V/Hz Control for Motor Drives (Full Lecture)
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- เผยแพร่เมื่อ 27 ม.ค. 2017
- In this lesson we'll take a brief look at V/Hz control for motor drives. We'll define base and maximum frequency. We'll learn motor drives not only vary excitation frequency but also applied voltage for excitation frequencies below the base frequency. Additionally, we'll examine simple linear V/Hz profiles and more advanced quadratic V/Hz profiles, dual slope V/Hz profiles, and special purpose V/Hz profiles for reduced or increased torque applications. Finally, we'll preview how the V/Hz profile must be used in combination with frequency as function of time plot during acceleration and deceleration events. (Full Lecture)
NOTE: Alternative V/Hz profiles include "constant boost" which is similar to a linear profile, however, includes a initial starting bias at low frequencies it is then linear to the base frequency. Additionally, a "start boost" profile includes a constant boost at low frequencies then it flat lines until it meets the normal V/Hz profile. Other application specific profiles exist.
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Motors and Generators • Motors and Generators
Motor Drives/Variable Frequency Drives (VFD) • Motor Drives
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Cheers Jim what you've put up here is at a fundamental level 'gold' for those building an understanding of VFD's.
I have come to like your videos. It's helping me develop my career in Africa. Many thanks Sir
Thanks great video. Great review.
Imagine how well lab would go if applications (cars, airplanes, motorcycles, boats) were added into the lecture. They all require different curves and regimes of acceleration/deceleration rates. I hope this is address individually at one point in time. Other than that excellent lecture.
Insightful exploration
thank you
Great!
I have a question related to induction motor reg breaking. How do put a motor in reg breaking mode. In other word how is the voltage and frequency set, relative to base values, in order to put the motor in regenerative breaking mode ?
Check out the "Motor Drive Deceleration Methods" lecture at: th-cam.com/video/P4bDpyJCzp4/w-d-xo.html
Thanks
Pablo I told you about this awesome lesson didn't I?
Does hertz change the amount of voltage? If something is running off of 480v at 60 Hz and is then changed to run at 10 Hz, does that make the colts lower?
If the motor drive in question is using a V/Hz ramp, yes, 10Hz should have a lower magnitude than 60Hz. Presuming linearity (which it isn't) you might expect 80V at 10Hz (ie: 1/6th of base). This being said magnitude, frequency, and, for that matter phase shift and phase sequence, are independent properties and can be varied individually.
Jim Pytel Thank you Jim, i appreciate you taking the time to answer.
Hi Jim, why does current remain a constant when voltage and frequency are decreased together? I'm trying to figure out the relationship between current, voltage, torque and frequency. I see books and your videos talk about the torque and speed curve, the voltage, frequency and time curve, but where does current come in play? Would you mind recommending books that cover these in great detail?
Check out the inductive impedance and AC Ohm's Laws lectures at:
th-cam.com/video/xowCaVH44MU/w-d-xo.htmlsi=y2G4ULz6BYfjLAzz
and
th-cam.com/video/F2DpVRdsU0Y/w-d-xo.htmlsi=-NGQrWS9I3IWzjMR
Long story short inductive impedance is a frequency dependent phenomenon. As frequency increases inductive impedance increases. Therefore one can apply higher voltage at higher frequencies given opposition (impedance) is higher thus current remains manageable.
When will i understand these things ... vfd made things tough in industry
I have a question that if I make a v/f curve in which the v/f is initially 5 volt/frequency tiill 30 hertz and then 6 v/f ratio till 45 hertz and 7.6 hertz @ 50 hertz for a three phase motor. Will the motor run at higher speed or not? I saw a yaskawa vfd drive example to understand this. I have a very unreliable voltage from the grid and I intend to keep running the pump. I have a 25 HP motor that is running a 15 HP load. Motor has amperes rating of 40 Amperes and it is taking a load of 28-30 amperes. So, should it be alright? Will it be able to increase in speed? the torque required at low speed is not much and certainly less than the full rated torque
There are plenty of different multi-slope V/Hz profiles out there. A lot of them feature a little "boost" at low frequencies just to nudge the rotor to a start. Others, like the one you're suggesting, might feature a concave type slope. Not sure what you're asking about "higher speeds" though because a motor's speed is dependent upon frequency. Higher frequencies result in higher speeds. This being said, there are practical limits and mechanical considerations of motors designed to operate at specific speeds. Don't exceed a manufacturer's recommendations.
@@bigbadtech thank you. I have a grid supply that is very unreliable in summers. I need 380 voltz at 50 hertz and my ideal speed is 2900 rpm but since I have a vfd its speed reduces so much as the grid voltage is just 230-250 volts in three phase in summers. So, I want to keep frequency high and make the curve into a concave as you explained to keep the speed above 2300 rpm and keep getting water out of it. I am considering the overmodulation function of my vfd but I am scared that if I use it to increase output voltage it could be dangerous. What do you say? should I go for overmodulation as well as changing the v/f curve?
I get it. You might need a speed V/Hz profile that flatlines at a desired maximum voltage beyond a certain frequency rather than a continuous concave slope.
@@bigbadtech So, do you think I can increase speed by weakening the field (sacrificing the torque even at lower frequency to get desired frequency at low voltage?
Still not exactly sure what you're asking. Ideally a motor drive has a certain input range (example: 200-240V @ fixed 60Hz). As long as input remains inside this range, the motor drive should be able to produce the desired output voltage magnitude and frequency capable of driving a properly sized motor at a desired speed. One can think of the fixed magnitude and frequency AC input as being "isolated" from the variable magnitude and frequency AC output by the DC bus. Check out the "Rectification and Inversion in Motor Drives" lecture at: th-cam.com/video/FZIVhHNdYnY/w-d-xo.html
is it possible to go under the 6HZ? if yes, what will happen?
Explosion! Just kidding. Depends upon the motor drive in question. Anything lower than 6Hz is almost DC anyways. Speaking of which some motor drives offer DC injection braking: th-cam.com/video/P4bDpyJCzp4/w-d-xo.html
@@bigbadtech thanks man appreciate that!!