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Induction motor controlled by variable frequency drive

I am currently producing a surface mount permanent magnet motor for the oil and gas application in Canada. Since China is controlling the NdFeB magnet and the price of the magnet has gone up 5 times since a year ago, I have to look into induction motor to fill my customer's need.

The existing motor is a 75Kw 16 poles PM motor that produce 1000 ft-lb from 30 to 450 RPM. This motor runs really cool at any speed because of its high efficiency 97.6%. With a induction motor, I am thinking that 16 poles is a bit difficult to do, so I am thinking of making a 12 poles motor. The existing PM motor has a stator diameter of 550mm (21.6 inch) and stack height 320mm(12.6 inch). It is way over sized for its power rating. I am hoping that I can keep the similar dimension so all the mechanical parts will fit.

I am using a Gozuk GK3000 VFD and I added a sine wave filter to it -- so the motor does not see the PWM frequency at all and that helps to keep the motor cool.

Certainly a 4 pole motor is very common. This motor is going to run at constant torque (not constant power) from 30 to 450 rpm. Since this is a constant torque application, at 30 RPM, the power output is very low and it reaches full 75Kw power at 450rpm. It is a continuous operation, but the torque many change from weeks to weeks. I am only concern about the cooling between 30 to 150 RPM, at 150 to 450 rpm I think the efficiency of the motor is high enough not to be a concern.

For a 4 pole machine, the motor frequency will be from 1Hz to 15Hz - will such low frequency be a problem? I have never run an induction motor at 1Hz for extended period of time, so I don't know what it will behave. If that is not an issue, I should give that a try. With a 12 poles machine, it will be running between 3Hz to 45Hz - that is something I more familiar with.

Since this motor is VFD driven, the poor power factor will be an issue between the variable frequency drive and the motor that will not affect the line.

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