3-phase motor torque and VFD technolgy - Page 1
Forum section: electronics / projects

author: Jester
author_rank: Frequent Contributor
timestamp: January 04, 2025, 01:32:30 pm
content: This might be a lively discussion......Assuming a particular 3-phase motor is rated at say 1HP and 1800RPM, and powered by a VFD.My understanding is as follows, please correct if I'm missing something:1) Generally speaking low end and perhaps early generation VFD's use V/Hz type technology, so at lower frequencies, the voltage is reduced and as a result torque is reduced.2) Generally speaking better VFD's often offer Sensorless Vector Control technology that measures the back emf of the motor and as a result can safely provide higher currents (and hence more torque) at low speeds compared to the simple V/Hz type technology. At really slow speeds the back emf measurements become less than ideal because of the slow updates.3) If one needs to squeeze the absolute most torque out of the motor at low speeds an encoder on the shaft can give the VFD accurate positional information and so even greater low speed torque can be realized.4) Inverter rated motors typically have better insulation, cooling and bearings to improve reliability because the motors are intended to operate outside their nominal speed rating.Yesterday I was discussing this topic with someone and they claimed that what I really needed for maximum torque at slow speeds was to order a very expensive triple the cost "constant torque" motor that had its windings wound in some magical way that would provide maximum torque at slow speeds even if driven by a V/Hz type VFD. Claimed it was all about the motor and little to do with the VFD. I'm skeptical. I do understand that dual winding motors are available that allow running at a higher and lower synchronous speed by changing the number of poles, and if running at the lower speed yes the motor will have more torque in the lower speed mode.So do these magical constant torque (when driven by a V/Hz type VFD) motors exist and if so can someone expand on what makes them constant torque?

author: CaptDon
author_rank: Super Contributor
timestamp: January 04, 2025, 02:10:30 pm
content: My VFD experience falls into two catagories. First is the industrial setting. A big use of the VFD is to allow optimal speed settings based on load or air handling requirements. I can make process control much more efficient and save energy. It is also useful in slowly ramping up high inertial loads. The better insulation and better bearing part seems to be mostly mandated by severe mechanical resonances as the motor comes up to speed. We had a 300 horsepower blower with combination axial and centrifugal stages on the same shaft. The unit had so many mechanical resonances as it went from 0 to 3600 RPM it sounded like it was going to rip itself right off the mountings. The PWM of the VFD resonating at various points in the spool up was insanely loud!!! With no ear protection (which would be a stupid thing to do) it seemed to cause a sense of anxiety. The second experience is with locomotives. As for constant torque...we didn't do any winding transitions on our A.C. locomotives which were basically squirrel cage induction motors. They were driven 'as needed' with anything up to full rated input even at the stalled condition and the maximum available input power was tempered by measured motor temperature, calculated power vs. time temperature rise, and any wheel slip conditions. We ran one inverter stack per motor, our competition ran one inverter stack per truck which could have two or three motors. It would seem that a motor with more poles would have better low end torque. Didn't exactly answer the question, only injecting practical experience in the field.

author: schmitt trigger
author_rank: Super Contributor
timestamp: January 04, 2025, 03:57:26 pm
content: I believe that the following images will be relevant to this discussion. From the book Electric Machinery, by Fitzgerald, Kingsley and Umans. McGraw-Hill 1990.The torque of a three phase induction motor is governed by its slip. Meaning the difference in the frequency of rotation between the field and the rotor itself.With this in mind I would make an educated guess that VFDs designed for maximum torque would use an external tachometer, to accurately measure the rotor speed, and adjust the field frequency to be at the slip’s point where the torque is maximum.Now, those special wide torque band motors have existed for a while; they are the double-squirrel-cage rotors. Its operation is an extremely clever way to control the cage’s effective resistance via the changing leakage flux as the slip varies. Again, described in detail on the same book, and I attach an image from the explanation.

author: jmelson
author_rank: Super Contributor
timestamp: January 04, 2025, 06:22:20 pm
content: Most modern VFDs have "torque boost" that allows some inflection points in the V/Hz curve, so that more voltage can be applied at lower speeds, then at higher speeds they merge back into the straight-line V/Hz at the top end. I had an el-cheapo Chinese VFD that had this feature and worked great at low speeds, but I couldn't figure out how to get it to merge at the higher speeds. So, it caused overcurrents at higher speeds. The documentation with the cheap unit was just ONE PAGE! I had to buy a more expensive unit from Automation Direct. It came with an ~100 page manual, and it was clear how to program this feature.Jon

author: Jester
author_rank: Frequent Contributor
timestamp: January 04, 2025, 06:40:43 pm
content: Double-Squirrel-cage rotor..... Very interesting, if I have learned one thing about electric motors is that they are not quite as simple as they appear!Now I'm really curious how a Double-Squirrel-cage motor compares (low speed torque wise) to a single cage driven with a Synchronous Vector mode VFD?

author: Picuino
author_rank: Super Contributor
timestamp: January 04, 2025, 06:42:52 pm
content: In motors as small as the one you mention, if you need a lot of torque and precision at low revolutions in an industrial installation it is best to use a brusless servomotor. It will not be much more expensive and you will be able to drive the motor not only in speed, but also in position with maximum torque (above 100% for a limited time).In more powerful automotive motors, techniques such as the one you mention are used to achieve high torque at low and high speeds.

author: bdunham7
author_rank: Super Contributor
timestamp: January 04, 2025, 07:10:28 pm
content: Quote from: Jester on January 04, 2025, 01:32:30 pmInduction motor design is a very specific sort of thing, generalities are not really that useful when it comes down to actually making something. Dual winding/speed motors could have more or less torque on the lower speed and there's no reason to assume one or the other. Two-speed pool pump motors, a common implementation, have much less torque at the lower speed.QuoteI can't provide details on what is specifically available for VFD applications, although I'm sure there are designs optimized for starting torque, constant torque, etc. For regular fixed 50/60 Hz designs though there are many different torque/slip curves. For some, like a fan or fluid pump, you want maximum torque as close to the synchronous speed as possible and these motors are said to have a high breakdown point. Others, such as an overhead crane, will be optimized for maximum starting torque, these have a low (or zero) breakdown point but don't have any torque near synchronous speed. These are definitely characteristics of the motor.As far as expanding on how the designs differ, a guy named Charles Steinmetz wrote a book on this long ago and I can't think of anything to add to that.

author: Siwastaja
author_rank: Super Contributor
timestamp: January 04, 2025, 07:17:30 pm
content: I could be wrong, but I get a feeling the guy is just old school enough to not understand what VFD truly is and what it does.If you want to run with fixed frequency (directly off mains), then surely the motor construction is everything.But there is reason why those curves that we see are torquevs. slipcurves. They arenottorque vs. rpm. With fixed frequency, they are the same thing, really, so I understand people used to see motor torque curves before the VFD era get confused.But with VFD, the whole point is keeping the slip atoptimumvalue (nearly but not exactly constant), so how the curve behaves near the left edge does not matter. Efficiency is shit there anyway, with some motor type maybe a bit less shit, but VFD avoids that region completely.

author: bdunham7
author_rank: Super Contributor
timestamp: January 04, 2025, 08:06:01 pm
content: Quote from: Siwastaja on January 04, 2025, 07:17:30 pmWhich guy? Charles Steinmetz has been dead for over 100 years. Me? I understand what a VFD does and why but I don't pretend to understand the exact math and art behind designing the motors. Someone else? For those that do seem to misunderstand the issue, I think it is more a case of they don't fully understand what an induction motor is doing, not that they don't undertand the VFD specifically.

author: Siwastaja
author_rank: Super Contributor
timestamp: January 05, 2025, 07:33:56 am
content: Quote from: bdunham7 on January 04, 2025, 08:06:01 pmThe mysterious guy mentioned in the opening post (the whole point of the discussion was Jester's argument with this (assumed) guy).

author: IconicPCB
author_rank: Super Contributor
timestamp: January 06, 2025, 05:29:13 am
content: Dual winding squirrel cage rotor is a variant of the deep bar rotor. Deep bar technology grew out of wound rotor slip recovery motors and is now augmented by VFDs.

author: Jester
author_rank: Frequent Contributor
timestamp: January 07, 2025, 08:27:18 pm
content: I don't really have a sense how common these double cage motors are?I called tech support at various motor and VFD vendors to see how well a double wound squirrel cage would compare to a good sensorless vector control VFD driven regular squirrel cage motor and most of them had no clue what I was even talking about. Until I called Yaskawa.Yaskawa, was excellent in summary there GA500/800 drives (open loop sensor less) will produce 200% of rated torque down to 0.3HzSo I think the take away is get a good drive and a inverted rated motor with good low RPM range and all will be well

author: Oleg.normalniy
author_rank: Newbie
timestamp: July 18, 2025, 06:47:30 pm
content: https://youtu.be/eB84X8UQla4

author: MariuszD
author_rank: Regular Contributor
timestamp: July 19, 2025, 06:22:20 am
content: Quote from: Jester on January 04, 2025, 01:32:30 pmThe impedance of the coils decreases with the decrease in frequency. The voltage must be reduced to prevent excessive current increase and motor burnout. Both V/F=const and FOC reduce the voltage.The torque is determined by the currents in the rotor and stator, not the voltages. FOC can control the torque because it controls the current in the rotor. With V/F=const the torque is determined by the motor characteristics.At low voltage, which occurs at low frequency, winding resistance causes significant voltage and current drop. FOC can prevent it.At low RPM motor has inadequate cooling so maximum current/torque should not be used. If the motor always runs at low RPMs, it means its power has been wasted. Power is the product of torque and RPM.

author: Oleg.normalniy
author_rank: Newbie
timestamp: July 19, 2025, 07:39:59 am
content: https://disk.yandex.ru/d/DBaLhJqc1zkzCAhttps://disk.yandex.ru/d/8PzEnnFpvrgX7Qhttps://youtu.be/v0n7D76zQz4https://disk.yandex.ru/d/WpeEnnMvlOIpNg

author: Doctorandus_P
author_rank: Super Contributor
timestamp: July 19, 2025, 10:21:32 am
content: Quote from: Jester on January 04, 2025, 01:32:30 pmYes, but that does not mean it's bad.With V/Hz, there is usually also a setting to either boost or lower the voltage at low frequency.I put a frequency inverter on my Rong Fu (Round Column) mill, and I purposefully reduced the voltage setting for low RPM. Now I can put an M6 tap in the chuck, then turn up the rpm untill the motor has just enough torque to tap into a blind hole, and when the tap bottoms out, the motor just stalls without breaking the tap.Quote from: Jester on January 04, 2025, 01:32:30 pmAny sufficiently advanced technology is indistinguishable from magic (According to Arthur Clarke). And for stupid or ignorant people, that threshold is a lot lower.They probably boosted the voltage for low RPM. If you do this, do realize that motor heating is mostly caused by current though the copper winding. And more heat while the fan does not work properly because of the low RPM can be a cause for the motor overheating. It's common to add a separate fan (with it's own motor) in this case.

author: johansen
author_rank: Super Contributor
timestamp: July 19, 2025, 09:04:47 pm
content: Quote from: Jester on January 07, 2025, 08:27:18 pmthey are more common on anything that needs more torque at startup than the standard single bar induction motor which is usually optimized for efficiency at full load, or its optimized to get the torque for a given temperature rise. i've melted down a number of induction motors and only ever found single bars that are slightly trapezoidal in shape, with straight iron bars.As for the vfd, it is usually programmed to operate the motor at minimal slip, so the skin effect problem doesn't exist and the current flows uniformly in the rotor.as for the OP's constant torque. what a constant torque motor means is that the fan is oversized and the motor will not overheat, down to the specified ratio. so for example, i had a 76 pound 5hp motor that had a very lightweight aluminum finned case and oversized fan and it was rated for a 7.5:1 ratio. meaning it could deliver full torque down to (1750 divided by 7.5) rpm.