On 1/22/20 10:51 AM, David Sharpe wrote:
Jim Do you presently have 120/208 “house power? If you want 3 phase usually horrible expensive from utility. Another out there house power option is 240 high leg delta (you get 120/240 from one Xfmr, 3 phase from all 3, and no ungrounded system risks since 120/240 xfmr establishes N-G connection.
Nope.. no 3phase house power. And, being portable is why I was kind of heading the generator route.
At really large powers I’m like you I’d love a really large MG set from 25 -2500HP and up. Flywheel action provides 2.5-5X momentary overload capacity and who needs a stinking variac, when I can very simply control output voltage by field control of generator at O.5-1% of output load. The only advantage of a solid state voltage control like I’m working on (IGBT across Full Bridge, PWM control) is SS can be EPO electronically in simulated 2ms with fault detection. A large MG set even with vacuum bottle CB technology will be an order of magnitude slower, AT BEST. If your looking for a generator I’d insist on a brushed rotor so you can manually adjust Vo. Most generators have on board regulators that will shut down unit if you exceed compliance ranges of generator. Irregardless, you have to stay within the PQ stability limits of your unit or you will cause thermal damage to stator or rotor depending on under over excitation, and reactive power tolerance and unit input output limits.
Is that something that is typical on these smallish generators? These days, I think they're all brushless.
For instance, a Generac 25kVA trailer mounted says: Marathon Electric® o Brushless o 4 pole o Class H insulation Voltage regulation +/- 1% with Marathon SE350 Voltage RegulatorLooking at an MQ whisperwatt (which would be WAY more than I want to spend, but is a common rental unit in the LA area), they say Brushless excitation with AVR.
https://www.multiquip.com/multiquip/DCA25SSIU4F.htmI did find an example that calls out "Direct" for the excitation (whatever that means), and another that says "brushless 12-lead"
I notice, too, that you pay a substantial premium for 3 phase in generators, which sort of surprised me - the engine is the same, and I would have thought the alternator cost was about the same for a given power rating - after all, it's driven by copper and iron costs and manufacturing, and that's pretty similar. Must be a "volume" thing - they sell lots of 20-25kVA single phase 240V generators for residential and office backup duty, so those are $4-5k and the 3phase units are $8-9k.
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