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4 pole motors run at 100 Hz 1

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geefer

Industrial
May 20, 2008
13
Does anyone have any experience of running a large AC 4 pole induction motor at 100Hz this design has been offered to drive a pump with a 3000 rpm top speed < however the pump will normalyy run in the 2200 to 3000 speed range . The reason I was offered this solution was that the 2 pole motor originally offered was in reasonace at 1800 to 2600 rpm. A suitable 2 pole stiff rotor design does not appear to be available. We would be controlling the motor Via a Robicon VSD. Would we have torque issues above 50 Hz or can a motor be wound to reflect 100Hz as being its as manufactured top speed.
 
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geefer,

I think the motor V/f will be controlled by the VFD and if the motor is designed to withstand the higher voltage (twice the rated in this case), there shouldn't be any problem. If the motor is matched with the drive (which it should be), the above aspect would have been taken care.
Speed is proportional to speed, so at 100Hz, you should have no problem reaching 300rpm with a four pole motor.
Torque is proportional to Vsquared. Hence, I guess the torque while working 3000rpm should be four times the rated torque at 50Hz and 1500rpm.
 
The question of motor surviving centrifugal forces at 3000 RPM and of the bearings rated for 3000 RPM, are best addressed to the manufacturer.
 
As long as you don't run out of hp to run the pump and the motor has a cast aluminum rotor and is 200hp or smaller, I'd go to 100hz without asking any further questions.
 
You have to think of mechanical effects - what will happen to coupling? -

Bearing? - doubling the speed will have effect.

Cooling will increase - assuming a fan will run at same speed as motor.

VFD need to set properly for V/F, A, V, I limits and temperature protections.

NC

 
Mechanically there should be no major problem using a 4 pole double its rated speed but you will not get the same torque output at 100Hz with the 4pole model as you would using a 2pole. Most 4 pole motors use the same casing and bearings as 2 pole motors. However, your 4 pole motor will get full voltage at 50Hz and therefore full power/torque. As the frequency increases beyond the base speed then the voltage will remain constant but your torque will drop off quite dramatically as the motor is in the region known as field weakening.
Rewinding a motor suitable for 100Hz as base frequency would require a very specialist knowledge in my view and chances are it would not be practical.
Unless you increase the size of motor and VFD then it will not produce the torque you need assuming the motor/VFD are sized correctly. So on this basis, it would be more prudent to look at a decent 2 pole motor.
 
Electrically, you would run the motor at 2X it's rated voltage at 2x it's rated frequency. For example, connect a 240V motor to a 480V VFD and then program the VFD that the motor is 480V, 100hz. In this way, you would maintain the V/hz ratio of the motor right up to 100hz.

You would also get rated torque out of the motor for the full speed range up to 100hz. This means that a 100hp motor, for example, would be producing 100hp at 50hz and 200hp at 100hz.

 
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