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Induction Motor control relays

I have a couple questions about relay selection (mechanical and solid state) to be used to switch power to a 2hp 120V single phase induction motor. The motor is in a dust collection system for a woodworking shop. Don't know what the starting current would be on the system, only the HP rating of the motor (haven't bought the dust collector yet). Looking at the ratings for motor loads, I see contactors rated at 2hp at

240V while they are only rated for 1/2hp to 1hp at 120V. Would it be acceptable to parallel both sets of contacts on a DPST contactor to safely handle the 2hp motor?

About using a SSR... Looking at an Opto22 model 120D45, rated for 45A at

120V; would this be an appropriate SSR to switch power to the motor? It's a zero-cross SSR, and has an internal snubber for its rated current at 0.5 PF. The datasheet is at
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Thanks for advice

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Dave M
masondg44 at comcast dot net
Reply to
Dave M
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No. One contact will close or open first, putting all current through a single contact at the worst (highest current or opening arc) time. Also not code compliant.

The proper way would be to use a motor load rated contactor (NEMA size

1 for 115 VAC single phase 2 HP motor) if and only if your motor has built in overload protection, or a magnetic (as opposed to manual) starter (which is a contactor with overload trip relay) if it does not, with overload elements selected for the rated full load current of your specific motor. (The overload elements are fairly cheap and easily replaced.)

As a permanent part of a woodworking shop, code compliance trumps what would work at the lowest cost, do not use anything not specifically rated for your motor.

If possible, order the system with a 240 volt motor or reconnect the motor for 240 VAC if it is dual voltage, and use the smaller and cheaper (NEMA 0 or 00 ?) contactor or starter.

Reply to
Glen Walpert

Thanks for the instruction, Glen. I thought about the possibility of the contacts opening and closing at different times about 5 minutes after posting the question, so I understand that issue. I'll have to contact the store that sells the dust collector and see if we can open one up to see exactly what kind of motor we're dealuing with. Don't think it's dual voltage, but will check to be sure.

Thanks again for the advice.

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David
masondg44 at comcast dot net
Reply to
Dave M

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