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If I take all three windings of a three phase induction machine and put them in series, it acts as a single phase induction machine with three times the number of poles. This scheme is used to perform zero sequence braking. The synchronous speed of the single phase machine is then 1/3 of the synchronous speed of the original three phase machine (due to the higher pole number).

My question is, if I take two or three windings of a three phase induction machine, and put them in parallel with each other, will this also operate as a single-phase induction machine, and what will the pole number and synchronous speed of the resulting machine be?

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    \$\begingroup\$ The answer is no. You did mess evertyhing and electrodynamic braking is special case, where motor acts as generator and phases are connected together to form a short circuit. \$\endgroup\$ – Marko Buršič Oct 31 '15 at 12:25
  • \$\begingroup\$ No, zero sequence braking is where ypu put all phases in series to make a single phase machine, applying the same ac voltage to all windings.they are not short circuited. I would like to know if it would also work with all phases in parallel instead. \$\endgroup\$ – crobar Oct 31 '15 at 14:46
  • \$\begingroup\$ In a 3 phase machine the three poles are at 120degrees (electrical) apart. If you short them out in series or parallel the 3 poles effectively cancel each other out as a rotating magnetic field and it basically becomes a stationary (AD or DC) magnetic field. There is no rotating field and @MarkoBuršič is correct when he says it can only be used for braking. \$\endgroup\$ – KalleMP Oct 31 '15 at 17:54
  • \$\begingroup\$ When you ask "will this also operate as a single-phase induction machine," do you mean operate as a motor or operate in the zero sequence braking mode? \$\endgroup\$ – Charles Cowie Oct 31 '15 at 20:27
  • \$\begingroup\$ I'm interested in braking with generation, so the a mode like the zero sequence braking mode. Motoring is not as important. \$\endgroup\$ – crobar Nov 1 '15 at 12:03
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Yes. You can do something like that. In order to get the motor to start, you will need to connect a capacitor in parallel or in series with one of the windings. The number of poles and the synchronous speed will not change. The starting torque will be reduced. The slip and associated losses will increase. The safe operating load will be reduced. There are more complicated connections using multiple capacitors that can improve the performance to some extent. One such arrangement is called a "Smith Connection."

Since this use of a three-phase motors is not what they are designed for and since motors that are designed for single-phase operation are better suited for the purpose, there is very little detailed information available about motor performance using these types of arrangements.

Here are some diagrams and additional information. Reference Link

Edit: When I said "losses will increase," I should have said that the consequence of increased losses is reduced efficiency.

I assumed that you were asking about operation as a motor. The motor will also operate in the zero sequence braking mode with a parallel winding configuration with the same synchronous speed as for the series connection. You would need to apply a lower voltage. Switching from motor to brake operation would be more complicated. The easiest implementation involves a motor that is delta connected for motor operation with the delta opened and voltage applied to the two opened ends for braking.

For a wye connected motor and a parallel connection for zero sequence braking, I assume that you would disconnect from power, connect the three motor phase terminals together and apply voltage between the phase terminals and neutral.

Edit 2: Since you are only interested in braking, you may have some kind of continuous braking application in mind. Zero sequence braking will return some of the braking energy to the supply and thus reduce rotor heating, but the percentage of the baking energy that is returned will apparently be rather small. In addition, the braking motor will be be injecting harmonic current into the power system. Keep in mind that the applied voltage must be lower that the normal winding voltage to avoid overheating the motor. Connecting the windings of a delta connected motor in series reduces the winding voltage to 1/3. Since that scheme is apparently practicable, the motors must be able to tolerate that for occasional braking operation. For continuous braking, further voltage reduction may be required.

Reference

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  • \$\begingroup\$ Yes, very poor efficiency is to be expected in this configuration, not what the OP was asking about. \$\endgroup\$ – KalleMP Oct 31 '15 at 18:04
  • \$\begingroup\$ Adding a capacitor on three phase motor you can expect unbalance, because windings are shifted geometricaly by 120 degrees, while single phase motor has an additonal winding for capacitor displaced at 90 degrees. \$\endgroup\$ – Marko Buršič Nov 1 '15 at 11:16

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