Troubleshooting Contactors

Are all three motor power cable protection fuses intact?

As PWSlack said. Cut the power and check each phase to make sure all three are working: take out each fuse and check it.

After you verify that the fuses are good, check the condition of the contacts for dirt, pitting, or excessive wear.

Incoming voltage... is it ok?

Is contactor coil energised by incoming voltage?

As others have said, the incoming voltage is insufficient since (a) the contactor is unable to pick up and (b) the motor is unable to start even when the contactor is manually closed.

If incoming voltage is all ok, then there is a short-circuit somewhere, which is draining it somehow when you want to start the motor. The sparks you see almost certainly indicate a very abnormal situation.

If you are likely to get fired for not being able to solve this, you may look for another option, it is better. You have at least come to CR4 to ask... it shows your commitment. What is your boss doing ? Playing golf?

GA Elroy .................thanks for being so clear

<...If your boss wants to fire you for something you havn't been trained to do, I would be looking for a better boss. It's better to lose your job than your life....>

Absolutely.

Please check the following :

Cable connections to the contactor.

Wiping action of contactor.

Clean the contacts of contactor.

Check the condition of the springs to ensure snap action of contacts of the contactor.

Check the fuses( HRC fuses) in the circuit.

There is one shorting band placed in the core of the contactor: please check it.Please check condition of the coil of the contactor to identify whether there is any intertun shorting in the coil winding.

MOTOR:

Please check IR value with insulation tester.

Plese check balance values of percentage inductances and resistances of three phases of the motor by Electric Motor Checker( EMC) instrument.

Identification of rotor bar cracks by EMC instrument.Please check electrical connection at motor terminal board

By checking the above points and after taking necessary corrective actions your peoblem may be solved.

First check whether the motor is freely rotating.Measure the voltage at input to the contactor.Measure the resistance between input & out put of each contact when pressed by hand,it should be zero.When start signal is given measure the current in three phases and voltage at the output of contactor.You will know the problem.

Sir, most likely when a three phase motor hams a single phasing occurs. when contactor fails to energize check the coil maybe it is damage, check also power that supply the coil and the connections.

When you manually push the contactor and there is an output on each line then check the line and connection between the motor and the contactor. Otherwise your motor have a problem.

sparks are usually caused by loss connection or contacts. Clean or Replace the contactor if the spark are too big.

Your question is not very clearly stated: What is a multimeter doing here so that it makes the contactor to hum and ... Sparks...Where?

BUT from what I can guess...: Your Contactor coil might be rated to run between 2 pahses (380V ?) and One of the phases is missing either a fuse is blown or a really bad contact. Therefore, the conatctor fails to close because either the same failed phase is used for it or, a bad contact making a voltage drop enough to fail closing it. In such a case, if you close by hand, you still have a missing phase (or a bad contact on that phase) and your 3 phase motor will hum and not rotate.!