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Single phase to three phase sinewave converter

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T3sl4co1l:
It's same frequency. :)

Shifting frequency is actually really hard to do, electrically!  If you needed, say, mains at 50Hz, you could use a synchronous machine with the stator driven at 60Hz, rotating it at 10Hz (600 RPM) via gearing from a normal (synchronous) motor, and taking 50Hz off the rotor.  Otherwise, a motor-generator set, or these days, using a VFD.

Tim

Yansi:
Doubt that rotary converters use synchronous motors (difficult to start), or that they use gearing to correct for the slightly under-synchronous speed of the induction motor. Because I think there is no need to correct for the slightly lower frequency in the first place.

The will very likely be a induction motor and synchronous generator on a single shaft. Hence the output frequency MUST be slightly lower, due to the slip of the induction motor.

Gearing is added cost, complexity, noise.
Synchronous motor is a problem to start up, not to mention I haven't seen a single phase synchronous motor bigger than a couple of watts.

T3sl4co1l:
No, rotary converter doesn't fall out of sync.  How could it?  Two of the three phases are supplied directly by the mains input!  The rotating mass is only there to supply the third phase.  Do you mean the third phase has a frequency shift?  But then its phase would be rotating relative to the mains, and a rotary phase converter it ain't. :)

The field in the rotor does slip, but it slips relative to the rotor itself.  That field, as seen by the stator, is rotating at mains frequency.  Slip manifests as a phase shift, not a frequency shift.  The phase shift won't be exactly 120 degrees, but a bit more because of the slip.  AFAIK, reactance dominates, so you need compensation capacitors anyway, and you probably can't measure this phase shift in operation, just under contrived circumstances (no load?).

Perhaps you're having a confusion similar to the "missing dollar paradox"?  In that one, the listener is misled to subtract quantities when the correct step is to add; in this case, the slip might be there, but it subtracts back out when you look at the field seen by the stator. :-+

Tim

Yansi:
 What if you would try to first state what kind of electric machine the converter is, instead of insisting that i am not right (i think i am pretty spot on with those induction motor driven generators in the isolated converters)? I haven't noticed once you specify, the usual non-isolating rotary converters use an induction motor directly as a generator for the third phase? No other machine is present on the shaft.
Without stating the basic concept, we could argue over and over and both be correct.

But thats no problem, I have already done a bit of research and informed myself.  :-+

So, cheap rubish uses specially wound* 3ph induction motors fed by a single phase using the remaining leg as the third phase.
*to acomodate for the higher current in the input phase and the output voltage assymetry.
Then it makes sense.

God bless those with (and without) 3ph available! :-/O

//EDIT: Typo.

T3sl4co1l:
Ah hm, I thought this was the only kind but if there are others I'm not aware, that would be a problem!

http://www.phase-a-matic.com/images/Rotary-Converter-Connection.jpg

Where "rotary converter" is just an induction motor (no load) with capacitors as needed.

Or, better yet, this: https://electronics.stackexchange.com/questions/280486/diy-rotary-phase-converter-with-starter-motor-cutout-relay-in-parallel-with-loa

An "isolated" converter that is literally no more than a genset would indeed require a synchronous machine to produce synchronous output, and one made from induction motors would... hmm, I'm not sure offhand which way the field slips in generator mode actually, but that's rarely used anyway, so, a pair of sync machines it would be.

Cheers!

Tim

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