VFD - Dynamic Brake Resistor - Cable Length

[DrDrreeeaaa]

Hey All,

As some of you may be aware one cannot have too much inductance present in a dynamic brake (whether it's in the brake itself or the connecting cables).

I am trying to confirm my understanding of the reason behind this.

My understanding is that too much inductance causes overvoltages on the DC bus due to the high frequency of the brake chopper and the inductance presented by the brake resistor and cables not fully discharging and acting like a boost converter which will present overvoltages and potentially damage the resistor and/or the drive.

Is this correct?

If so, what can be done to limit the affects? Will any such overvoltages be seen by the drive and trip the drive, in a similar manner to braking a load with too much intertia?

Thanks.

 

[itsmoked]

Nothing fancy here. The chopper is there to bleed off excess energy piling up on the DC link caps. If you have too much inductance it will do its inductance-thing which is NOT allowing fast changes in current. Since the chopper circuit turns the current ON/OFF rapidly and the inductance resists this there just won't be much current carrying away the excess energy. The end result is the DC voltage will climb anyway and trip the drive off. No boosting, no feedback, nothing fancy. Same as using a braking resistor that has too high a resistance, the DC link voltage will rise to a trip point.

Keith Cress
kcress -

http://www.flaminsystems.com

 

[Drivesrock]

Hi 123MB

In my experience, a 'high' inductance in the DB circuit caused the DB unit itself to fail. Particularly the fast-acting anti-parallel/freewheel diode to protect the transistor as in that case it was not integral to the device. The VFD would have tripped on DC link overvolts after the DB unit diode failed (and fuse opened) and could not dissipate the regenerating energy anymore. I didn't get damage to the VFD.

The solution was to make the 'long' cables from the DB unit to the resistors that had been laid apart, in to a twisted pair. Problem solved. These cables should be laid together at least.

DB resistors should also be specified as low / zero inductance to prevent any overvoltages being induced across them too by the rapid switching of the DB transistor.

VFD DC Links are designed to get the inductance as low as possible to prevent internal damage when switching the inverter IGBTs. I think your most likely issue with inductance in the DB circuit will be a DB unit failure.

Drivesrock