Electrical – Using motor circuit breaker buttons to start/stop motor

acautomatic-circuit-breakercircuit breakermotorthermal-cutoff

I want to add some protection to an ac 3/4 hp motor. I did some research and found that I could use one of these motor circuit breakers:

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My question is, if I use one of these, can I use the on/off buttons it has to start/stop the motor?

A few guys told me I could, but others tell me I should add a contactor with a couple of no/nc push buttons. But if I need to add a contactor, shouldn't I be using a thermal relay instead of the motor circuit breaker?

Best Answer

Yes, you can use that directly all by itself. The other more descriptive term for it is a "Manual Motor Starter" (MMS), because it is specifically designed to be used exactly the way you want to use it. There are however two things it does NOT do for you:

  1. Operate remotely. With a contactor and push buttons, you can mount
    those push buttons somewhere else. With this, you are right there
    where the power is being switched, it's a little less safe overall
    (but done all the time).
  2. Low Voltage dropout. With a contactor and push buttons wired for what is called "3 wire control", if the utility supply power fails, the contactor drops out and does NOT come on again when power is restored until you hit the Start button again. With an MMS like this when power comes back on, the buttons remain in their last state and your motor starts, suddenly and unexpectedly. THAT can be a very serious safety problem. You can add an accessory to the MMS however called an "Under Voltage Release" coil that will accomplish the same function. If it's a dangerous machine that can hurt someone (as most are), I highly recommend that option.
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