froesen
Electrical
- Aug 15, 2011
- 2
Hello all....long time lurker, first time poster. I'm an electrical technician at a Canadian power utility, and have a few questions regarding VFD's, with which I have limited experience.
First, a little history on what led up to this question; 275Mg powerhouse crane, main hoist with a Magnetek G+ driving a 100hp Marathon Blue motor (600v-3ph). This drive is equipped with 3 dynamic braking modules/resistor banks. A failed (melted) terminal block inside one of the resistor housings caused a fault on the DC bus, through one of the resistor braking units. This occurred while lowering a 266Mg load (VFD should have been running ~ 45Hz, IIRC).
After discovering and repairing the fault, we attempted to restore power, only to have (several) breakers trip with a seemingly larger fault than originally existed. A follow up investigation revealed that all 3 input rectifier diode blocks (2 diodes per) had one diode failed shorted (polarity feeding the fault). I had checked the input to the drive (at the input reactor) for shorts both to ground and ph-ph and found none (simple ohmmeter) so we suspect that one diode was failed shorted after the first fault, and caused the subsequent failure of the other two diodes at the second fault. Does this make sense? A half-wave ph-ph-ph short would cause the larger fault, so it seemed logical. Currently waiting for a parts time estimate.
Now, for the question I came here to ask; with the cranes I'm accustomed to, extended partial-speed lowering of near-capacity loads with wound-rotor speed control, one had to be careful to not overheat the resistor banks. However, my understanding of the VFD braking system is that the DB modules use transistors driving into resistor banks to keep the DC bus voltage in the drive at an acceptable limit, feeding into the resistors to "absorb" the energy. Does it even matter what "gear" the hoist motor is operated in? Would a comparable amount of energy not have to be dissipated into the resistor banks, regardless what the speed of the motor is with this type of system, or does the old rule still apply, being the slower, the lower the duty cycle?
As I said, not all that familiar with VFD's. Any insight is appreciated. Sorry about the long-winded post.
First, a little history on what led up to this question; 275Mg powerhouse crane, main hoist with a Magnetek G+ driving a 100hp Marathon Blue motor (600v-3ph). This drive is equipped with 3 dynamic braking modules/resistor banks. A failed (melted) terminal block inside one of the resistor housings caused a fault on the DC bus, through one of the resistor braking units. This occurred while lowering a 266Mg load (VFD should have been running ~ 45Hz, IIRC).
After discovering and repairing the fault, we attempted to restore power, only to have (several) breakers trip with a seemingly larger fault than originally existed. A follow up investigation revealed that all 3 input rectifier diode blocks (2 diodes per) had one diode failed shorted (polarity feeding the fault). I had checked the input to the drive (at the input reactor) for shorts both to ground and ph-ph and found none (simple ohmmeter) so we suspect that one diode was failed shorted after the first fault, and caused the subsequent failure of the other two diodes at the second fault. Does this make sense? A half-wave ph-ph-ph short would cause the larger fault, so it seemed logical. Currently waiting for a parts time estimate.
Now, for the question I came here to ask; with the cranes I'm accustomed to, extended partial-speed lowering of near-capacity loads with wound-rotor speed control, one had to be careful to not overheat the resistor banks. However, my understanding of the VFD braking system is that the DB modules use transistors driving into resistor banks to keep the DC bus voltage in the drive at an acceptable limit, feeding into the resistors to "absorb" the energy. Does it even matter what "gear" the hoist motor is operated in? Would a comparable amount of energy not have to be dissipated into the resistor banks, regardless what the speed of the motor is with this type of system, or does the old rule still apply, being the slower, the lower the duty cycle?
As I said, not all that familiar with VFD's. Any insight is appreciated. Sorry about the long-winded post.