# running single phase motors 120 degrees out of phase

Discussion in 'General Electronics Chat' started by strantor, Mar 18, 2011.

1. ### strantor Thread Starter AAC Fanatic!

Oct 3, 2010
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I want to run single phase 120V fans from power that is not 180 degrees out of phase. It is 120 degrees out of phase, as it is coming from a step down transformer from 3 phase power. Is there a problem with that? The fans are the blowers for this motor, & the website & datasheet don't give a whole lot of info about the fans. thanks!

2. ### GetDeviceInfo Senior Member

Jun 7, 2009
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Your 'single phase' is in response to the voltage potential of the input. It knows nothing about 'phase'. It is simply the sinsodial sum of the inputs.

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3. ### strantor Thread Starter AAC Fanatic!

Oct 3, 2010
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Initially I read this and just said "oh, ok. makes sense" but the now I'm thinking about it and (my knowledge of the theory of motors isn't too great) doesn't the magnetic field rotate around inside the motor, following the phase? Isn't that why "single phase" motors need a start or run capacitor (or they would lock up due to the phase going only 1 of 2 directions) whereas a 3 phase motor does not? I was thinking that 120V motors are specifically designed (poles & windings oriented in a certain manner) to be operated with the phase 180 degrees offset and that running it with the phase 120 degrees offset might pose a problem. So initially I wasn't too concerned about the Voltage potential as much as I was about the phase offset. I was thinking that the magnetic field inside the motor would rotate from 0 degrees to 120 degrees, following the phase, then there would be a dead zone, then it would pick back up at 0 degrees. Effectively the motor would gyrate back and forth between 0 and 120 degrees and never completing a full revolution. Is there any merit to my concern?
Thanks!

4. ### GetDeviceInfo Senior Member

Jun 7, 2009
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Some single phase motors have an auxiliary winding offset from the main windings. Creating a shifted phase current through those windings assists in smooth torque development, not like a 3 phase, but closer towards it.

Offset from what?. There is only one phase, so it's not offset from anything. It's doing it's own sinusoidal thing. It's potential, as mentioned is a mathematical sum of the phases that create it. If you view it with an oscilliscope, it is a sinusoidal wavefrom. Nothing to reference it to come up with an 'offset'. Unless of course you create a shifted phase as with the cap/resistive start motors referenced previously. Now you have a second phase you can reference to your first and say, the start (in some cases run, and or both) phase is shifted in reference to the main winding. All this is doing is adding a little torque bump (pole) in between at assist in the starting torque and/or make for smoother running.

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5. ### strantor Thread Starter AAC Fanatic!

Oct 3, 2010
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If anybody else come across this and has their mind stuck in a hole like I did, go here and punch in the numbers. You will see graphically what GetDeviceInfo is describing. When you add 2 sine waves of the same amplitude and frequency 120 degrees apart, you get a single single sine wave of the same amplitude directly between them. This only happens at 120 degrees phase shift and 240 degrees. At any other degree your amplitude would go up or down.