AC Induction Motors - most useful speed range?

A

Thread Starter

AlCan

Hi all,

I have read a number (of the many posts in this forum) that more or less relate to my question, but can't say I have seen any that really answers it.

The scenario is that I have a customer who operates a commercial bakery. They make a range of products that until now have mostly been small in size - biscuits, jam tarts etc. These require relatively short bake times.

They now wish to expand their range by baking fruit loaves. They wish to be able to vary the oven conveyor speed to give bake times from 10 to 100 minutes. They want to do it with an ordinary AC Induction Motor and Variable Speed Drive. (They want to use the most "cost effective" means...)

We are proposing to alter the gearing of their conveyor to better suit their requirements. (The gearing was wrong before, but it will be much worse now.)

Starting with a small (0.55kW) 4 pole, 50Hz, 400V, 3 phase induction motor and a fixed (expected in the range of 3,000-7,000:1) reduction chain, we can achieve a motor speed range from say 5Hz (~140RPM) to 100Hz (2800RPM). The motor size is rated (at 50Hz) at more than 5 times the power that is required to drive the conveyor, to cover the fact that the power output drops at speeds away from the rated frequency.

I know that torque drops off at both ends of the speed range, and that motor heating likewise increases.

We are offering an auxiliary single phase cooling fan to provide constant cooling to the motor, irrespective of rotor speed, and we propose to use a Sensorless Vector drive to give it the best chance of reliable operation.

My question is, where should we operate the motor to obtain the best performance? I know they don't provide frequency / torque curves for ordinary AC induction motors...

My feeling is that 6-60Hz would be better than, say, 10-100Hz - because the motor winding impedance will cause the torque to fall off faster at the high end of the speed range - which is where the power is most needed - than it will at the low end.

Or is that completely incorrect? Is the low rotor magnetisation efficiency at low frequencies as big or even bigger a problem?

Is there any type (winding style?) of induction motor that is best suited to this application?

Please inundate me with your comments!
 
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