# Potential Relay Rating



## Billy Bob (Aug 20, 2011)

I have a 230v 2hp, 1 phase motor with start and run caps. 

I want to put a potential relay in the start winding to remove start winding cap after start up. 
What do you use, the motor nameplate voltage & amps, or the start cap voltage rating, for sizing the potential relay?

Thanks, Bill


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## John Valdes (May 17, 2007)

They do not size relays in Micro Farads. Are you trying to use a relay in lieu of the centrifugal switch? You will need a stout relay to carry the start up current.
You will have to figure out how many volts and how much current that winding will see during start. Capacitor specs mean nothing if you are using a relay instead.


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## Billy Bob (Aug 20, 2011)

I was refering to the voltage rating on the cap. I figured that since the relay was in series with the start cap, the relay would see the same voltage as the cap?


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## Big John (May 23, 2010)

Billy Bob said:


> I was refering to the voltage rating on the cap. I figured that since the relay was in series with the start cap, the relay would see the same voltage as the cap?


 The relay only needs to be able to tolerate the RMS voltage applied by the circuit, and possibly less depending on how you wire it. 

The voltage rating on the capacitor is based on the peak voltage the capacitor will see for each cycle and shouldn't be used as an indicator in specing your relay.

-John


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## Billy Bob (Aug 20, 2011)

John, 
Quote "The voltage rating on the capacitor is based on the peak voltage the capacitor will see for each cycle and shouldn't be used as an indicator in specing your relay".


Thanks for that knowledge. Thats sorta what I figured but wasn't sure. I didn't want to buy two relays to find out. 
Thanks , Bill


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## Motorwinder (Dec 30, 2010)

Any motor shop can order you the right relay. Just give them the HP and running voltage.


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