jimlux jimlux at earthlink.net
Wed Jul 15 20:26:36 PDT 2009

john at kk9a.com wrote:
> I though that MOVs degrade with each protection event and that their 
> breakdown voltage decreases.  Is a 45v MOV appropriate for a 33v system?
> John KK9A

More that the leakage current increases.  Imagine a whole bunch of 
little tiny MOVs in parallel.  When you get an event, the lowest voltage 
MOV turns on, and slightly dies in the process, becoming a high value 
resistor.  The next event kills another microMOV, now you have twice the 
leakage, etc.etc.etc.

This isn't how it actually works, but is a model for the behavior.

I've run some big MOVs on a 480V Variable Speed motor drive which had 
long wires and transients of several kV.  In this kind of application, 
you're getting a transients on every cycle (actually, at the PWM 
switching rate,but it's the ones at the top of the sine wave that are 
the ones that cause problems)..  we could kill MOVs in a day.. the 
leakage current would rise and the VFD would shutdown with a ground fault.

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