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Isolator Output


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It is not advisable to isolate the output of a variable speed drive through a power isolator or a contactor. We are working on a project where there are quite a few conveyors which would be VSD driven. The plant manager is requesting an isolating medium near the conveyor for security purpose during maintenance. I have got two questions

1. Basically what happened when the output of a VSD is suddenly isolated,and

2. Is there any regulation which prohibits use of isolatoror contactor on the output of VSD ?


Thanks for any comment.



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Hi Bob


There are potential problems with switching on the output of an inverter. When the inverter is running and you operate a disconnect (either a contactor or a switch), there willbe arcin on the contacts because you are swtiching an inductive load. This arcing can cause very high voltages and rates of rise of voltage which can damage the output devices of the VSD. Some manufacturers state that it can not be done, others state that it can sometimes and others say that there is no problem. I would ensure that you have it in writing from the manufacturer before I would contemplate such an operation.


The other problem is if you close the switch on to a running drive. This will both cause problems due to contact bounce and associated arcing, and cause a very high current to flow (like Full voltage starting) if the inverter is operating above 5Hz when the switch is closed.


I would strongly recommend an early break contact be fitted to the switch and that this be wired back to the drive enable input to immediately shut the drive down when the switch is opened, and to prevent the drive from operating when the switch is open.


Recommendations :

1) gain positive statement from the manufacturer in regard to switching on the output.

2) Use early break auxiliary to interlock with drive enable.


Best regards,

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