DC bus overvoltage at random times
DC bus overvoltage at random times
(OP)
We're struggling with a Beijer BFI P2 90kW VFD right now. 3 phase 400V. It's driving a large rotating drum for washing fishing nets. This means it's seeing a very varying load through a gearbox at roughly a 240:1 ratio. It's a tried and tested application, which to my knowledge has never produced a DC overvoltage fault. Now one of our costumers are struggling with frequent alarms, 2-3 times every 5 hours of runtime. It seems to be happening at random, both during acceleration, switching directions, running steadily at 50hz, whenever. The problem started gradually happening this last month.
There's a decently oversized brake resistor connected, and it's in good shape. The HMI stores a log at 250ms intervals of the DC bus voltage during a trip, and it seems to just be a single sample of ~670V inbetween ~570V. I'm thinking theres a short spike that almost slips inbetween the samples for some reason. Had a multimeter across two input phases when it happened once, and there were no irregularities.
I've spent the day trying to replicate the fault, but it's nearly impossible. Very intermittent. The weird thing is that it's only happened at the end of a preprogrammed "cycle", where it spins one way for X minutes, and an extractor fan turns on the last Y minutes of the X minutes. The fan is also driven by a much smaller VFD in the same cabinet. I'm trying hard to figure out how the fan running could possibly cause spikes on the bigger VFD's DC bus, or if it's just coincidence. They are both fed by the same supply and share the same ground.
Appreciate any input.
There's a decently oversized brake resistor connected, and it's in good shape. The HMI stores a log at 250ms intervals of the DC bus voltage during a trip, and it seems to just be a single sample of ~670V inbetween ~570V. I'm thinking theres a short spike that almost slips inbetween the samples for some reason. Had a multimeter across two input phases when it happened once, and there were no irregularities.
I've spent the day trying to replicate the fault, but it's nearly impossible. Very intermittent. The weird thing is that it's only happened at the end of a preprogrammed "cycle", where it spins one way for X minutes, and an extractor fan turns on the last Y minutes of the X minutes. The fan is also driven by a much smaller VFD in the same cabinet. I'm trying hard to figure out how the fan running could possibly cause spikes on the bigger VFD's DC bus, or if it's just coincidence. They are both fed by the same supply and share the same ground.
Appreciate any input.
RE: DC bus overvoltage at random times
RE: DC bus overvoltage at random times
Once on acceleration, it tripped while the load was unevenly distributed inside the drum and started over-hauling the drive faster than the drive accelerated.
And once while it was running at a constant speed with a very light load. At the end of the shift with 2 minutes left after 2.5 hours of steady operation, I noticed the DC bus readout started fluctuating randomly with every rotation of the output shaft. Still at constant speed. The VFD tripped after 1 or 2 minutes of this unstable voltage. Also some erratic "DC Ripple" logs, but I'm not quite sure what the mechanics behind that are.
The braking/regenerating in itself seems to work very well, the brake resistor barely gets hot. It's stopping completely and reversing direction every 25 minutes, but the faults dont seem to follow that interval.
RE: DC bus overvoltage at random times
It is because the moment of inertia of the drum and the load causes the speed to be higher then set speed and the motor becomes a generator instead, and starts to charge power back to the frequency converter.
You didn't say what kind of motor it has, or which kind of speed control is used, but whit a application like that when a accurate speed might not be the most important, I would choose something like >
It could be a mechanical backlash in the gearbox or in the clutch or in the wedge on the motor shaft or whatever is used.
“Logic will get you from A to Z; imagination will get you everywhere.“
Albert Einstein
RE: DC bus overvoltage at random times
Other options may include disabling regenerative braking in the run mode and only utilize it while stopping.
RE: DC bus overvoltage at random times
RE: DC bus overvoltage at random times