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HELP 2 SSRs used on 230V motor--do I need to protect against 110V?

Discussion in 'Electronic Design' started by Mark Main, Sep 14, 2008.

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  1. Mark Main

    Mark Main Guest

    For my boiler heat at home I have a 230V 1.75A AC water circulating
    pump (GrundFos UP26-116F)
    Here are the specs for it:$File/Up26116f.pdf

    Unfortunately the pump runs 24x7 during the heating season even when
    all 4 of the thermastats are open (which shuts down each of the water

    I have 2 Omron SSR (DC-RR010), here are the specs.,...ays SSR 24-480V AC, 25A SSR user's manual.pdf

    I want to use the SSRs for the 2 hot leads of my 230V motor, which is
    simple enough... but what happens if one of the relays fail? Does the
    motor have built in protection of it's only receiving power from one
    of the power leads (in otherwords it becomes 110V), or do I need to
    build a circuit that protects the motor?

    And if so, then how?

  2. Mark Main

    Mark Main Guest

    The circuit breaker at the service panel is a "dual breaker" that
    covers both circuits going to the motor... so, if only 1 of these
    lines is hot and the motor is turned on, will that trip the breaker?
    It was my assumption that all 230V motors would be designed so that it
    won't burn out if only 1 of the circuits is hot... and if this is
    true, then it seems like I'm protected two different ways.
  3. whit3rd

    whit3rd Guest

    [about a 230V appliance]
    There's a misconception buried in there: a circuit
    is a full current-out/current-return path, and the usual
    230V appliance has only ONE circuit. It has two
    LIVE WIRES, not two circuits.

    A turnoff switch needn't disconnect all live wires from
    the motor to perform its function. One SSR will control your
    motor (or water heater). To make the wiring safe, however,
    your circuit breaker DOES disconnect all live wires.
    In case of fault, to prevent fire, that's useful. In case
    you want to work on the wiring, to prevent shock hazard,
    it's useful.

    The SSR (solid-state relay) isn't reliable and fail-safe
    the way the circuit breaker is intended to be, so it
    cannot do the same job as the circuit breaker in any case.
    Thus, "double SSRs" isn't useful.
  4. Jamie

    Jamie Guest

    If its single phase ( 2 wires only from service) you need only to open
    one side. Your service should have a dual breaker for this line so even
    if the motor should short to ground the lead that stays connected, it'll
    simply popped the breaker for you in which case you would need to
    service the motor any ways.

    You may want to consider a motor starter contactor over using a SSR.
    they come with various selections of coil voltages to energize the
    relay. You can even op for a motor over load on the starter."
  5. PeterD

    PeterD Guest

    Why would one of the lines at the box be hot, and the other one not?
    Why would it trip the breaker? Unless there was a short, the motor was
    locked rotor, or some other failure.
    Why are you saying "1 of the circuits", as if there were two? There's
    one circuit, a 240 v two wire feed. That other, third wire, is a
    I'm not sure what you are protected against!
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