Ac motor torque
Ac motor torque
(OP)
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!





RE: Ac motor torque
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.
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RE: Ac motor torque
RE: Ac motor torque
Gunnar Englund
www.gke.org
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RE: Ac motor torque
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RE: Ac motor torque
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.
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RE: Ac motor torque
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.
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RE: Ac motor torque