I have a dc motor of 60hp rating 1750 rpm used in a 3 roll calendar machine of a tyre industry. The dc motor is now running at the maximum speed and now I have to replace this dc motor with an ac motor and a VFD with sensorless vector control. Gearbox and other transmission systems has to be retained. So how should I calculate the rating of the motor and since the speed has to be 1750 rpm ( to match the existing speed) the squirrel cage motor that I have to use is 3000 rpm synchronous speed.

Regards,
Mike

 

Hi Mike,

From your '3000 rpm synchronous speed' I assume you are working with 50Hz and 3 phase.

Assuming the 60HP 1750 RPM DC motor was working OK, the simplest solution would be to substitute a 60HP 1500RPM (4 pole, nominal 1500 rpm @ 50Hz) AC motor, and adjust the VFD to give the correct speed. I doubt the wisdom of trying to slow a '3000 rpm synchronous speed' motor down to 1750 RPM and run it that way for a long time. You would probably have to increase the rating of the motor and drive at least 33%. Cheaper to just buy a new AC motor.

Rob

 

Small comment:

Beware of starting torque!

You did not indicate what type of load you have. A DC motor develops higher starting torques at zero speed than an AC motor. Was your DC motor of the shunt, series or compound type? If it had a series winding in it, then most probably you need a high starting torque.

An similarly rated AC motor may not develop the same amount of starting torque, and so may never take off.

Check your torque requirements before you attempt the changeover.