2 Speed 2 Winding 3 Phase Motor

[jdogg05]

I am trying to understand 2 speed induction motors, so I figured the 2 speed, 2 winding, 3 phase induction motor would be simpler to understand. From what I learned in my electrical engineering classes a few years ago, the more times a conductor is wound, the stronger the magnetic field and thus the higher the torque. I was then trying to understand the workings of a 2 speed motor in our plant and I was told that the winding that resulted in more volts/winding would result in the higher speed. That means, connected to the same bus that the winding with less "winds" would actually be the faster speed and would require more power (it's powering a fan). I don't understand how this works. I mean, taken to an extreme, the fastest, most powerful configuration would then be 1 wind, and the slowest, weakest configuration would be infinity... I am sure I am looking at this wrong but I can't seem to wrap my head around it.

 

[ScottyUK]

The speed changes by virtue of changing the number of magnetic poles in the stator. The number of poles is always an even number: 2, 4, 6, 8, etc. In the 50Hz world the synchronous speeds are 3000 rpm, 1500 rpm, 1000 rpm, 750, rpm, and so on. Induction machines run slightly below synchronous speed.

Some motors have two individual windings with differing pole counts, which are not linked by any specific ratio. The two speeds can be chaosen independently of each other. Other motors have only one winding and use switching of winding taps to reconfigure the winding to behave in a manner akin to a machine with two separate windings. This latter method always has a 2:1 ratio between speeds.

 

[LionelHutz]

The operating speed of every successfully installed 3-phase induction depends on the number of pole pairs created by the winding arrangement and the applied frequency. The motor will also have a small speed variation depending on the shaft load.

The actual number of turns in each coil in the winding is part of the motor design based on the motor power rating and applied voltage.

"the more times a conductor is wound, the stronger the magnetic field and thus the higher the torque" is a complete oversimplification that you really should not use.

 

[jdogg05]

OK so it has nothing to do with how many times the conductor is "looped" on each winding. I think I was confusing windings, which was meant to be number of poles, with loops on each winding... Which would totally explain it. Just to cement this, the two separate windings in a 2 winding configuration COULD have roughly the same total length of conductors??

 

[petronila]

Hello jdogg05

"The two separate windings in a 2 winding configuration" is a two speed motor with two different windings, usually this kind of configuration was made when the speed ratio is not 2.
Example(60 Hz): 1750 RPM and 1154 RPM (4 and 6 poles) or 892 RPM and 345 RPM (8 and 20 poles). RPM = (120*F/Poles) - Slip.

When the speed ratio is 2 you could use the Dahlander conection, this one is very usefull when you need two speeds with only one winding.

Like Lionel sayds, all motor parameters of each winding depends of machine design.I will add the main parameters for winding design : Turns, Span, Wires on hand, Connection type (D or Y) all are calculated taking account the power output and are based on frequency, supply voltage, core dimensions and magnetic density in back Iron, Air Gap and Tooth.

About the total lenght of conductors in some case could be the same for both windings or you could find the less power winding is more small than the bigger power winding (The lenght depends of how the winding was made in factory or in shop repairs), that lenght and wire sizes are important for inductance and looses calculations.

Hope this can help.

Regards


Carlos

 

[jdogg05]

Yes that helped a lot, thank you.