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Soft Starter For Slipring Ac 3 Phase Motors


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#1 pma

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Posted 02 February 2008 - 04:57 PM

How we can use soft starters for slipring motors. The application required for compressor motor 120HP, 415V,3 phase, 50Hz with slip ring. Can we short slipring and convert the same to induction motor.

#2 marke

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Posted 02 February 2008 - 06:48 PM

Hello pma

Welcome to the forum.

If you short out the rotor of a slip ring induction motor, you will have a very low locked rotor torque and a very high locked rotor current.
When you apply a soft starter and reduce the start current to an acceptable level, you will have a very low start torque and will probably not be able to start a machine.

With a slip ring motor, provided that the load does not require a very high start torque, you can connect one stage of resistors to the rotor and bridge these when the motor gets to full speed. The resistors should be the equivilent of the final stage stage resistors from the existing slip ring starter.

If the load requires a very high start torque across the full speed range, then you are best to use a proper slip ring starter. - it is the only way to get the high start torque.

There are some soft starters that offer a "cycloconverter" type of start and this can sometimes be used to provide a start on a shorted rotor, but the current flow is pulsed and can cause interference and voltage problems on the supply and damage to the motor.

Best regards,

#3 pma

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Posted 11 August 2008 - 05:13 PM

Thank you, Mark for your guidance. To use soft starter for slip ring motor what is way? with modern controls. This should reduce the mentainance of conventional starters and cabling.

Best regards

Dharma Patil

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QUOTE (marke @ Feb 2 2008, 06:48 PM) <{POST_SNAPBACK}>
Hello pma

Welcome to the forum.

If you short out the rotor of a slip ring induction motor, you will have a very low locked rotor torque and a very high locked rotor current.
When you apply a soft starter and reduce the start current to an acceptable level, you will have a very low start torque and will probably not be able to start a machine.

With a slip ring motor, provided that the load does not require a very high start torque, you can connect one stage of resistors to the rotor and bridge these when the motor gets to full speed. The resistors should be the equivilent of the final stage stage resistors from the existing slip ring starter.

If the load requires a very high start torque across the full speed range, then you are best to use a proper slip ring starter. - it is the only way to get the high start torque.

There are some soft starters that offer a "cycloconverter" type of start and this can sometimes be used to provide a start on a shorted rotor, but the current flow is pulsed and can cause interference and voltage problems on the supply and damage to the motor.

Best regards,



#4 marke

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Posted 15 August 2008 - 09:40 PM

Hello pma

Adding a soft starter to a slip ring motor with one stage of resistors does reduce the complexity of the slip ring starter, however you still have the rings and brush gear in service and these need regular maintenance.
In effect, you are changing the slip ring motor to perform in a similar manner to a standard cage motor, but you still have the brushgear so more maintenance than a standard cage motor.

Best regards,
Mark.




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