We have a 125hp 480V fan motor which is started with a VFD. We have recently had problems with the drive tripping on overvoltage during startup of the fan. When the drive operated fine it had a long ramp up time of about 3 minutes or so. Somehow the program got dumped and the ramp time defaulted back to a ramp time of 10s. This is when the problems started occuring.
My question is in theory how a smaller ramp time can cause DC bus overvoltage as oppposed to a longer one? Can a fan overhaul itself during startup?
Is it possible that other fans in the same ductwork are pulling a draft which is spinning this fan faster than motor frequency? Wouldn't this get worse for a longer ramp time however since the fan would spend more time at slower speeds while the draft was pulling it?
This drive has a DC brake which I'm not sure is not dumping this excess voltage. Should the chopper and brake be active during startup? I'm double checking to make sure that this brake is enabled.
My question is in theory how a smaller ramp time can cause DC bus overvoltage as oppposed to a longer one? Can a fan overhaul itself during startup?
Is it possible that other fans in the same ductwork are pulling a draft which is spinning this fan faster than motor frequency? Wouldn't this get worse for a longer ramp time however since the fan would spend more time at slower speeds while the draft was pulling it?
This drive has a DC brake which I'm not sure is not dumping this excess voltage. Should the chopper and brake be active during startup? I'm double checking to make sure that this brake is enabled.