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Vfd's And Slip Ring Motors


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#1 Merlin26860

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Posted 26 May 2011 - 11:39 AM

One for the Boffins,
Has anyone had experience in using VFD's on Wound Rotor Motors?
We are intending to short out the rotor prior to the Slip rings so as to ensure there will not be any sparking and resistance changes on the rotor circuit.
Using a Open Loop Vector drive, we can expect to get up to 200% Torque from a Squirrel cage motor (250% Torque using a Encoder), however, would we be able to get a similar value from the Wound Rotor Motor/ VFD combination?
As the motor is used on a winch, (with a high reduction gearbox) we will require a high torque level for raising and lowering the load. The drive will also be required to hold the load while brakes are released and engaged.
If you believe this will be marginal, 1st stage rotor resistance values can be provided.

Any experience in this field would be appreciated


#2 marke

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Posted 28 May 2011 - 06:46 AM

Hello Merlin26860

Yes, we have controlled a number of slip ring motors with the rings shorted using a VFD.
I have not used an open loop flux vector system, but see o problems provided that the software is able to work with the different rotor characteristics.
With the rotor shorted, the motor exhibits a very high LRC and low LRT. It is possible that some VFDs may have problems with the extreme values for the rotor inductance and rotor resistance, but I expet this willnot be an issue.

Best regards,
Mark.

#3 Tractor

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Posted 10 February 2012 - 06:02 PM

Wound rotor motor work well with inverter drives however. If this is a retrofit, fitting a VSD / VFD to a system originally operated with slip ring resistors retain one stage of resistors. You may find that the torque developed by the rotor is low if the sliprings are shorted out, this can be increased by connecting some resistance into the slip ring circuit. Once the machine is comissioned they cab be left in circuit.

Inverter drive's are very agressive and can damage motor insulation with the high dv/dt developed by the pulse width modulated output, I recommend fitting an output choke between the inverter drive and the motor.

#4 marke

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Posted 10 February 2012 - 09:23 PM

Hello Tractor

I am interested in your comments in regards to resistors in the rotor circuit when a slip ring motor is used with a VFD.
My experience has been to just bridge the rotor with a VFD and use resistors with a soft starter.
A VFD does/should not operate under high slip conditions so resistors hould not be required.

Best regards,
Mark.




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