# AC Motor question.



## Stator (Oct 31, 2009)

Sorry not sure where to post this.

Just a quick question for the motor guys that are on here. What is the purpose of having 2 or more start windings on single phase AC motors? I understand that it's impossible to get rotation out of just one phase, so by adding a capacitor in seriers with the start winding we now have created the current to lead the voltage and in tern achived a "split phase" condition, and maximun starting torque is developed. 

I can make sence of why there would be a need for mulitple run windings since each run winding would be connected to let's say 2 stator poles and another could be connected to 4 stator poles in order to achive different speeds. But start windings..... Hmmmm:no:

Thanks guys.


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## micromind (Aug 11, 2007)

If the motor is dual-voltage (115/230), all windings are 115 volt.

If the windings are numbered, and there's one start winding, there'll be 6 wires out. We'll ignore stuff like capacitors, overloads, and start switches. Typically, these wires are labeled T1, T2, T3, T4, T5, and T8. (T6 and T7 are internal wires connected to the capacitor and start switch.)

T1 and T2 are one of the run windings, T3 and T4 are the other. T5 and T8 are the start winding. 

If we connect T2 and T3 together, and apply 230 volts to T1 and T4, each winding will see 115 volts. What about the start winding? There's only one, and nothing to series connect it to. 

Since the run windings are wound on the same core (the stator), they'll act like a transformer. Meaning that they'll maintain constant voltage. Therefore, the start winding can be placed in parallel with one of the run windings. Usually, it's T3 and T4. 

The obvious problem here is that if T3 and T4 are paralleled with T5 and T8, and these are series connected to T1 and T2, the current through T1 and T2 will be huge during starting. One solution (usually found on larger motors, 2HP and up) is to have two start windings. In this case, one start winding parallels with T1 and T2, the other with T3 and T4. 

A motor with two start windings typically has more starting torque, and the start windings can be engaged for a much longer time, but it's more costly to build. 

Rob


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## Stator (Oct 31, 2009)

Thats great! I did think that It might be somthing to do with the fact that there is a huge starting current and with just one start winding there could be a problem. Thanks for clearing that up Rob!:thumbsup:

What I'm finding though is that typicaly there are not that meny stator leads present (Still talking single phase). The last motor I worked on was single speed capsitor start induction run, it was connect to 120v and had 3 stator leads. There was no labeling on the leads. Black, White, Red , Brown. 

The black was 120v the white neautral red was start brown was run. Now the only way I could make sence of this is that the connections were made in the stator Instead of bringing out both start leads and run leads they only brought out 2, the other 2 were connected internally. 

Could that be a possibilty?


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## micromind (Aug 11, 2007)

A single phase single speed reversible motor would need 4 leads brought out. two of them would be run, the other two would be start. 

To reverse such a motor, the start leads would need to be swapped. 

The run winding of a capacitor-start or split-phase start motor is bi-directional. Meaning it'll run in either direction. The phase relationship of the start winding determines the direction it'll go. If the start winding is disconnected, the shaft can be turned by hand (quickly), and it'll run in that direction. 

This is why these motors are reversed by interchanging T5 and T8 (the start winding). 

Rob


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## subelect (Nov 25, 2007)

Micromind,
I have a quick question to satisfy my curiousity.
When I am trying to figure out a single phase motor, especially one that connects the run and start caps. in various arrangements, I always have to stop and draw it out on a piece of paper to figure out the ckt. I still have a tough time looking at some of the more complicated ones and knowing exactly how the motor ckts are layed out.
Can you look at a motor connection and figure out the winding arrangements or do you have to draw it out first?
Rick


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## micromind (Aug 11, 2007)

I work with a lot of motors, all sizes. 

I don't even remember the last time I looked at a connection diagram. I guess I just know from the number of leads, (and difference in lead size in some cases) how to connect just about any motor.

The stuff I wrote above about single phase motors was from memory; I didn't look anything up at all. 

Something rare, like a 10 lead single phase motor, or a two speed 3 phase motor, I'd have to either look it up, or draw it out. 

Rob


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