Single Phase Motor in Series

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fifty60

fifty60

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I have a 230V single phase supply and 2 115V PSC motors. I am trying to evaluate what will happen if I connect the motors in series. The only way I can think to evaluate would be with 2 inductors in series. In this case the inductances would add, the voltage would split between the 2 motors, and the current through the motors would be the same through both motors.

I know there has to be more to this than the above simple analysis. What extra precautions arise from wiring AC motors in series?
 
fifty60 said:
I have a 230V single phase supply and 2 115V PSC motors. I am trying to evaluate what will happen if I connect the motors in series. The only way I can think to evaluate would be with 2 inductors in series. In this case the inductances would add, the voltage would split between the 2 motors, and the current through the motors would be the same through both motors.

I know there has to be more to this than the above simple analysis. What extra precautions arise from wiring AC motors in series?
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The biggest caution is that for an even voltage division between the two motors, they must be identical in design AND be coupled mechanically to exactly the same load. Under any other combination of circumstances you can have the two motors consuming unequal amounts of power only be having different PF or having different applied voltage. If you have different applied voltage, you will probably be damaging one of the motors.

Look at the simple example of lighting loads: If I put two sets of 120 volt lighting loads in series connected to 240 with no neutral, what will happen?

If both load sets are identical incandescent bulbs, it will work for awhile. But as soon as one bulb burns out, the rest of the bulbs in that set will see overvoltage (since they will be forced to carry the same total current with one fewer bulb) and you are in trouble.
If you have 200W of incandescent and 200W (real power) of fluorescent, you will have a problem from the start.

What you are missing from your analysis is that each motor is not modeled by a fixed inductor or by a fixed inductor and a fixed resistor.
The actual values will vary with the speed of the motor shaft (slip) and with the torque being supplied by the motor. Hence unless they are on the same shaft they will not track well.
 
That makes a lot of sense. I will need to dive back into rotating machine theory to understand load, current, and torque relationship. Let me try to think this through...load increases for one motor, rpm's decrease and torque increases. The increased torque will increase stator current and induced voltage. Will this same increased current be seen by both motors? Will the increased torque in one motor decrease the voltage in the other motor?
 
fifty60 said:
That makes a lot of sense. I will need to dive back into rotating machine theory to understand load, current, and torque relationship. Let me try to think this through...load increases for one motor, rpm's decrease and torque increases. The increased torque will increase stator current and induced voltage. Will this same increased current be seen by both motors? Will the increased torque in one motor decrease the voltage in the other motor?
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You are forcing the current through the two motors to be the same, both the inductive and the resistive component. All that can change between the two is the applied voltage across them.
The result will be confusing enough that I am not going to stick my foot any deeper for fear it is already too close to my mouth.
 
GoldDigger said:
Hence unless they are on the same shaft they will not track well.
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Which is exactly what you have with dual voltage motors, two coils in parallel at the low voltage rating with identical load conditions on them, and when connected to high voltage same two coils are series but voltage across each equally divides because the load equally divides across them. Put those same two coils on different loads somehow and they will not balance unless the loads happen to be identical.
 
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