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| Synchronous motor rotor excitation effect on current draw |
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| jbb:
Well, it doesn’t look like you’ve got enough information to do detailed calculations. So maybe they’re looking for a couple of reasonable assumptions (eg does an excitation current of 10A mean anything special?) and a qualitative answer. |
| duak:
I remembered that synchronous motors can be used to provide an adjustable leading or lagging power factor by varying the excitation current. See https://en.wikipedia.org/wiki/Synchronous_condenser I don't think the AC losses change much as the excitation is varied so, to conserve energy, the input power should be constant. After all, it is a synchronous machine that runs at a constant RPM. The VARs (reactive power) change as per the V-curve. The stator current is at a minimum when the PF is 1. I'll bet that the rotor will pull out of lock with the rotating field and stall if the excitation is dropped too low, especially if it's 100% loaded. |
| RudiB:
Am I right in thinking that the PF will become leading if the excitation is reduced to 5a, and that when reduced to 10a it will be lagging and to the point of becoming unstable? |
| duak:
RudiB, no, see attached V curve graph. Nominal excitation (10 A) = unity PF. Over excitation = leading power factor = capacitive reactance. This graph also shows the stability limit for excitation and load. Underexcitation reduces the magnetic flux developed and thus reduces the output torque leading to a stall. |
| schmitt trigger:
FWIW, I scanned a page of the book Electric Machinery by Fitzgerald, Kingsley and Umans, related to steady state synchronous machine operation. |
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