecker to C16 Breaker
Or even c20 or C25. I want to make one wall socket for C25 in my garage so my angle grinder and welder wouldn't Trip the breaker.
Since you have a 3-phase induction motor lying around, why don't you make a soft-starter for that as well? Or do a DIY or BUY video for a 3-phase induction motor soft-starter?
I like how it's 2021 and he's still using torn pieces of paper to annotate things in real life.
Cheers.
You should have used a mechanical relay to bypass the TRIAC after softstart. The would have allowed you to use a smaller heatsink. And at 3000W, the amount of current flowing through the TRIAC might heat it up a lot, so having the circuit inside a closed housing with no airflow might cause it to overheat.
True, adding a relay would have been a nice addition.
I had a similar problem 20 years ago. Solved with a 2kW heater in series with the transformer that was shorted by a relay. It takes some tens of ms for the relay to close the contacts and this time is enough to limit current surge
Informative video.
I didn't realize that Solid State Relays have a zero crossing circuit.
This should fix my Siglent Bench PS from tripping the UPS on my router.
Just a tiny note, and I hope it's not rude, but "Such a" refers to something you've already described. Like "A resistor is a device that resists current flow, such a device can be used to..." Not something like "you can use such a resistor". Instead "a device such as this resistor" or "we can use something to limit the current such as this resistor" etc
Thanks for the feedback
Well, I have a 6kVA variac and while my solution is not as eloquent as yours, it works. =)
In industrial applications we would replace the breaker with a C type breaker. those breakers can handle a higher inrush current! A Hager MCN116 would be the replacement for that. That costs only 15 euro's. That display on the auto transformer might break when using a soft starter on the main's voltage. Maybe do a video about the karakteristics on circuit breakers?
An autotransformer isn't really a transformer. It's really a potential divider, with inductors instead of resistors. The inductors have reactive impedance, so they don't dissipate power, but that's really how it works.
3000 jigawats!? 3000 JIGAWATS?!? GREAT SCOTT!
in industry CB type K are used in this regard . good job dude
How would I go to... instead of a push button, use the thermostat of the ac unit make on and off...
So instead of a momentary switch have it for a toggle switch at line voltage ?
Great work thanks !!!
Why make it so complicated? I made softstarter to my 10kVA transformer by pre magnetizing the coil through power resistors for some milliseconds before directly powering the coils. Very simple and works.
I've been watching you since I was 12
Brilliant :-) I hope you learned something along the way
And now you are? 12?
Nice project I have a couple questions. Does this work for 120V? I've seen a couple other soft start projects which claim that a relay is a more reliable long term solution than a triac? Also would like to see a soft start which starts up automatically when the load device is powered on. Thanks.
I think that using a relay when done with a softstart would be a great idea, because of the loss on the triac.
Love the project with a practical example built right in
That's why power supplies, such as the ones in computers, have a big thermistor in the input
Err.. we used to do this for big ham amp power supplies with a resistor and a time delay relay. Cheap simple potter and brumfield socket relays set to 3 sec or something like that. Those were supplies with big caps to charge though.
You can use another curve circuit breaker too.
Like D-curve Circuit Breaker.
He lives in Germany. They are only allowed to use B-type breakers at home.
True
@@sergeyk9060 Thats not true, we also have C-Type breakers which are rated for higher inrush currents
@@legominimovieproductions thank you for correction. Im not fsmiliar with German standards. Pretty sure its recommended to use B for home appliances tho. Is it easy to justify using C's with electrical regulators?
@@sergeyk9060 It depends on the total grid impedance. You have to make sure a C-type circuit breaker will trip instantly (
Your explanation of the inrush current on an inductive load isn't quite correct; what you describe is more like how a capacitive load acts than anything. For motors, the inrush current is due to the mechanical inertia; when first turned on, the motor isn't spinning and so there's no back EMF to limit current flow, but once it gets up to speed the back EMF lowers the effective voltage across the coils. For transformers, it's actually caused by connecting *at the zero crossing*, which can even saturate the core and cause its inductance to fall dramatically, further spiking the current upward. I'm not sure how your zero-crossing switch fixed the problem, actually... The ideal moment to switch a purely inductive load on is actually at the peak of the AC supply voltage. Your transformer may have a significant capacitive component to it, perhaps? Could be a parallel capacitor added for power factor reasons; I doubt it's parasitic winding capacitance.
Hmmmm I will have to think about that. Not quite sure....
@@greatscottlab It's a real puzzler; I wonder what's inside that autotransformer.
Great video, although using a microcontroller, a triac and a lot of other components seems like a massive overkill. A NTC thermistor with a relay that bypasses the thermistor after around 1-2 seconds would do the job as well, but with way less components. But of course, with the microcontroller route you learn a lot in the process.
The core of the transfer is probably saturated when turned off in a certain point in the AC cycle.
The next time it is powered and the current is in the same direction as the created saturation, a huge current flows through the transformer, that trips the breaker.
I've got a 4kVA variac, if I switch it on it's 50:50 whether it'll trip the breaker or not. I use a big 7kVA mains conditioner which soft-starts it and prevents tripping.
Thanks so much for your efforts in teaching us for free (I´m going to design one of this for my grinder!) Stay teachly!