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I'm building a circuit to control an AC motor using a relay.

Since the motor is an inductive load, there will be a voltage spike when the relay opens.

It seems that best practice for dealing with this voltage spike is to connect a capacitor across the load (or the relay), with a series resistor to limit inrush current to the capacitor.

why not just put a cheap, beefy MOV in parallell with the relay contacts? One example of such a MOV is the "Panasonic ERZE14A391": https://www1.elfa.se/data1/wwwroot/assets/datasheets/ERZE_series_eng_tds.pdf

What are the pros/cons of this solution?

Is the main problem that the MOV allows the voltage to rise too high, thus putting more wear on the relay than a capacitor-based solution would?

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4 Answers 4

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Metal Oxide Varistors (MOVs) are cheap but will wear out and fail shorted. Properly rated capacitors as part ofa snubber will last indefinitely. Both will allow a significant voltage spike. See, for example, Electromagnetic Compatibility in Medical Equipment: A Guide for Designers ... By William D. Kimmel, Daryl D. Gerke

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MOVs are more appropriate to deal with occasional spikes rather than continuous clamping applications, where they are appropriate at all.

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  • \$\begingroup\$ According to the datasheet for the ERZE14A391, it will last >1 million cycles for 10ms transients at 2A. This is much longer than the relay will last. Would the MOV be okay for snubbing a motor drawing 1A current? \$\endgroup\$
    – avl_sweden
    Commented Jul 20, 2015 at 21:05
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    \$\begingroup\$ It should be okay. Probably safer to put it across the motor rather than the contacts. \$\endgroup\$
    – Spehro 'speff' Pefhany
    Commented Jul 20, 2015 at 21:16
  • \$\begingroup\$ So that it doesn't start a fire while the motor is off, if it fails to low resistance? \$\endgroup\$
    – avl_sweden
    Commented Jul 20, 2015 at 21:20
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    \$\begingroup\$ Or spin the motor without control, which might be not be good if it's controlling rotating blades or something. \$\endgroup\$
    – Spehro 'speff' Pefhany
    Commented Jul 20, 2015 at 21:27
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MOVs do allow a lot of voltage for relay contacts which could experience arcing on opening or contact bounce.

MOVs also have a wear-out mechanism meaning that over time they are less and less able to clamp to the specified voltage with the specified current flowing in them.

An AC TVS would have a tighter breakdown without a wear-out mechanism as long as you don't exceed its ratings. It might be a smaller solution that a snubber.

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  • \$\begingroup\$ The TVS-diodes I've found, such as the "ST 1.5KE400CA" ("Transil"), don't really seem suitable over mains. This TVS, which is more expensive than the MOV I referenced in the question, is only rated for 200A, compared to 10000A for the MOV. I'm worried that a surge will fry the TVS. \$\endgroup\$
    – avl_sweden
    Commented Jul 20, 2015 at 21:16
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    \$\begingroup\$ Here you can see some 10,000A TVS diodes: bourns.com/ProductLine.aspx?name=power_tvs_products TVS diodes are absolutely more expensive than MOVs which is why you find MOVs in the vast majority of consumer surge protectors. The problem is that if you live in an area with lots of surges the MOVs wear out and your surge protector no longer protects. \$\endgroup\$
    – John D
    Commented Jul 22, 2015 at 5:47
  • \$\begingroup\$ Cool! Looks promising. \$\endgroup\$
    – avl_sweden
    Commented Jul 22, 2015 at 6:49
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Beware of MOVs the way that they fail short circuit can be very bad they can go on fire in fact I set up a discharge using a halve sine current pulse and got a short duration flame to shoot out My associate digicamerad this showing the flame to be 20 cm long and posted it on the web some years ago SO using the MOV isn't simple when you have to take fire precautions

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Others have mentioned the dangers of having the MOV in parallel with the relay when it fails short, but keep in mind that if you instead put the MOV in parallel with the load, when it fails short, it'll simply throw your circuit-breaker -when you switch on the next time- without the risk of catching fire. It also avoids leakage current into the motor if it has only partially failed (which could damage the motor otherwise.)

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