VFD for decanter centrifuges 1

[gaga45]

I am trying to spec a control panel and motor for a decanter centrifuge using a VFD. I have normally used a 40 HP star delta motor or a standard 25 HP NEMA B motor with a fluid coupling. Starting time is about 2 to 4 minutes. Both of these are fine if starting from scratch with a new design.
I am trying to fit a vfd to a used decanter centrifuge which has a standard NEMA B design 25 HP motor. I need variable speed capability.
Do I need to spec a different motor?
What would be the correct VFD type or make for 460VAC?
More important, do I need a resistor bank when stopping or deccelerating the centrifuge?

 

[GordS]

You do need to be concerned about the motor being compatable with the VFD. The options involve either specifying a different motor, rewinding with spike resistant wiring ( likely more cost effective to buy a new motor at this low HP), or install output filtering.

Consider also the impact on motor cooling at reduced speeds - the cooling due to the motor fan at reduced speeds needs to be sufficient to remove the heat generated at reduced speeds. If you buy an inverter duty motor, it should have a nameplate that tells you the lowest speed you can go to for a given type of load torque.

Some of the major VFD specs you need to be concerned about are:
1. the system voltage and VFD rating must match
2. VFD must have a current rating at least that which will be required by the motor.
3. VFD must be able to supply whatever feedback you want ( e.g. speed, torque, power, amps...)
4. The control signals must be compatible with what you have ( e.g. 4-20 mA)
5. VFD must be able to supply the speed range in which you want to operate.

You don't necessarily need a resistor for braking but you do need some way of absorbing the kinetic energy. The centrifuges I work with have a regen front end so I don't use resistors.