Ac motor torque 2

[smavo]

I have a question about the torque of an A.C. Induction motor. Would changing the v/hz ratio or running it in vector control actually give the motor more locked rotor torque than an across the line starter. I am trying to get a better understanding of the v/hz ratio effect on a motor. Thanks in advance!

 

[jraef]

Maintaining the same V/Hz ratio maintains the same FLT of the motor That's what a VFD does at it's basic level. Without Vector Control, you can alter the V'Hz ratio to gain additional torque, but it also increases the flux, which unnecessarily heats the motor.

Vector Control, through complex manipulation of controllable variables, provides the ability to "tweak" the output in such a way so as to increase the output torque up to and including the Break Down Torque*, which is the PEAK torque the motor is capable of, at any time; temporarily. In a way, the "tweaking" is akin to changing the V/Hz ratio, but it is way more complicated than that. However the effect is similar in that it is not something that can be sustained indefinitely without deleterious effects on the motor and/or drive.

On a common Design B motor, Break Down Torque is generally higher than Locked Rotor Torque. LRT = 150-160% of FLT, BDT = 200-220% of FLT. So if you were wondering if a Vector Drive can accelerate a motor faster than an Across-the-Line starter, the answer is yes, provided the load can be accelerated within the time the motor and drive can tolerate the effects of an increased output.

"Will work for (the memory of) salami"

 

[smavo]

Thanks for the reply. I understand it better now. Basically I gather that to increase the torque of an induction motor you have to alter the v/hz ratio. Obviously there are limits but this is what vector control does also.

 

[Skogsgurra]

An excellent answer by Jeff! I have nothing to add. It is three o'clock in the afternoon over here. And even if Jeff is in SF CA (6 or 7, am?), he bet me to it! Well done - in several ways!

Gunnar Englund

http://www.gke.org

--------------------------------------
Half full - Half empty? I don't mind. It's what in it that counts.

 

[mikekilroy]

another useful/distinguishing tidbit is the current each takes: for that 150-160% LRT, across the line will take 500-600% LRA current, while on vfd it will only take similar 150-200%. ditto even the higher BDT will not pull much more than maybe 220-300% current either.

http://www.KilroyWasHere<dot>com

 

[xnuke]

smavo,

It's not entirely correct to say that vector control alters the V/Hz ratio. V/Hz is proportional to the magnitude of flux - this can be derived from Faraday's law. Vector drives work by controlling both the angle of the rotor flux vector relative to the angle of the stator flux vector and the magnitude of the flux vectors. Since the torque produced is proportional to the sine of the angle between the two vectors, when these vectors are 90 degrees apart, you get maximum torque. If I remember correctly, vector drives usually keep the angle at 90 degrees so the maximum torque for given flux magnitudes can always be provided, even in the locked rotor condition. The magnitude of the torque produced can then be adjusted by controlling the magnitude of the flux vectors. This is all done by controlling the current flowing to the stator. Sensor vector drives have a way of measuring the rotor flux vector magnitude and angle, and sensorless vector drives develop a mathematical model of the motor that then allows them to infer the rotor flux vector values.

I tried to keep it simple, but I'm sure there are some drives experts on here who can improve on my post. I'd be curious to hear any additional comments that could add to my description. If anyone really wants to get into this theory in a deep way, they should learn about dq0 representation of induction machines and how we can control them in the dq0 representation by controlling the components of the stator current along the d and q axes.

xnuke
"Live and act within the limit of your knowledge and keep expanding it to the limit of your life." Ayn Rand, Atlas Shrugged.
Please see FAQ731-376 for tips on how to make the best use of Eng-Tips.

 

[mikekilroy]

Actually, also vector control is not required to get this higher BDT than LRT from the motor - at any speed above about 10% of base speed (assuming you have a vfd with enough current as explained in previous posts).

With v/hz ratio not varying, the motor WILL produce up to the BDT, irregardless of vector control or skalar (v/hz only) control.

This can be seen in simple motor speed/torque/amp curves like shown here; all the vfd does is move that 100% point up and down in speed. The vector control allows much better control of amps into the motor and a lot quicker, so with it the BDT can be had at the lower speeds of less than about 10% nameplate.



www.KilroyWasHere<dot>com

 

[smavo]

I appreciate all of the replies. I learn more on this website than I did in college. X nuke I will investigate the dq0.