Frequency change on an induction motor

[MikeSimpson]

Can you please settle a professional dispute! A subcontractor is supplying a hydraulic powerpack driven by 3 x 3 phase 50Hz induction motors. We tested them at 50Hz and recorded current readings, my arguement is that this will reduce at a higher frequency. I base this on the inductive reactance increasing as the frequency increases. Rotor impedance will also increase, net result motor taking less current. Please advise if I am right or am I taking the client stand!

 

[MikeSimpson]

Just to add to my thread the increase in frequency I am refering to is from 50 to 60 Hz

 

[CJCPE]

See

http://www.eng-tips.com/faqs.cfm?fid=1224

What is the motor nameplate voltage and frequency? What voltage did you apply for test? What voltage do you intend to apply at 60 Hz?

 

[electricpete]

The biggest factor in answering the question would be the load. If it acts like a centrifugal pump/fan, then increasing the frequency by 20% would increase the mechanical load by almost 60% (neglecting slip), which would certainly increase the current.

If you kept the mechanical load constant, there are still two effects to consider: power factor and efficiency. I suspect power factor will be the bigger of the two. Looking at the equivalent circuit, it’s not clear which direction it would go. Increasing the series leakage reactance tends to reduce power factor while increase the parallel magnetizing reactance tends to increase power factor.

I seem to recall a figure showing possible effects of change in voltage and frequency for typical motor but I can’t find it.


=====================================
Eng-tips forums: The best place on the web for engineering discussions.

 

[CJCPE]

The general rule for changing frequency with an AC motor is keep volts/hertz constant. If the nameplate says 380 volts and 50 hertz, the motor wants v/hz = 7.6. If you raise the frequency to 60 hz and raise the voltage to 60 X 7.6 = 456 volts, the current will not increase unless the load torque increases.

Look at:

http://www.reliance.com/mtr/pwrspymn.htm

http://www.easa.com/mo/TN-pdfs/TN08-397.pdf

 

[aolalde]

The performance is complicated due to the load performance.
If the hydraulic power pack motors drive direct coupled pumps, the mechanical power demand will increase due to the increased speed with the frequency increase.That will increase the current.
If the voltage is icreased to keep the motor strength ( V/HZ=400/50 = 480/60) and the load is regulated not to exceed the nominal motor HP load, the current will be reduced almost at inverse ratio to the voltage increase.

 

[aolalde]

If the voltage at 60 HZ is not increased to keep the V/HZ ratio constant, the motor will be too weak and the current will increase in spite of not exceeding the nominal HP (assuming the load could be regulated).

 

[CJCPE]

If the torque remains constant, the Hp will only increase by a factor of 60/50. If the voltage is increased by a factor of 60/50, the increased Hp will be delivered with little or no increase in current.