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Contactor At Load Side


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I have seen panels with contactors on load side (output) of VSd, and some panels with contactors on both line(input side) and Load side of VSd.


I personally do not use any contactors ( Load/line) for VSD.


What are advantages/disadvantages of using contactors at the output and input of VSD?


Any thoughts?






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Hello chaterpilar


Switching on the output of drives was always considered a definite NO. Modern drives are less prone to damage by such switching, but it still varies from drive to drive.

The switching on both input and output is generally to enable the drive to be bypassed in the event of a drive failure.

Personally, I would avoid this if at all possible. I do not believe that there is a 100% guarantee that there will be no problems. I am always concerned that if the control voltage to the contactor coils reduces, there could be copntactor chatter and a blown drive.

Best regards,


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There are valid reasons for having contactors on the load side of a VFD however.

One that is quite common is in what is called a "3 contactor bypass". In this application, both a line and load isolation contactor is used only when the motor MUST be started by a 3rd Across-the-Line (DOL) starter that is run in parallel to the VFD. This way, the VFD can be removed for servicing while the motor continues to run. Similar to this is when you just use ANY kind of bypass starter because the line voltage must NEVER be applied to the load side of the VFD.


Another valid application is when you are going to use one VFD to start multiple motors sequentially; essentially "recycling" the VFD once a motor reaches full speed. The isolation contactors then are preventing the VFD from applying power to more than one motor at a time.


The important issue in all of these applications is that under NO CIRCUMSTANCES must the contactors be opened while the VFD is in operation. That is usually done with electrical and mechanical interlocking.

"He's not dead, he's just pinin' for the fjords!"
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