Question on phase angles

----------- Actually it won't- except for losses - the power is supplied electrically, not mechanically - >

-------------------- That would require a bit of calculation but the "supply" machine must be able to handle its own real and reactive needs, as well as those of the load motor from the single phase line without smelling funny. It will not have balanced currents. If "large" with respect to the load, it is not a problem. If you have access to cheap, large 3 phase motors it is reasonable. If you have to spend $$ to set this up, it is not and electronic single to 3 phase converters are more attractive. The case that I have seen were where someone got his hands on a surplus 3 phase motor for a fire sale price (probably ex-military stuff shipped to East Boondock and not worth the cost of shipping back out as often happens - the farmer lived in Moose Wallow which is somewhat north of East Boondock ).

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I agree- it is a fix dependent on what is available and cheap at that time and location.

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---------------- It's a point but --if you could start the first machine somehow (supply one phase and use a string on the shaft to spin it up or whatever) then the starting method might be actually easier for the smaller "load" machine - why bother with the first machine?

--------------------- If you have a 3 phase machine and a smaller single phase machine as well as some way to get the 3 phase machine starting to turn in one direction or the other, you can try it.

Cheers

Reply to
Don Kelly
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|> | Be careful here. The rotor field will only lag if the *shaft* is | loaded, |> | *not* the 3-phase output. If the motor is run unloaded so the only |>

|> You're saying that no matter how much power is drawn from the generated |> teaser phase, this won't put drag on the motor? | ----------- | Actually it won't- except for losses - the power is supplied electrically, | not mechanically - >

So how does the power get into the other two windings once the motor is running from single phase, without the mechanical rotation being involed?

Reply to
phil-news-nospam

electrically,

Reply to
Don Kelly

|> |> | Be careful here. The rotor field will only lag if the *shaft* is |> | loaded, |> |> | *not* the 3-phase output. If the motor is run unloaded so the only |> |>

|> |> You're saying that no matter how much power is drawn from the generated |> |> teaser phase, this won't put drag on the motor? |> | ----------- |> | Actually it won't- except for losses - the power is supplied | electrically, |> | not mechanically - >

|>

|> So how does the power get into the other two windings once the motor is |> running from single phase, without the mechanical rotation being involed? | ---- | single phase stator excitation produces the effect of two rotating fields- | forward and backward - rotor is turning and the effect of the forward field | is enhanced (any single phase motor can be started by mechanically giving it | a mechanical boost in the desited direction)- dominant effect is the | induction of voltages in the rotor at slip frequency- these produce currents | leading to a field at slip frequency rotating at slip speed with respect to | the rotor and at synchronous speed with respect to the stator- inducing | voltages in each phase. Essentially you have a form of transformer. All the | power input is from the single phase stator excitation, not from the | mechanical system. Any power from the mechanical system would cause an | increase in slip- that is true- but the only available source of energy is | the single phase supply. | Without doing further analysis I cannot say that slip will be constant. What | I can say is that it will not affect the frequency of the output.

I would not expect it to affect the frequency, other than for short term ups and downs during a changing phase. But I would expect a phase change of a few degrees in that extra leg. What I have been trying to think about is a way to automatically compensate for it and swing it back to the right phase angle, and the right vector length (voltage).

Reply to
phil-news-nospam

---------------- There may be a slight phase change as well as magnitude unbalance simply because the currents will be unbalnanced and different IZ drops occur in the different legs. Capacitors will help. If the loads are more than usually sensitive to unbalance- it may not be feasible- with normal loads it is possible. The following message sent to me by Scott Falke may be of interest. Thank you Scott.

Reply to
Don Kelly

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