I'm doing some experiments on a synchronous machine with follwing data:
- nominal power: 5kVA
- nominal stator voltage: 380V phase-phase
- nominal stator current: 7.6A
- nominal frequency: 50Hz / nominal speed 1500min-1 -> 2 pole pairs
- connection stator coils: Wye
- excitation circuit connected to 1kW controllable D.C. power supply
- an external DC machine connected via the shaft drives the generator
Now I start up the generator with no-load until it reaches 1500min-1 and turn on the excitation current to get nominal voltage at the stator terminals. Then I suddenly close a short circuit with a switch at the terminals an measure the resulting current in each phase. (Circuit sketch below)
I'm wondering about the "strange" curve of the currents, which should in my opinion look like a sine wave. Once I saw a simillar form namely when a transformer is saturated. But my measurements look the same even when I'm doing the test with no-load voltage of 25V before the short circuit switch is closed. At this point the generator should definitely not be in saturation.
My question is now: why do I have this unexpected short circuit current curve? Perhaps somebody had a simillar issue.