# What is the synchronous speed of single phase induction machine from 2 phases of 3 phase machine in parallel?

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?

• 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. Oct 31, 2015 at 12:25
• 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. Oct 31, 2015 at 14:46
• 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. Oct 31, 2015 at 17:54
• 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?
– user80875
Oct 31, 2015 at 20:27
• I'm interested in braking with generation, so the a mode like the zero sequence braking mode. Motoring is not as important. Nov 1, 2015 at 12:03

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.