torque pulsation

[royclh]

i understand the single phase system can cause the torque pulsation in motors and prime mover (generation). i dont know what exactly what torque pulsation means? How it will have impacts on the motor and prime mover. Single phase motors seems quite common and how these pulsation problem have been resolved?

please help.

 

[davidbeach]

In excel, MathCad, or other program with such capabilities, plot out a voltage and a current at an 80% lagging power factor. Then using that information, plot out the resulting power. Torque is proportional to power. See the torque pulsations? Now do the same again for 3-phase.

 

[JBinCA]

Davidbeach's suggestion is a good way to answer your main question.

Regarding the question about how the pulsation is resolved - It's not. If you apply a standard single phase motor, you've chosed to live with the torque pulsations inherent to single phase.

Regards,

JB

 

[electricpete]

I think david's suggestion is a good intuitive proof. But I think there is an assumption required which is not satisfied. The assumption is that the stored energy within the fields of the motor is constant. If that assumption were met, then we can say that th einstantaneous power output is equal to the instantaneous power intut at any time.

But the reactive elements of the motor can to store energy, and I don't think the total stored energy in a single phase motor is constant.

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[electricpete]

But a minor modification to the explanation resolves that concern.

Just consider the voltage and current accross the element of the equivalent circuit R2*(1-s)/s

Then as was explained, the power varies at 120 hz, which is in fact the frequency of torque oscillations in a single phase motor.

For three phase motor, we have three circuits 120 degrees apart, and the total of the three sums to a constant non-time varying total torque.

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[electricpete]

I also wonder about why we don't see torsional problems on single phase machines.

There is no doubt that torsional resonance is considered in design. But it is also considered in design of large machines, and for some reason it seems to me there are many more problems with torsional resonance in large machines.

There are of course certain types of equipment that have unique risk factors for torsional vibs. Reciprocating machines (with excitation), vfd driven machines, long flexible shafts, gear trains.

But beyond that it still seems to me that torsional problems appear far more on large machines than small. Maybe the torsional resonance frequencies are generally higher than the torsional excitation frequencies in smaller equipment.

It think that for single phase motor/load systems that the torsional resonance will likely be far above 120 hz.

I have attached below a simple example calculation of a resonant frequency for a system of two disks connected by the shaft:

http://home.houston.rr.com/electricpete/eng-tips/Torsional.xls

One disk would represent the motor rotor and the other would represent the load. For simplicity, it is assumed everything is made of steel. You input the shaft dimensions and the two disk dimensions and the weights etc are calculated from there. No coupling is shown which I think is typical of single-phase machines where load is mounted direct to motor shaft.

It seems to me difficult to find any reasonable values to get the natural frequency down to 120hz. You can change the input number in green and try for yourself. Maybe if you were driving a fan with a very high inertia.

I offer no guarantee that this calculation is correct. You can see the formulas used in column E. If anyone finds any errors in the calculation, please let me know (of course I realize it could be enhanced with a more detailed model accounting for flexibility at the fits, mass of the shaft, etc).


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