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Is this relay appropriate

Discussion in 'Electronic Basics' started by Eric R Snow, Oct 2, 2005.

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  1. Eric R Snow

    Eric R Snow Guest

    I have a 120 VAC 1 hp motor that I want to control with a solid state
    relay because they are sealed. Looking at solid state relays with 3 to
    32 VDC control voltage I found a couple that might work. 1 is rated
    240 VAC 25 amps. The other is rated 240 VAC, 40 amps, and is zero
    crossing. And twice as much as the 25 amp model. Since the load is an
    AC induction motor does the zero crossing relay need to be used? Is
    either relay appropriate? Will these relays rated at 240 volts be OK
    switching 120 volts as long as the amperage is within spec?Here's a
    link to the relays:
    http://www.aaaim.com/u/web/aaaimc/cgi-local/shop991/shop.pl/SID=59872674/page=RELS.htm#RL2052
    The stock numbers are RL9801and RL2052. They are located near the
    bottom of the page.
    One other thing comes to mind. Since these relays are solid state
    devices and they may be sensitive to voltage spikes (I don't know if
    they are or not). If they are, and fail in the ON mode, it won't be a
    safety hazard because the motor can run continuously without burning
    up. The motor runs a circulating pump so the water would just get
    circulated more.
    Thank You,
    Eric R Snow
     
  2. John Fields

    John Fields Guest

     
  3. Eric R Snow

    Eric R Snow Guest

    Greetings John,
    I should have described the situation better. I'd make a lousy
    teacher.
    The pump only needs to run about 30 minutes 5 or 6 times a day. So,
    while running the motor non-stop won't hurt the system or be a safety
    hazard, it will use electricity.
    Eric
     
  4. The problem running motors from any switching mechanism is the locked
    rotor current that occurs for a few cycles at start up, and lots
    longer if anything actually jams the rotation. Have you compared the
    locked rotor current rating of the motor to the surge current rating
    of the relay? Otherwise, the only problem you may have is that the
    TRIAC switches in many solid state relays don't handle inductive
    loads, like motors very well. The inductance causes the current to
    shift phase relative to the voltage. This causes a problem at turn
    off. When the current goes through zero and the TRIAC switches off,
    there is significant line voltage at that moment that appears across
    the TRIAC, almost instantaneously, and tries to turn it back on by
    capacitive coupling of the rising voltage to the internal gate. So
    you may need a pretty big capacitor in series with a current limiting
    resistor connected across the TRIAC or motor. Maybe a 1uF or 2.2uF
    line rated cap in series with a 10 to 22 ohm 1 watt resistor.

    One other question: A 1HP motor seems pretty big just to move water
    around in a circle. Must it be so large? Is the circulation being
    used as a heat source?
     
  5. Eric R Snow

    Eric R Snow Guest

    Yes, it is. And the relay it currently uses has failed a couple times.
    The contacts fail. Everything else has been checked and it seems that
    the relay it came with is undersized. No wonder the company that built
    it is now out of business. I had a friend who installs and repairs
    HVAC units looked at the control and said the relay current capability
    was too small for the surge. Since the relay is in a humid enviroment
    I thought a nice sealed solid state relay would be best. My friend who
    does the HVAC stuff doesn't know enough about SSRs to tell me if this
    application is OK. I guess I'll just get a machanical relay with the
    proper rating.
    Thanks,
    Eric
     
  6. ehsjr

    ehsjr Guest

    Good move - it will be cheaper. And add an R/C snubber across
    the contacts to prevent/lessen arcing and contact burning.

    Ed
     
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