Motor selection

[bob]

Hi,

 

I have one application where a 400 kW 750 rpm 400 V motor is driven by a VSD. The motor drives a fan. The VSD can only be configured for V/F mode and is not a problem for a fan application. My concern is as follows. The company has earmarked a 400 k W 3000 rpm 400 V motor as a spare one for this application.

I am pretty sure that the VSD running the 2 pole motor at 750 rpm will be a problem because the motor will be insufficiently fluxed at that speed and will not deliver the required torque and power to drive the load notwithstanding that the motor will require force cooling at taht speed.

Any comment.

 

Bob

[marke]

Hello Bob

 

If you operate a motor below rated speed, you will get (in theory) rated torque output from the motor from all speeds down to approaching zero. At low speeds, if you operate at rated torque, you will need to have extra cooling in order to stop the motor from overheating.

As the motor speed is reduced, because the torque is limited to the full speed rated torque, and as the power is proportional to torque times speed, you will have a reducing power output from the motor.

In other words, if we take a 2 pole motor and run it at 8 pole speed, we will get only one quarter rated power from the motor. In your case, you will get around 100 KW from that motor.

If you can reduce the speed by mechanical means, i.e. a gearbox or beltdrive, then it will be OK, but if the motor is operating at less than full speed, your power output will be down.

 

The rated torque of an eight pole motor is four times higher than the rated torque output of a two pole motor.

 

Best regards,

[bob]

Dear Marke,

 

Thank you for your reply. I agree with your comments. However, would the matter had been different if the drive could be configured for sensorless vector.

 

Bob

[marke]

Hello Bob

 

No, an open or a closed loop flux vector drive would allow a higher transient torque at close to zero speed, but will not affect the continuous torque output of the motor at a quarter speed. The limitation is the flux in the iron. To get more torque, you need more flux and this will cause saturation and motor overheating.

 

Best regards,

[kens]

Hi Bob, I would imagine that there will also be a problem with the physical size difference betwen the motors. The 8 pole and 2 pole motors will have different frame sizes.

 

Ken