Question about dual voltage motors, series and parallel stator wiring

[jason sleeth]

I am teaching motor calcs and have a question. In dual voltage motors such as a 240/480 motor I was taught that the low voltage is always wired in parallel and the high voltage is always wired in series but I am not sure why? Why can't you wire the low voltage in series and the high in parallel?

If I am looking at a motor with 24 ohm windings in a nine lead motor and If apply 240 volts in a parallel config the amperage draw will be 10 amps. correct? If I wire it to 480 V in series it will draw 5A? Why can't it be wire at 480V (20A) or in parallel or 240 (2.5A) in series?

 

[stevearne]

motor windings

motor windings

The windings of a single-phase dual voltage 240/480V motor are 240V rated windings. They must be put in parallel on 240 so that they will be connected at their rated voltage. Likewise, if put in series on a 480V supply, the 480V divides across the two equal resistance windings, so 1/2 of the 480V (240V) is applied across each winding.

If you put the windings in series on 240V, you would only receive 120V on each winding, which would not be enough to provide the proper magnetizing effect to develop turning torque.

If you put the windings in parallel on 480V, you would be putting the full 480V on each 240V rated winding, and you would burn the windings up.

You can't really do just a simple ohms law by reading the winding resistance and calculating it out, because there is much more involved as the motor begins to turn and generates counter EMF which opposes the input current flow.

This is a simplified version, but maybe it will make some sense.

**Edited for spelling SA

 

[jason sleeth]

Thanks steve,

That makes perfect sense. Thank you for your explanation and examples.

 

[tom baker]

Hey Jason heres a really cool experiment that will illustrate the above.
Get a small electric drill, an old 1/4" from the second hand store, a pigtail socket and a 300 watt lamp.
Wire up the pig tail socket so its in series with the drill.
When you start the drill, the impedance of the windings will allow the lamp to light.
What will happen when you hold the drill chuck to keep if from turning?
The lamp gets really bright as there is no back emf. If you understand why, you know why a motor draws max current at start up.