I have a 7.5 hp, 3-phase motor I am using as a generator on a grid-tied micro hydro system.
The system works well most of the time and generates roughly 4400 W or about 100 kWh a day.
The connection scheme is shown in the attached picture. There are three capacitor banks, one leg of each 300 μf capacitor is connected to one of the three legs of the motor, and the other three legs are all connected together.
When the motor is rotated up to speed, the unit begins generating wild AC power quite readily. This output from the generator is connected to a 3-phase full-wave rectifier and the output from the rectifier to a grid-tied inverter.
The system is designed so that when the grid fails, dump loads are connected to the generator to prevent overspeed. When I manually disconnect the grid, the system works as designed and when the grid returns the inverter reconnects and the dump loads disconnect.
For some reason the field is lost at random times and the motor stops generating and spins into hyper rpms, far above the rated 1800 rpms of the motor. When I reset the system it behaves properly. Some other points to note:
- I can't duplicate the loss of field situation manually.
- The capacitors have been tested and test as being good.
- This loss of field occurs even when the grid is stable.
- All electrical connections appear to be good.
- Once the field is lost, the generator will not generate again until the resistive load is removed. Once this resistive load is out of the circuit the motor becomes a generator again, like nothing ever happened.
I am hoping someone has ideas as to what might be wrong and how to stop this. I fear some part of the system will be damaged by this out-of-control situation.
