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Thread: Motor starter

  1. #1
    tryoung is offline Plastic
    Join Date
    May 2010
    Location
    Oklahoma USA
    Posts
    12

    Default Motor starter

    i was using a 7.5 hp RPC to run my 5 hp 3 phase table saw. I acquired a 7.5 hp 3 phase radial arm saw. When I first tried to start it using the RPC, the contactor in the motor starter sparked and chattered. I replaced the contactor, and the saw motor started, but slowly.However, there was no sparking or chattering from the contactor. I decided I needed a bigger RPC, so I built and tuned a 10 hp RPC. Now when I try to start the saw, the contactor that worked before sparks and chatters. I tried a different motor starter rated for 7.5 hp 3 phase, but that contactor sparks and chatters as well. So, the question is, do I need a contactor with a higher amp rating? Or is this a problem possibly related to the RPC? I'm really stuck on this one. Thanks in advance for the help.

  2. #2
    chuckey is offline Stainless
    Join Date
    Jul 2005
    Location
    Wensleydale, UK
    Posts
    1,118

    Default

    The contactor in a motor starting box is to "latch" the action of the ON button. A motor starter is a normally short circuit OFF button feeding the incoming supply to the ON button which goes to one side of the contactor coil. The other side of the contactor coil goes to neutral (normally). As well as the motor supply contacts the contactor also has a pair of "auxilary" contacts that close when the contactor is energised. So when the ON button is pushed the contactor pulls in an the AUX contact shorts out the ON button so keeps the motor running. Also there are some current sensing overloads that are connected in series with the motor. when the motor takes too much current for too long, a contact in it opens and breaks the feed to the AUX contacts so the contactor drops out and the motor stops.
    The most likely problem is that the contactor is using a "live" that has been generated by the rotary, so it is too low to keep the contactor pulled. The other problems are that the contactor needs to be cleaned (action and contacts). If a problem occurs in the overloads/On/OFF button loop the contactor should drop out immediately.
    Frank

  3. #3
    tryoung is offline Plastic
    Join Date
    May 2010
    Location
    Oklahoma USA
    Posts
    12

    Default

    Thanks for the help. One of the lines to the contactor was the generated leg. I switched them and everything is fine now

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