Contactor Question

Discussion in 'General Electronics Chat' started by gregzactly, Sep 25, 2006.

  1. gregzactly

    Thread Starter New Member

    Sep 25, 2006
    I run a small print shop and am having a problem with my paper cutter. I was adjusting the brake mechanism because the cutter blade was stopping late in its return cycle. Similar to a automotive disk brake, I apparently over tightened the adjustment and put too much of a load on the main motor upon restarting(220 3 phase). The internal breakers kicked off (3 - 20amp). I re-adjusted the brake with plenty of clearence, but now when I energize the circuit, it runs for about 10 seconds and then shuts off. Also, it is turned on with a push button switch that also powers up an air pump motor, which will energize even though the main motor won't. Internal breakers are not popping off now.

    Question: Could I have damaged the contactors when the circuit initially overloaded? It appears it needs a "recovery" time before it can be restarted again.I have looked for overload heaters to reset but find none. Electrically, everything was fine untill I wrenched on it...Any help would be appreciated.
  2. mrmeval

    Distinguished Member

    Jun 30, 2006
    You'd need to measure current draw at the motor. You could have a shorted winding inside causing an overload.
  3. gregzactly

    Thread Starter New Member

    Sep 25, 2006
    Thanks...I'll check the current draw tomorrow. If it's not higher than normal, any other possibilities for the problem?
  4. awright

    Well-Known Member

    Jul 5, 2006
    Are you sure that the brake is still not putting too much of a load on the motor? That kind of delayed shutoff suggests the possibility of slight overload, but not nearly as much as during your oiginal failure.

    One possible test is to measure the temperature of or feel the heaters of the overload relay immediately after the motor shuts off. DO NOT DO THIS WITH POWER ON THE STARTER!. Immediately after an automatic shutdown, PULL THE PLUG or open the main switch and feel the contactor heaters. If they are very hot, that could indicate that an overload is still occurring.

    Also, on some motor starter heaters you can see a tiny ratchet on top of the heater. That ratchet is what releases the latch holding the contactor closed when the low-temperature melting alloy inside the heater melts. If you see the ratchet spin and release the contacts, you probably still have an overload condition.

    Check for leakage resistance from the motor windings to ground. Should be many megohms. That's a simple test you can perform in place.

    Finding shorted turns in a motor winding is much more tricky. You have to take the rotor out of the motor so you have access to the stator poles. Winding testing is generally done with a "growler." A growler is just a coil that you energise with reduced line voltage and hold next to the poles of the motor and move around the stator. If you pass over a shorted turn, the shorted turn acts like a shorted transformer secondary and draws heavy current. As I recall, the high secondary current in a shorted winding will cause a hacksaw blade held near the pole to vibrate noticeably. This doesn't happen with a healthy winding. You can find a description of this test in almost any book on motor repair. Since it involves motor disassembly, you want to exhaust all other possibilities before resorting to the growler test.

    Good luck.