VFD BRAKING RESISTOR

[transient]

Lets say I have a VFD with a braking resistor unit attached .

I use it as per requirement from 9AM to 5 PM .

One night due to some reason , lets assume the braking resistor cable( from DC bus bar to breaking elements) or braking element breaks (physical damage) .
Thats means breaking resistor will now not work .

So , next morning , when somebody goes to start the VFD , should he will be able to run the motor connected to the VFD ,
or
will the VFD will give me an error message , which will mean braking resistor is open circuit /faulty?

Second question , is same , but the nature fault is changed ,just assume , now the breaking elements got grounded.

Regards
Transient.

 

[Jraef]

Lets say I have a VFD with a braking resistor unit attached .

I use it as per requirement from 9AM to 5 PM .

One night due to some reason , lets assume the braking resistor cable( from DC bus bar to breaking elements) or braking element breaks (physical damage) .
Thats means breaking resistor will now not work .

So , next morning , when somebody goes to start the VFD , should he will be able to run the motor connected to the VFD ,
or
will the VFD will give me an error message , which will mean braking resistor is open circuit /faulty?

Second question , is same , but the nature fault is changed ,just assume , now the breaking elements got grounded.

Regards
Transient.

The drive should be fine to run if it is connected properly, but may trip off as soon as the Braking function is engaged. The brake resistors are not connected directly to the DC bus (at least they should not be), there is a braking transistor that is controlling that circuit. When the VFDs DC bus voltage rises above a set point, that transistor fires and dumps the excess energy into the resistors to be burned off as heat. When running normally that transistor is off, so the drive has no idea that the resistor circuit is damaged in any way. But as soon as it is called for, a short to ground or otherwise will cause the brake current to spkike and most drives will detect it and trip (not all will though). If the circuit is open, the DC bus voltage will have nowhere to go and the VFD will trip on high DC bus voltage.

If you know the circuit is damaged, you can simply reprogram the drive to not provide braking, ie coast to stop, and it will run fine. Some more advanced drives will even allow you to program a digital input to enable or disable the braking functionality with a switch closure.

 

[transient]

VFD BRAKING RESISTOR

Mr Jraef

As usual you are always spot on with your replies with VFD's

Thanks

Transient.

 

[oregonshooter]

An added point with BR setups is to be sure the BR is wired through a starter that is only on with the drive's run circuit. I've had them burnt apart on the weekends when the plant was down and the voltage went high.

This was on an old Saftronics but I've used a separate starter to isolate ever since due to the fire risk. Another option is to use a button T/C over the resistor to open the circuit on overtemp.

 

[Jraef]

An added point with BR setups is to be sure the BR is wired through a starter that is only on with the drive's run circuit. I've had them burnt apart on the weekends when the plant was down and the voltage went high.

This was on an old Saftronics but I've used a separate starter to isolate ever since due to the fire risk. Another option is to use a button T/C over the resistor to open the circuit on overtemp.

If your line voltage is going high enough to trigger the DB transistor to fire long enough to burn up the resistor, you have a MUCH more severe problem. Putting a contactor ahead of the resistor will have solved that issue, but moved the problem to the DC bus and capacitors, leading to drive failure. That would happen anyway if the contactor was not there and the resistors fried of course, but my point is that curing the symptom is not curing the disease. If your line voltage is going that high, I would put a contactor ahead of the entire drive and control it with a voltage monitor relay.

To give you an example:
DC bus voltage on a 480V drive is typically around 676VDC, the ratio is 1.41:1. The default trigger point of most DB chopper transistors in Yaskawa drives (old Saftronics) is around 750VDC. To get there just from the AC line being high, that means your AC incoming voltage must be over 530VAC! Like I said, bigger problems than just the brakes.

And by the way, I hope you used a "starter" rated for 750VDC, because that is what is going out to the BR. Off the shelf AC contactors are NOT rated for DC, a 750VDC contactor is a huge ugly thing.

 

[oregonshooter]

Jraef,
Good info and I agree, but no I didn't on all accounts. It was a 600vac rated starter and was speced by Saftronics. They actually had it as a "recall" fix at the time. The drives did not fail in the next year or previous 3 years. They were G3 models.

That was in 1995, today at the current plant we have several drives using standard NEMA 600 vac starters doing the iso on all our VFD braking grids and have had no issues for more than 7 years. Plant voltage is currently only 2-5% high on weekends here though, I can't recall what it was at the previous mill.

It's been my experience in the wood industry that mills like the quick fix / cheap fix, usually not the correct / best one.