[TowerTalk] STEPPIR QUESTION
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
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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