MCB Trips with De-energizing of Magnetic Contract

What is your MCB rated Amps?

You probably have the wrong trip curve on your MCB. When a contactor opens, there is a brief moment when the circuit is arcing across the contacts. At that instant, the current spikes very briefly, as in sub-cycle. But if the MCB has a low trip curve, such as a B or A, it may be enough to cause a nuisance trip. You need a D crve for a motor.

Since your MCB is rated at 125Amps, normally, the MCB should not trip once your motor had stopped or once you have pushed the "Stop" Push Button of your DOL Starter. I suggest you to check your DOL starter contactor contact point to see if the contact point is wear out or if there is some dirt accumulated at the moving contact point.

This tripping may be one or two of your contactor contact point does not release fast enough and cause the tripping of the MCB. (Flash over internal contact during de-energizing of the contactor)

Other method is to check the voltage at the Overload terminal by a Volt meter to ensure that there is no voltage after you had push the "Stop" Push Button at the DOL Starter.

The final method is to increase the MCB from 125 Amp to 150 Amp rating. (I would not recommend to do it, but you have to conform that all the method are O.K.)

If you are saying that the load of the mixer gets too heavy and the mixer stops the motor, that is a different question entirely.

Just checking, since some times the words don't translate the meaning so clearly. We do try hard to understand, and I think the answers are excellent so far, but I did wonder if we got the question correct.

Regards, CJM

Thanks CJM,

The Mixer with gearbox was running normally, they are belt driven by the motor. It is not stopped by the mixer, its is a normal stopped by the DCS.

Thanks Simon Wan,

I am planning to change the contactor too, this is a DCS run loop. On the suggestion to increased the MCB rating higher , noted ,will do a sturdy.

The symptom look like a restart, immediately after the stop signal, in m-secs. Agree with the theory there is a surge upwards upon stopping and quick restart.

Will test this out with the DCS engineer on the loop configuration.

I recall we had a similar tripping in our compressor starting, upon analysis by an expert using hi-speed recorder we really saw a drop of signal in one of the interlock relay, causing a start defect , but within m-secs restart, this cause the whole sub station to trip, due to long run up time, an operation close to the max setting of the overload set for the SS. (motor back emf)

But this time I had a run to stop and the MCB trip. Can this be the same as the compressor scenario.

Dear all, thanks , plse to fill in, will let the community know after I change the contactor.

Rgds.

Dear ALL,

Problem solve, We were right about the immediately re-start that occurs within m-secs, The DCS guys admitted that setting is at one value eg. value of 1000kgs the mixer will be cut off. They initially said that stopping and starting is at different value, ie stop at 1000kg re-start at 1200kgs. So now a deadband of 200kg is installed as what the setting should be.

Thanks to all for your inputs.

Thanks for getting back to us. CJM