2 Speed 2 Winding 3 Phase Motor

[lukebarber]

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.

 

[Mr. T]

Speed (rpm) in a three phase induction motor is dictated by the number of poles (not windings) and frequency. Changing the voltage does not change the speed.

If you want it to run faster you need fewer poles or higher frequency. Slower, more poles or lower frequency.

 

[vinnygalbo]

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 chosen 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.

 

[lukebarber]

Here's a good post on 2-speed motors.

https://precisionautomation.net/2019/04/05/a-primer-on-two-speed-motors/

 

[vinnygalbo]

The operating speed of every successfully installed 3-phase induction motor 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.