back emf explained
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- เผยแพร่เมื่อ 20 ต.ค. 2016
- Back emf is a situation called self inductance and occurs in electrical motors. It results in power loss that is unavoidable, since it is a result of physics principles.
This video covers the basic of why it occurs.
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Suggestion for the future videos. Whenever you are talking about a hand rule, add a small picture that shows how the hand rule is supposed to work.
+Fearofthemonster good suggestion. Thanks
I produced a video on hand rules to help
th-cam.com/video/GmURfG3tAhg/w-d-xo.htmlsi=rrAiloqdT7lUqZv6
Without back emf, all the electrical power will go to heat dissipation and the mechanical torque would be a free lunch. Back emf comes from energy being conserved, it lowers the armature current, making it look like there is an extra resistor, but instead of dissipating power through heat like a real resistor, the dissipated power the shows up in the torque.
Tq
If the energy conserved during the work done against the back EMF is actually converted into torque then shouldn't there be more torque when the back EMF is more
@@bethanyrumkui2293 Higher rpm causes higher back emf which acts against the voltage from the power supply to lower the current and the torque. Since mechanical power is rpm * torque, back emf has a regulating effect on the power draw. Without back emf, a circular saw hooked up to 110V outlet would blow a fuse, but the back emf brings the current down to safe levels.
@@karilentz5000 as I understand it, this is how potential relays work with a start capacitor. Ive went to school for and worked in the HVAC field for 11 years now but am still learning.
Other than getting the right hand and the left hand mixed up, probably the best explanation video on TH-cam.
Thanks. There are different hand rules. I use the palm rule which uses the left hand for induction. The Fleming rule which you refer to uses right hand. I have a video on different hand rules.
th-cam.com/video/GmURfG3tAhg/w-d-xo.htmlsi=rrAiloqdT7lUqZv6
Great explanation, especially at the end when you mention why there is a special circuit designed to protect the motor from burning (to absorb the spike in current due to the sudden absence of back EMF when the load is suddenly disconnected)
Thanks for the explanation on the relationship between back EMF and flyback voltage spike.
This really helped! Thanks :)
thanks so much for this explanation.
Thank you for the clear explanation !!!
That's also why motors have such a huge current spike when they first start.. Because at first the full voltage is essentially back EMF since there is zero movement-- but once the movement starts, the current will drop proportionally to the RPM. On larger motors, the inertia is so great that the current spike has so much back EMF that the motor can't even start... I'm assuming that's why they will require a 'starter capacitor".. such as in HVAC motors.. which use a "run capacitor and a second "starter capacitor. (I could be wrong about that. :)
Great explanation thank you!
Awesome explanation.
Thank you so much, my physics exam is tomorrow and my useless physics teacher didn't cover this, you're a life saver
I think the voltage that face resistance (as you said back emf) is like drain of voltage which gets stored in the form of magnetic energy and released when input voltage is going down (forward emf)
Good explanation of the back emf
Thank u sm i never understood it but u saved me Paul the physicist :)
thanks for posting. some thoughts. imo, when talking about a BRUSHED motor being damaged when it is forced to stop while still receiving power, the voltage and current spikes are caused by a short circuit through the commutator contacts and armature windings. (the only resistance present in the circuit is a few ohms in the windings.) i have a theory about back-emf in a coil. take a look if it suites. cheers
Thanks! Tony!
Thanks for the explanation. Electronic crash course with a voice and accent so close to Ironman's voice, made my day
Haha. I hope that’s a good thing.
Legends on TH-cam🤙
thank you that was very usefull
Please elaborate on the last point regarding speed vs current?(How?)
sucha goated video
thanks professor!
Finally someone with an Aussie accent that doesn't speak at 300 words/second
I thought he was South african tbh lol
Actually Dutch by birth. 😎
High School Physics Explained ohh that makes sense
GREAT EXPLANATION!
Thanks!
Subscribed😍
I RARALY COMMENT, THIS VIDEO IS VERY GOOD!
great
Excellent video 👍🏼
Thanks
I have a question. In an elevator door operator using an open loop DC door motor, the speed is set with resisters, resistance added or removed to vary the speed in the open or closed cycle. If I added a spring force in the closed direction, would the additive force increase the closing speed? For example, think of a tape measure with a coil spring that pulls the tape back into the body.When the door motor is commanded to close, the speed is x. If the coil spring had an force of 10 or 100 pounds force, with the added coil spring force, would CEMF limit the motor speed (without feedback) or would the rotational motor speed increase?
You mixed up Fleming's rules but thank you!
Fleming’s rules are one way to determine vectors but not the only way. I have another video that compares Fleming with the rules I use.
We aussies use the right hand palm rule :)
For electron theory, it's left hand for generators and right hand for motors.
Aaaaahhh That's why it was backwards when I tried it... Is this a conventional flow vs.. actual flow thing?
Convrntional
great work
make video on hall voltage
Thanks for the videos paul! I have one question. How come the current also decreases when the back emf takes effect decreasing the voltage. I think i am confused because in a transformer question for hsc, energy must remain the same, so if voltage drops, then current will decrease. Could you please explain why the current drops too? Thanks
thanks for the question. Don't confuse a transformer with a motor. In a transformer the input emf is separate to the output emf and are connected through flux linkage. They are separate circuits. This is where LCE applies, and as you correctly note, that if there is a drop in voltage, we get an increase in current (in. the secondary coil)nt
In the motor we only have one load, which is the motor, so a back emf is generated that lowers the net voltage. Since it is one load, Ohms Law now applies, so as voltage decreases so does current (assuming the wires are ohmic resistors.
Thank you for the explanation, but I don't understand why it rotated counterclockwise because all I understood is that the thumb points to motion, the index finger points to the direction of the magnetic field, and the middle finger points to the direction of the current. Therefore, I think the index finger should point to the right side, and the middle finger should point to outward as B (because the currents flow from left to right), then the thumb should point to the upside. According to this (my opinion), it should rotate clockwise. Please let me know what I missed, Thanks
First of all thanks for the simple clear explanation second can you pleas explain why there is no back current !
There is actually a back EMF (not current), but did not discuss it here. Here is a video I have on back EMF - th-cam.com/video/5mf4NmmLWnE/w-d-xo.html
When a generator runs and no load is connected at the ends of the terminals, does electric current produced in the conductor.? I understand when we connect a load forms a closed circuit allows the electrons to flow but what really happening on the conductor when there is NO load?
Did you mix up when to use left and right hand rules in this video? I thought left hand was for motion and right hand was for electricity but you seem to have done it the other way around?
When you say current decreases with speed, (at around 5 minutes) you mean that it actually increases but decreases proportionally. ie. it doesn''t increase by as much as it would without the back EMF but it does increase?
You're not saying the current actually decreases whilst the voltage increases, are you?
Just making sure because if you are I need to do some re-thinking.
Thanks.
thank you sir
THANK YOU THANK YOU THANK YOU
Hi, thank you for this video. I would like to know what happen if I cut the applied voltage while the motor is running. I will only have the back emf due to the inertia and it will cause a reverse current right?
You are correct, though it is not back emf now, but simply an induced current. but due to Lenz Law, it will also cause a braking effect, and slow the motor down
Thanks for this question, Helen! I was curious about this too 😊
Since your comment was written five years ago, I wonder if you've found how to deal with this current that flows in the opposite direction.
Everybody talks about freewheeling/flyback diode, but that only deals with an inductive kickback that flows in the same direction as the original power source.
Is it because we don't need to do anything about the current generated by the rotational inertia?
Please help!
Is the back emf caused due to the north CW spin field accelerating towards south CCW field so as the current is induced by the number of free electrons attracted to the field creates a counter rotation thus generating electricity? since voltage drives the dielectric field divergence of free electrons in current which causes energy dissipation ( heat inside the wires ) . which of course applies to the field of the ether phenomena ...
What happens during the off phase when we control the motor in PWM (DC=50%)? Because now due to the inertia we will have the coil rotating inside the magnetic field. From oscilloscope measurements with a brushed motor I have seen that there is a current in the opposite direction and a voltage generated (don't know what is the correct term for it; still back EMF?) that is the same polarity as that of the battery (acting like a generator or something). Why does the current change polarity but voltage stays the same? If you could explain would be highly appreciated. Thank you so much!
In a single phase compressor motor, why is the back voltage on the start winding higher than the supply voltage? I am asking with respect to the use of potential relays.
thank you
What would happen in your electric drill example if back EMF wasn't accounted for?
direction of induced emf is found using flemings right hand rule!!
Thank you for the video though
+TMS that is true but using the left hand is equally helpful. I chose that way since the fingers are consistent for magnetic field lines when comparing with the right hand palm rule for motor effect. I also like the complementary aspect with the motor effect - right for motor, left for induction and you can tie in Lenz law. What ever works best for you, stick to it.
In many texts the RHR is used for the motor because the thumb signifies conventional current. This is opposite to electron flow.
(Obviously the reversal for induction)
what if, i store the energie of the back emf appearing into a coil ,to a capacitor,with a certain timing,and so if i re injected it from the capacitor into the next coil? i learnt back emf peaks (when disruption happen)are between 10 to 30 times more powerfull than the original current that made it,so in all logic, the fact to stored and re injected it (always at a certain timing),change his direction,to appear as a "conter-back emf" current, made it usable for a more efficiency rotation?
tell me what you think about it....thanks to care
thanks for your video mate ;)
Suppose I apply 12V to a DC motor and let it achieve it's max speed, then it's back EMF is somwhere around 11.5V. If I were to observe the voltage between the motor pins on an oscilloscope, It would show 12V.
I have 2 questions.
1)Where would I see the voltage being 12-11.5 = 0.5 V?
2) If i turned off the motor, only back EMF would remain, would that mean the polarity of the voltage seen on oscilloscope would change?
1) its unlikely for you to get a back EMF of 11.5 V. More like a few volts. Did you attempt measuring the voltage? You should get a reduction in voltage. I have done this experiment multiple times and admittedly I have always measured current, but that always drops when the motor spins. That can only occurs is voltage drops (resistance remains constant, barring any temperature fluctuations)
2. Yes, if you switch the supply off and the motor keeps spinning, in essence you will have a generator and this you will register a negative voltage. (though do not call it back emf at this stage. That term is only applicable WHILST a input voltage is supplied.)
Thank you for a fast response!
From the data sheet of my motor: At no load speed = 4008 rpm = 420 rad/s. The back-EMF constant for given motor is K=0.027232 (V*s)/rad. The back-EMF is equal to shaft speed*EMF-constant, thus 420*0.027232 = 11.4V.
When i tried the second experiment, the voltage just decreased from 12 to 0 in about 0.5 seconds, no polarity change seen on the oscilloscope?
Aren't the causes of back-emf and "generator effect voltage when stopping" the same? Is it just wrong to name the second voltage back-emf?
hmmm, I will have to look into that. Bear with me
- Disclosure. I am Physics teacher, and not an electrical engineer. :-)
In negative voltage
For a BLDC, your ability to see the bemf will depend on how the motor is driven. If you have a PWM/commutation scheme where any of the phases is floating at some point, then you'll see the bemf on the floating phase ...
It's a good start - but the use of the term 'right hand palm rule' for Flemings Left Hand Rule and the 'left hand' for Fleming's Right Hand Rule is still very confusing.
See comment above - though I do have a video on the hand rules - th-cam.com/video/GmURfG3tAhg/w-d-xo.html
In this presentation, the actual voltage increases from 10 V to 11 V, so the current should also increase apparently, instead of decreasing. I suppose we should assume that the actual voltage (input minus back emf) in fact decreases...
When at 5:00 you say the current decreases, do you mean that it's rate of increase is less than might be expected or that it actually goes down? I mean will the current at 11 volts be less than that at 10 volts? Surely not?
Also, thanks for another excellent video.
I had the same question. I think he means rate of increase goes down.
Someone else asked and then said he already answered it to a fourth person. So I'll try find that answer.
He answers Frank (who used to be 1011) with this: "The current does decrease as rate increases but my example with my back emf value is too low and hence looks wrong." So for his example the current would increase since the resistance is constant.
You say increased speed will reduce current. However, at 5:00 you show that increasing the speed by increasing the input voltage from 12V to 14V actually INCREASES current as the voltage across the motor winding increases from 10V to 11V.
Is this a mistake? If not how can this difference be understood?
I just noticed that 10111 already commented this and you replied so feel free to ignore.
I dont get why the current decreases .The voltage increases from 10V to 11V, since I=V/R , should I be bigger as you increase the speed of the motor?
You're voice is like iron man.
Hi. Just wondering if the back emf ever equals the initial voltage, and thus if current ever actually reaches zero, or if it just approaches zero.
You can only hypothetically reach zero, if there is no load on the motor. Considering that the motor will still have a load in the form of frictional forces, therefore work still needs to be done. This means that the supply voltage always be greater than the back, EMF, giving you a net voltage to do that work.
is there back emf in induction motors?
Does back emf have in ac motor?
What is the result say both faceing ends of the magnet in your diagram are the same polarity.
So is 5:1 actual ratio of forward emf to back emf? What percentage usually is back emf relative forward emf?
No it isn’t. It was just an illustrative example.
For Motor, we need to use the left-hand rule. Right-hand rule is for the generator.
Yes if you use Fleming’s rule. I use the palm rule which is reversed. See my video on hand rules.
i understand that you use your own hand rules, but deviating from the standard as someone far out of highschool trying to learn this was terribly confusing even after viewing your hand rules video, other than this it was helpful in understanding emf
How do you fit in faradays law here??
I have a real question: what is inside a magnetic line in between the 2 poles N and S can you tell?
hye.
when motor is starting it creates B.EMF. Say for a 220V motor, 1000W, 1500 rpm,what will be the Back EMF?
Does this EMF permanently induced when the speed is maximum or subside to 0V when the speed is max?
to calculate the running current, is it I = V/R or I= (V- Vemf )/R
+Adam Mukhriz I'm not sure about calculating the back emf, have to look that up but the back emf does not subside. It's dependent on the rotational speed. It's all about Faradays Law.
The running current is determined by the second formula you gave.
High School Physics Explained just wondering it should be a small fraction of the supply voltage or almost near to supply.
secondly when we measure using a meter, is it a supply or V- Vb? For a motor which is running definitely we got 220V at the terminal.Hence it seems the terminal voltage is the one causing the running ampere ( which ia the full load ampere) to flow .If the voltage causing the current to flow is V-Vb, the current will be smaller and not the same as the nameplate. Unless the Vb os small so that the net voltage wont be too far away from the supply hence the current will almost be the same as the nameplate.Hope you can clarify.
or if the running motor is supplied by 220v, when we measure at the terminal we still get 220v and not lower.If Vb is presence V measured shoyld be < 220V, unless Vb~0.
How does that even make sense? You're saying as it speeds up the overall voltage increases like in your example from 10 to 11 but then you say according to this the current decreases. V=IR and this has constant resistance so I'd expect the current to increase but you say it decreases.
10111 you are correct. I'll have to fix that. The current does decrease as rate increases but my example with my back emf value is too low and hence looks wrong.
Thanks
Can there be a time or stage in the process where back emf equals the applied emf and net current is zero??
No. It’s self regulating. The more back emf the more resistance. Thus then lowers the spin rate. But then that lowers the back emf.
Thank's doc!
Lenz law is nuts
I wish somebody would explain this with oscilloscope
AKA "counter electromotive force"
Am i missing something, what kind of motor we are talking about? Because in an induction motor the speed only depends on the frequency you are using and not the current.
It’s about a DC motor, not induction motor.
Faraday Disk has No back EMF??? P.S. I'm remaking the Faraday's Disk (1831) even better with monopole magnets.
The copper disk is now a single pole, put the N&S copper disks together and spin on its face. Monopole materials are more conducive, and monopole magnets pull harder together.
You could say.....
“A motor is a generator, and when you spin one manually, it generates an AC voltage”
And since....the math is never perfect, and all these things are made in Taiwan, when you turn on a motor...it can actually generate some electrical pixies back into your circuit
The motor may appear to be spinning pretty consistently
But...under an o scope, there’s fucktuations between voltage and magnetic field and all sorts of stuff that wind up being kinda like a god damned shit-tastrophe
If you didn’t wire up some diode packs to stop these back fucktuations of electric pixies, it can potentially blow up all your fancy and delicate computer bits, turning your robot into a candle
Now....here’s some math....but remember, the math only works in a theory vacuum where the parts all have perfect tolerances
Which doesn’t exist
So, it’ll give you a decent estimate which can be super helpful
But will never be correct
Of course, back emf is great, but inrush current to coils and motors is way more likely to blow up your poorly designed electronics widget
And....inrush current effects back emf as well
Why do motors need a capacitor to get started.? Large motors like elevator motors?
Can we calculate the magnitude of back emf?
It can, by using Faradays Law
yes.. magnitude is same as emf equation of dc machine...back emf=¶NpZ/60A. where ¶= flux
@@bunnygalta5071
Thanks
Have in mind this does not apply to induction motors which work exactly because of back emf.
im confused why current will decrease if voltage e increases
I can die in peace now…..
Good video except that you've got your palms round the wrong way - right hand for gena(right)ors and therefore left for motors.
That’s only true if you are using Fleming’s hand rules. I have another video that discusses the different forms of the hand rules
@@PhysicsHigh - OK - thanks for sharing - FYI we only teach Fleming's hand rules and the right-hand grip rule here.
In a sense it would be advantageous to teach Fleming’s so it’s consistent across tertiary level but I find many time my students struggle remembering which finger is which. So using the slap rule with many fingers representing B field helps them. Thanks for feedback though.
I use the numonic 'Manchester Football Club' for Motion, Field, Current and name the fingers from top-down. I hope that's helpful.
...and remember which hand to use with gena(right)or.
Good other than getting hands mixed up lol
Hands are not mixed up. Using different hand rule. See my video on handrules. th-cam.com/video/GmURfG3tAhg/w-d-xo.html
The pointer is hardly noticeable, rather confusing when you speak about this wire this wire, a bigger and perhaps yellow pointer would have helped
your hand rule at the beginning is also wrong. When talking about motors, you use the LEFT hand rule, not the right. This is why it's so important to have a physical hand diagram right there.
Number one question. When you talk current, are you talking about conventional or electron current. This confuses every n00b.
Convention current. This is standard practice. If we talk about electrons we call it electron flow.
You made an error deriving the direction of the rotation. Sure, F=Q*E'=Q*(E+vxB), but Q = -e !
No error. Direction is determined by Fleming rule or palm rule which uses conventional current not electron flow.
Even with back emf how can someone says motors are above 95% efficient.
Hello high school Physics explained, i dont understand why the current would decrease. Could you explain it to me?
+Chris John think of input voltage as pressure. That allows current to flow. More pressure, more current. Thing is, with electrons flowing, they create a back voltage, pressure if you will. That means the NET pressure is reduced. Hence lower current.
thanks
hold on. shouldn't it be fleming's right-hand rule by standardization/convention when you're talking about induced current such as a generator/dynamo? why are you "back"ing the mnemonic technique? i'm here to gen myself up on some info i didn't know much but until i realized that you also messed up by giving a minor false info.....
Different hand rule. Not using Fleming’s. I have a video on hand rules
Sir can you Please Explain Semiconductor Working ?
I already have a number of videos on that start with th-cam.com/video/zdmEaXnB-5Q/w-d-xo.html
whihaout magnet the emf was here.
The direction of rotation is wrong.
No pay attention the red rows are the direction of current
Left hand rule for motors, right hand rule for generators
@10vanaqv different hand rule
See th-cam.com/video/GmURfG3tAhg/w-d-xo.htmlsi=rrAiloqdT7lUqZv6
i used my rhr but it rotates in cw direction
damn right! this guy is wrong big time
It worked for me. The right hand rule he is talking about is the right hand slap rule, where the fingers point towards B, thumb points towards I and the direction the palm would "slap" in is the force. It's a bit confusing, might want to google it.
😃🤓
I guess you are using electron flow and not conventional current?
Using conventional current.
@@PhysicsHigh Oh. By right hand rule do you mean right hand palm rule? I thought you meant Fleming's left hand rule but called it right hand to switch from conventional current to electron flow. My bad. I'm new to electrical engineering
Someone doesn’t know his left and right.
If you referring to hand rules there are two versions. I don’t use Fleming’s hand rule, but the palm rule.
@@PhysicsHigh I was just going to ask if that were the case. Took me a while trying out what I assumed you meant as Fleming's left hand rule and couldn't make it work. Of course.
Fortunately I've seen your video on the subject. Just a suggestion: could you mention which version of the rule you're using, ie. Flemings or palm rule, pls.?
Apart from that, thanks for another excellent video.
Flehmings rule explained wrongly
You should explain, in detail as you go, these "hand rules" you are referring to. I quit watching after the first couple of vague references which weren't any help.
I assumed a knowledge of hand rules as I made the videos in sequence of other topics. I have a separate video on hand rules and how to use them.
highschool ;_;
Very very poor visual explanation... we watch videos to learn visually, not to simply be lectured to.
To long and boring