I am going to build a protection circuit for a 240 volt pump motor. It is believed that when the relay cuts out the motor generates voltage back into the relay and burns out the contacts.

It has been suggested (on this forum) that a MOV and RC Snubber will save the day and relays.

Would someone help me with this? It is most appreciated.

Please see the original question.

P.S. This is a great forum. I am a materials engineer - but have learned so much from this forum that I missed in engineering school.

  • \$\begingroup\$ Relays wear out eventually. If they're used within ratings, the wear-out time is fairly predictable. Are your relays lasting less time than the datasheet (link please) would predict from the motor (data or link please)? And, by "time", of course I mean number of switching operations. \$\endgroup\$ – Spehro Pefhany May 12 '14 at 14:10
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    \$\begingroup\$ Just a quick note: This is not a forum. It is a question-and-answer site. \$\endgroup\$ – JYelton May 12 '14 at 16:46
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    \$\begingroup\$ This is not a wear out situation. They fail after a very short time sometimes just a few days. They fail open. \$\endgroup\$ – user40681 May 14 '14 at 17:41

a protection circuit for a 240 volt pump motor.

Actually you are wanting to protect the relay contacts (which you acknowledge).

You do not specify the size of the motor. But that is ok. For the larger audience, on larger motors, a Motor Starter is used rather than a simple relay. High power Motor Starters often contain an Arc Chute. More information can be obtained on Arc Chute using this link.

Essentially an arc chute uses the arc itself (current) to create magnetic field that draws the arc away from the contacts, extinguishing the arc on the contacts. (Very common on circuit breakers by the way).

In smaller motors and using relay's, you can find help on RC snubbers here on Electrical Engineering. Pretty common stuff. Essentially a 0.1uf (get one rated at 400 volts) and 100 ohm resistor (perhaps 2 Watts). Start with an RC snubber. Don't go for an MOV yet.

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