Motor and Drive Selection
Motor and Drive Selection
(OP)
I'm looking for assistance dealing with a motor configuration. I'm looking for ideas regarding continuous low speed operation of an induction motor, or what I think is low speed. The required motor is 700Hp at 100 rpm. I've got a supply voltage of 480VAC 3Ph 3Wire (delta). What I'm thinking of is a step-up phase shift transformer to 600VAC to a 12 or 24 pulse PWM drive. If I select a 14-pole 60hz induction motor, what are the ramifications of operating this motor at the low end continuously? Are there other configurations that I should entertain? The motor may eventually become a custom build. The motor initially will initially brought up to full rpm (100rpm) with near no-load and then full load or torque applied once speed has been reached.





RE: Motor and Drive Selection
RE: Motor and Drive Selection
RE: Motor and Drive Selection
If you reduce the speed of a motor using VFD, you reduce the effective power capacity of the motor with the speed.
In you your case, you are asking for 700Hp at 100RPM.
If you start with a motor that operates at 1000RPM at line frequency, then you are operating at 10% speed and therefore 10% power. That would mean that you would need to have a 7000hp motor at 1000RPM to achieve those figures.
It is best to use a mechanical speed reduction to get your base speed to your required speed with the motor operating at line frequency and then you can alter from there.
For your example, yous a 700HP 1000RPM motor with a 10 : 1 gearbox will work, and then you can use a VFD to vary the speed from there.
Best regards,
Mark Empson
L M Photonics Ltd
RE: Motor and Drive Selection
RE: Motor and Drive Selection
I don't think there is any "may eventually become" about such a motor being a custom build. If you really need direct drive, you should go to the people that can build the required VFD and motor and ask them to make a proposal. Those people would be ABB, Siemens and a couple of others.
RE: Motor and Drive Selection
A 14 pole machine will be a 'special' and will be very expensive. It will have a very high magnetising current too, meaning lousy power factor and high penalties or you'll need to install a PF correction system.
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If we learn from our mistakes I'm getting a great education!
RE: Motor and Drive Selection
RE: Motor and Drive Selection
RE: Motor and Drive Selection
RE: Motor and Drive Selection
The problem is not the PWM drive, it is the motor.
In order to do work, the motor must develop torque and the torque is a function of the flux in the gap. The maximum flux is limited by the iron saturation, so fro a given frame, there is a limit to the flux and therefore an limit to the torque. The design full speed torque of the motor is very close to the limit, so you will not be able to increase the torque from this figure.
As you reduce the speed by reducing the frequency, the torque stays the same and torque times speed equals power, so the power reduces with reducing speed.
Best regards,
Mark Empson
L M Photonics Ltd
RE: Motor and Drive Selection
Something like a car differential only more robust.
Roy