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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) enter image description here

This is the result I get (1st is stator voltage between one phase and N, last 3 are the stator currents): enter image description here

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.

Greetings!

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  • \$\begingroup\$ What does the excitation current look like plotted against the same time frame? \$\endgroup\$ – Andy aka Dec 14 '20 at 15:44
  • \$\begingroup\$ It's the second curve in the figure (with the 300Hz ripple). \$\endgroup\$ – joschi182 Dec 14 '20 at 16:59
  • \$\begingroup\$ I can't see what it may have risen to but, could it be exhibiting a saturation profile in its waveform? \$\endgroup\$ – Andy aka Dec 14 '20 at 17:05

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