vfd trip on outfeed ground faulting during DC braking

Howdy,

I was called onsite this afternoon to take a look at a Lenze SMV drive with a braking unit and resistor. Whoever had installed the braking unit before had not done it right and I had to straighten out the wiring. Also this installer had not programmed the correct parameters in the drive.

So i straightened the wiring and settings out to make it work like it was supposed to, but I had a problem with it faulting out right when we would shut the motor off. It would throw a F.OF1 code, which is for output ground fault. The motor would run just fine until we hit the stop button, it would immediately trip out with this code. I turned the DC braking voltage to 0 , and it still faulted out. The only way I could keep it from faulting is to set it to decel rather than brake to stop.

Any suggestions why it came up with this code during braking, but not during running?

What did you do to set it up for dynamic braking (which is where you use the external braking module)?

If you used P190-192, that's your problem. Those settings are for FLUX braking, which the drive does WITHOUT the external brake module. Flux braking is not the same as dynamic braking, i.e. with the resistor. For dynamic braking, all you do is decel, you don't "set" anything. That's why I ask.

If you set up Flux braking, the VFD pulls the kinetic energy off of the spinning load and dumps it BACK INTO THE MOTOR as waste heat by over fluxing the motor windings on purpose. If you do that AND have the DB module hooked up, power is going to flow to the DB module while the drive is fluxing too, so not all of the current going through the current sensors will return (because some is going to the resistor), and that will trigger a GF trip.

I set P111 to 1= Coast with DC brake. Then I used P174 and P175 to set the dc brake voltage and time. Even with P174 set to 0%, it would still trip. The only way I could get it to work was set P111 back to 2, like it was originially.

OK, if you have a braking unit and resistor, that is DYNAMIC braking, not DC Injection Braking. DC Injection Braking is NOT THE SAME as Dynamic braking using the resistors. They are two DIFFERENT and mutually exclusive functions. You don't use both at the same time because "Coast + DC Brake" means the DC braking comes on immediately, but if you have the Braking Unit, that's going to absorb that DC braking energy into the resistor, defeating the purpose. When you have the Braking Unit, all you do is set the drive to "Ramp", which is setting 2 of P111. It does the rest for you.

If it gets down to a very low speed and continues to coast too long, you can set P111 to 3, which is Ramp + DC braking. In that case the DC Injection comes on AFTER the drive ramps down to Zero Hz, but if the motor is still coasting, it will stop it. the down side of Dynamic Braking is that it suffers from the Law of Diminishing Returns in that the slower the motor turns, the LESS braking energy it has. So on low friction loads, that sometimes means the motor coasts on even though the drive is at zero speed. That's when you can use DC Injection Braking, because at that point the DB transistor has been turned off.

Ok. Thanks. that makes sense!