jraef
Electrical
- May 29, 2002
- 11,359
Hey fellow drive gurus, I have one that I have never come across before. I have a client wanting to use an 40HP (30kW) 460V induction motor on a water turbine pump and have the water overhaul it to turn it into an induction generator. So far so good, I’ve done that before. But the new twist is that they want to do this with a VFD, so that they can continue to get power out of it when water flow is lower than normal, also not a problem as I have done regenerative drives many times. The kicker for me this time is that they want this to NOT be connected to the grid, which means no excitation power, which means it cannot be self starting. They will feed the DC bus of the drive to another inverter to make AC power for a building nearby.
Their solution is to use a small cheap 120V portable generator to feed excitation power at start-up, and they seem to think they can do this with a small 3kVA step up single phase transformer that will connect 480VAC to two of the VFD line terminals, essentially a single phase input (which is possible with the drives they are using), then let that provide initial excitation energy to the motor windings (through the inverter of course) and once the motor is spun up and overhauled, it will provide its own excitation via the DC bus to be self sustaining.
The problem I am having is the AMOUNT of excitation current they will need to pull this off, and can a 3kVA single phase transformer supply that? As a general rule when I have done induction generators and have had to estimate excitation current (flux current) I have used 20% of the motor current as a safe bet. So using that, the motor is rated 50A, 20% would be 10A, which if coming from a single phase source, becomes 17A and the max amps out if a 3kVA 480V step-up transformer is just over 6A. I’m thinking this can’t work.
But I’ve never had to worry about this with a VFD before, because they have always been connected to a line source to where that is not an issue. In this case they say they had one that was doing it this way in the past, but the drives died years ago and never put put back in service, now there is nobody left who understood the original system. We just have drawings showing this 3kVA source input. So I can ASSUME it worked, but I’m unclear on the physics of excitation current in a VFD situation. Have I been over estimating the excitation current value at 20% for all these years because it didn’t really matter until now? I know that excitation current may be high because the PF is extremely low at startup, but maybe the VFD output might be using the DC bus caps to keep the RELATIVE line current much much lower?
Their solution is to use a small cheap 120V portable generator to feed excitation power at start-up, and they seem to think they can do this with a small 3kVA step up single phase transformer that will connect 480VAC to two of the VFD line terminals, essentially a single phase input (which is possible with the drives they are using), then let that provide initial excitation energy to the motor windings (through the inverter of course) and once the motor is spun up and overhauled, it will provide its own excitation via the DC bus to be self sustaining.
The problem I am having is the AMOUNT of excitation current they will need to pull this off, and can a 3kVA single phase transformer supply that? As a general rule when I have done induction generators and have had to estimate excitation current (flux current) I have used 20% of the motor current as a safe bet. So using that, the motor is rated 50A, 20% would be 10A, which if coming from a single phase source, becomes 17A and the max amps out if a 3kVA 480V step-up transformer is just over 6A. I’m thinking this can’t work.
But I’ve never had to worry about this with a VFD before, because they have always been connected to a line source to where that is not an issue. In this case they say they had one that was doing it this way in the past, but the drives died years ago and never put put back in service, now there is nobody left who understood the original system. We just have drawings showing this 3kVA source input. So I can ASSUME it worked, but I’m unclear on the physics of excitation current in a VFD situation. Have I been over estimating the excitation current value at 20% for all these years because it didn’t really matter until now? I know that excitation current may be high because the PF is extremely low at startup, but maybe the VFD output might be using the DC bus caps to keep the RELATIVE line current much much lower?