ISCM and VFD Torque.

[aolalde]

Is it possible to increase the Torque developed for a SCIM driven by a VFD?

The basic force developed in a rotor bar is proportional to F=i*( LxB) .
The flux density B is limited by saturation of the magnetic circuit.
L is a fix length of the rotor bar.
The current (i) is proportional to the induced voltage and inverse to the bar impedance. the higher the rotor slip, the larger the reactance of the bar.

How a VFD handles these parameters to increase the torque ( 300- 400%) as many have stated on previous threads?

 

[electricpete]

For steady state loading, vfd does not allow torque to exceed the "rated torque" i.e the torque which motor would see at full load when powered by line frequency.

Below rated speed, volts must be reduced in proportion to hz to avoid saturation. i.e. torque capability remains the same. So typically the steady state limit when operating at a frequency below line frequency is a constant torque, decreasing speed, decreasing power limit.

The motor also has capability to deliver momentary torque exceeding rated torque up to breakdown torque (on the order of 200% of rated torque). I don't think vfd changes breakdown torque.

I would be interested to hear which thread you are referring to.


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[jbartos]

Suggestion: Reference:
Bimal K. Bose "Modern Power Electronics and AC Drives," Prentice Hall PTR, 2002,
Section: 2.2.10 Variable Stator Current Operation
on page 47
Notice:
Te=K' x Is^2 x S x &[ignore]omega;[/ignore]e / (Rr^2 + (S x &[ignore]omega;[/ignore]e x Lm)^2)

 

[aolalde]

Epete:

I understand and agree with your opinion.

I have seen statements like “A Variable Speed Drive (VSD) will provide the most amount of torque per amp of any motor starting method available”. or “ALL the applications can be met by SQIM & VFD”.

That is why I would like to know if there is something that modifies the parameters of a SCIM while driven by VFD.

I think that by lowering the frequency at the start, the delta of load to synchronous field speeds is reduced. Allowing a reduced time to reach that sub-synchronous speed and then increasing gradually frequency in a similar pattern till full speed is reached avoiding a high frequency condition for the rotor and resultant high starting current.

By the other hand use of VFD introduces harmonics in the motor that could result in parasitic breaking torques.

 

[aolalde]

EP.

Please see thread " Can VSDs be used for slurry pump"

 

[gsimson]

In the vector version of VFDs the magnetising (field) current generating the magnetic flux is "independently" controlled both magnitude and phase angle.
Once the phase angle is at 90 degrees to the load current the maximum torque is developed. Also as the magnitude of the field current can be varied independently SCIM acts similar to DC motor.Hence the torque can go upto 400 % for a short time.
The magnetising current vector has to be constantly changed in relation to the 'actual' location of the rotor considering slip. Hence an encoder need to be coupled to the motor shaft.
Reducing the voltage in V/f control will not reduce the pullout torque as the pullout torque is proprtional to "volatage squared / sX2". s & x2 reduce in proportional to the frequency
Many articles are written by ABB, Rockwell & Siemens on this subject which are available in their website.