Braking with DC, graetz bridge rectifier blows up.

On a filtermachine we have a motor that needs to brake when stopping. The start-up is with star-triangle and the braking is done by stopping the motor and a few seconds later the braking contactor switches for putting DC on the motor. This works but, in 3 weeks we allready replaced 3 rectifiers. We allready putted a more powerfull one, without result...

Could it be the motor ?
Or should we increase the waittime before DC injection... It used to work, but the last 3 weeks not, is the inertia changed...

I'm thinking to place a Frequency Drive as a test. If it's the motor, then the drive should trip. If it's in the controlcircuit, then the rectifier should blow up again ?

Anyone who had the same problem allready and could tell me what else I can do ? Is it a good idea to place a frequency drive as a test ?

Thanks in advance,
Regards,
Combo

Adding a VFD is not a good idea. It will insert a bunch of additional variables into a situation you already don't understand very well.

That something has changed is clear even if you don't know what it is.

I would install a current recorder in the DC injection leads going to the motor and try to capture the current level that occurs. It may be higher than the bridge rating. It is common to have to add a series resistor to limit the DC amps while braking. If you don't have one, you may well be on the edge of both the motor and the bridge tolerance rating.

The rule I've always heard on DC injection braking is to limit the DC amps to 1.5FLA. That's to avoid damage to the motor's stator windings.

Hello Combo

Any details of motor and rectifier?

Has the rectifier been compared to original spec to find if it had been replaced with a reduced rating, perhaps... albeit you state a more powerful one has been tried?

Any wiring modifications?

Any load change?

Is the injection into one winding, if so which side of motor?

... or across two lines to the motor (U1,V1,W1 side)and utilising the star contactor (U2, V2, W2 side)?

Does the braking contactor open correctly?

Is any voltage getting from motor supply to braking voltage side of rectifier unit?

.

Hi

Hello,

Thank you all for your help. Today I had the time to check this out =>

We have a DC Ampere meter that you can put over the wires (donno the english word ). Didn't know that we had this, for AC this is a basic tool, but for DC...
So I measured when braking, it was between 33 Amps and 40 Amps. The Bridge is 35A. In the drawings it is also 35A. The load on the motor is is variable. The bridge becomes very hot.
Motor = 11 kW

We will place a bridge that can handle 60 Amps and mount it on a cooling plate.

Thanks.
Kind regards,
Combo

11 kw motor (YD start)= 22 FLA

1.5x = 33A ~35A within spec.

How long has this "worked" for before it started failing?

Problem

Machine from 2002.
problems started 2 months ago.

silva.foxx said:

11 kw motor (YD start)= 22 FLA

1.5x = 33A ~35A within spec.

How long has this "worked" for before it started failing?

Click to expand...

Some good info here: http://www.dmc-global-service.com/Main/DANweb/Vault/CommLit/ABP_GB_Gesamt_ms.pdf

bottom of p59

Idc = 2.5 x In

Silva, thanks for posting the link! Lots of useful info!

Kalle:book:

Indeed

Yeah indeed.
Thanks.

KalleOlsen said:

Silva, thanks for posting the link! Lots of useful info!

Kalle:book:

Click to expand...