PWM AC Drive torque ripples 4

[thinker]

When selecting AC induction motor to be fed from PWM inverter, what is the difference between usage of delta
or wye connected motor winding from the point of view
to get a minimum of torque ripples?

 

[electricpete]

rhatcher
From my perspective:
1 - Mathematically speaking:
unbalanced/balanced and harmonics/no-harmonics are two separate and unrelated things. A system can be unbalanced in the fundamental and have no harmonics. A system can be balaced in the fundamental and have harmonics. Those harmonics may be balanced(perhaps the 4th harmonics)/or more likely unbalanced, but that would be mathematically independent of the balance of the fundamental.

In-spite of the above, there may be some real-world effects that you are trying to describe. Like the fact that electronics generally introduce only odd harmonics into the ac system that feeds them.

2 - PWM drive produces little heating and torque precisely because of the high frequency you describe. The leakage inductance of the motor j*2*Pi*F&L goes up in proportion to frequency so higher frequency voltage harmonics result in very little current => very little torque and heating effect.

 

[jbartos]

Suggestion to the original posting:
1. It would be better if the PWM inverter be based on higher pulse conversions, e.g. 12, 18, or 24-pulse to minimize torque ripples.
2. The characteristic harmonics start with 11&13 for 12, 17&19 for 18 and 23&25 for 24 and tend to decrease THD significantly.
General comment: the even harmonics used to be generated by half-wave rectifiers. These are heavily discouraged by Utilities since the even harmonics are very dangerous to transformers, motors, etc.
Also, general nonlinear characteristics produce even and odd harmonics. Practically, every nonlinearity has a small trace of even harmonics. It is good to brush up on the Fourier Series.

 

[electricpete]

jbartos - do the terms 6-pulse, 12-pulse, 18-pulse and 24 pulse even apply to PWM converters? I'm remembering that PWM converter output stage consists of dc voltage switched between +/-Vmax and zero at a fairly high frequency in the khz range. Duty cycle or width of pulses is varied to give varying magnitude of the voltage fundamental. Motor inductance filters out the high harmonics.

 

[redtrumpet]

I believe 6-pulse, 12-pulse, etc. refers only to the input rectifier of the VFD, and has a bearing only on harmonics generated toward the line or supply side. On the load or motor side, the harmonic spectrum would be different for the reasons given by electricpete. Still, assuming the PWM pulses were symmetrical over a full cycle, I presume the even harmonics would be more or less suppressed.

 

[jbartos]

Suggestion: The 6-pulse, 12-pulse , etc. terms are normally used for the input section of AC-DC-AC converters/drives. Phase shifting transformers or active front ends (AFE) are used. Now, if one uses AFE to accept the electrical energy with the low harmonic THD, then one may use it for the active motor end (AME). AME could be implemented by multilevel inverters that may use PWM. See papers:
1. Newton C., Summer M. "Novel technique for maintaining balanced internal DC link voltages in diode clamped five-level inverter," IEE Proc.-Electr. Power Appl., Vol. 146, No. 3, pp 341-349, May 1999
2. Lin B. -R., Lu H. -H., "Multilevel AC/DC/AC converter for AC drive," IEE Proc.-Electr. Power Appl., Vol. 146, No. 4, pp 397-406, July 1999
3. Antunes F.L.M., Braga H.A.C., Barbi I., "Application of a Generalized Current Multilevel Cell to Current-Source Inverters," IEEE Transactions on Industrial Electronics, Vol. 46, No. 1, pp 31-38, February 1999
4. Tolbert L.M., Peng F.Z., Habelter T.G., "Multilevel Converters for Large Electric Drives," IEEE Trasactions on Industry Applications, Vol. 35, No. 1, pp 36-44, January/February 1999
5. Etc. There are many publications on this topic. Manufacturers of AFEs are ABB, and Siemens.