Commissioning Engineer (Water Treatment)

Energising the forward contactor while the motor was still spinning in reverse?

I think you are talking about a similar problem I ran into. Contactors are normally rated for AC and your brake operates on half-wave rectified DC. Switching a DC waveform causes a lot of additional arcing stress on the contacts which can easily melt the contacts together or pit them causing a bad connection, even after just a few switching operations. Switching to a suitably DC current-rated contactor and/or implementing an arc suppression circuit across the contacts will fix this.

The other possible cause is if you accidentally had both forward and reverse contactors on. This could cause excessive fault current through the contacts. Using a contactor mechanical interlock (its a standard contactor accessory) will prevent this happening.

I work for ABB and was involved in the manufacturing of electric motors.

On a standard design the brake was always driven by a full bridge rectifier with a MOV (metal oxide voltage arrester) connected across the coil. On 3 phase motors the rectifier is usually connected between the internal star point of the motor windings and one of the phases (the rectifier is usually supplied with 220/240V AC / 300V DC)

If a fast response is required during the braking operation, the rectifier is switched via an auxiliary contact on the forward and reversing contactor, the contactors are usually electrically interlocked (and sometimes mechanically or both). The auxiliary contact is switching the AC feeding the rectifier not the DC (to avoid contact arc).

Normally energizing the coil will release the brake, de-energizing the coil will apply the brake (by springs).

One of the problems when the brake is connected to motor windings is back EMF generated by the motor windings when it stops, this will feed back to brake coil via the rectifier and hold the brake coil on for a few milliseconds longer, by using an auxiliary contact this problem is avoided.

Nap

Thank you guys for your brilliant ideas!

Can we please further this topic? And I have a hard time looking for the trouble.

Actually, the motor is 3-phase, 400V, 50hz.

This is connected to a standard forward-reverse motor control.

Circuitry was checked at no-load, system is fine.

Motor windings as well are fine.

Cables were fine also.

But when load is connected, forward contactor and rectifier gets busted.

On the other hand, Half-wave rectifier (Input400V)was connected to the auxilliary NO

contacts before it goes to the magnet brake of the motor.

Is there a possibiliy that when motor starts, it sends voltage back to the rectifier?

I need your recommendations guys....thanks!

It can be very strangely,it seems that is everything ok,but J dont know,maybe problem exist in recitifier or that auxilary NO contact,or you have some mechanical problem with brake on the motor.If is it possible,tray to disconect brake on the motor,and run only motor without no load,than if it is ok put it back the brake on motor and run motor with no load.Here you can maybe to notice some problem if you have problem with recitifier or some mechanical problem with brake on the motor.It can be wery tricky,if you can when you do this to check ampers consumption of motor in all this stage,maybe you can find some big change.

Can you post a picture of the circuit diagram, it should help in finding the problem.

This might be a string of coincidences. Here is my scenario:

Motor lags to move under loaded condition (at least in a half-period).This clamps the back-emf. This means the motor coil behaves as short. At the same time, the brake coil is active and takes the half period of current via half wave rectifier and produce a self emk inversly equal the peak voltage of 400.√2 volts. This and the supply voltage (that is bigger than 800√2 volts why arching is formed at auxillary contact) appears on the terminals of rectifier, because the motor winding likely short (no emf at serial). This will slay the rectifier.

The short on the motor winding itself is already the cause of the busted contactor.

By my opininion the busted contactor will be change if you change the direction of rectifier, or if you change the phases those supply the brake coil.

I would like to study some experiments together but..

if you catch a point please post.

good luck