debugging a dc injection brake

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The brake manufacturer has sent some people out and installed snubbers on the SCRs.

The claim is that the power supply feeding the panel is "too stiff" causing a dv/dt problem. Whatever that means.

I did find this.

http://www.americanmicrosemi.com/tutorials/scr.htm

One serious limitation of the SCR is the rate of rise of voltage with respect to time, dV/dt. A large rate of rise of circuit voltage can trigger an SCR into conduction. This is a circuit design concern.
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gastoor said:
Motors are high efficiency and diodes and fuses were increased for the motor size, but still have failures. Could use anyones help and sounds like the problems are similar::-?
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Just a guess, but the higher efficiency motors will have more inrush and more of an inductive spike because the windings have more copper/lower impedance.

Not as big a factor as a larger motor tho
 
petersonra said:
The brake manufacturer has sent some people out and installed snubbers on the SCRs.

The claim is that the power supply feeding the panel is "too stiff" causing a dv/dt problem. Whatever that means.
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dv/dt= change in voltage/change in time, AKA rate of rise or slew rate.

I reckon a "stiffer" supply, with less sag during the abrubt SCR turn on, could add stress, but...

It's gotta work without relying on your power supply's impedance to regulate it.

Perhaps a capacitor type surge arrestor would help?
 
Let me see if I understand exactly what you have,

You have a 30Hp AC motor that is started with a contactor tied directly to line voltage (not a drive).

When the stop button is pressed the run contactor opens and the brake module supplies a DC voltage to the motor windings to stop it.

When the motor is stopped the DC voltage is removed.

Is the brake unit aftermarket?
Is the DC brake voltage supplied to the motor through a contactor or a solid state device(triac, scr)?
 
Psychojohn said:
Is the brake unit aftermarket?
Is the DC brake voltage supplied to the motor through a contactor or a solid state device(triac, scr)?
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I don't know what you mean by aftermarket in this context.

The brake unit supplies DC voltage to the motor, using SCRs. The SCRs are turning on while the contactor is on, which is bad.
 
Psychojohn said:
I mean did it come with the motor or was it purchased seperatly and added later.

Have the snubbers helped?
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Its not part of the motor. Its wired into the motor staret and inject DC when the contactor turns off to stop the motor.

The snubbers seem to have done the trick.
 
Psychojohn said:
Then it was probably transient noise on the line at motor start that triggered the SCR's on.
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I was down in VA last week starting up the machine. I had not seen it after the brake vender installed the "snubbers".

It appears that what they did was add a coil on each the output leads that the wound the output lead through 5 or 6 times.
 
Just out of curiosity, who is the manufacturer of the DCIB? Most have a requirement for an isolation contactor for that very reason. The "air coil inductor" is an interesting work-around to be sure.

My experience is that there is a dV/dt spike on the line for one reason or another, causing the SCRs to self-commutate as others have mentioned. I have seen it where a nearby air compressor with a 480V coil on the contactor was causing enough of a spike on the AC line that the SCRs commutated. I have also had the transition spike of an open-transition Autotransformer starter cause the brake failures. If it were a soft starter or a DC drive rectifier, the result would be irrelevant because it would only be on for a half cycle. But on a DCIB connected to a motor that's connected to the line, the result of inadvertent commutation is disastrous. That's why most mfrs insist on an isolation contactor.
 
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