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Vsd And Wound Rotor Motors.


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Hi all,


I have a number of pumps on a site with liquid filled resistance speed controllers on Siemens wound rotor motors. The resistance starters were a manually controlled affair. the motors are 110kW wound rotor motors.


I have heard a number of differing opinions on the possibility of fitting VSD's to this type of motor. Some stating the Insulation Class will be an issue and others saying that it should not be an issue depending on the choice of drive??. Some other information has been given regarding shorting the slip rings before connecting the drive if fitted.


Excuse my ignorance but i have had little exposure to wound rotor motors and am after any recommendations/experiences regarding the above. Further do the slip ring connections make it to the motor terminal box? So far I have been unable tio find a schematic.


Thanks to anyone who may help.!!

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Hello PBW28


Using VFDs with slip ring motors is commonly done.

First, you muse bridge out the rotor so that it has a permanent short. The motor is never operated under high slip conditions and therefore does not need rotor resistance.


The comment about the insulation is true for any motor and the older the motor, the higher the potential for insulation failure when a VFD is used.

You can add a sinusoidal filter to the output of the VFD to overcome the potential insulation issues.


Rotor Slip ring connections usually have their own termination box, typically at the non drive end of the motor.


Best regards,


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