Consequent Pole Motor

[bcorbin]

First, I would like to say, as a brand new reader of this board, and a 1-year veteran of the Electrical Engineering field, that the discussions I read here are so helpful to me when I run across things I haven't had the time to learn yet.

Here is a problem I ran across the other day...no textbook I've had really tells me what is really going on. In a 3-phase so-called "consequent-pole" motor, i.e., a two-speed, two-winding motor, how is the field wired? I understand that it starts in a low-speed parallel-wye configuration, and is then switched to a series-delta configuration for high-speed operation.

My main question is this: Are the two coils that make up the series connection of a phase separated by mechanical degrees? I'm hoping I'm on the right track here. Wouldn't this make them act as a single pole when in series, but two separate poles when in parallel? (provided it is not a true parallel...the wiring diagrams show three of the six coils simply as an electrically isolated wye circuit...i.e, not connected to the phases)?

Anyway, I was hoping somebody who actually gets to tear some of these things down and wire them rather than just look at books all the time could tell me if I'm correct. Thanks.

 

[winnie]

It's been a while since I've looked at the various speed changing windings, so my terminology may be off; I've been focused exclusively on inverter fed machines where the winding is simple and the complexity is in the control electronics.

The best advice that I can give for understanding the various windings is to _ignore_ the concept of poles and coils. Instead consider your stator as being composed of slots, each slot at a different mechanical angle, each slot driven by a different combination of currents from your various phases, and try to picture the distribution of current flow in the slots. The 'shape' of the current flow distribution will tell you the number of 'poles' developed.

Stator slots may include conductors from one or two phases. Also each phase can go through a slot in one of two directions. The net slot current is simply the vector sum of the individual currents in each conductor in the slot. So you have six 'phases' to work with for any given slot conductor (the three phases, with two directions per phase). All of the different possible windings are simply ways to connect the slot conductors.

Because motors are actually built with coils of wire, each slot conductor will be matched by a corresponding 'coil half' in a different slot. But if you thing of the individual slots as carrying current, and the entire aggregate current flow in the stator as what develops the magnetic field structure, then trying to figure out which 'consequent poles' come from.

In a consequent pole machine, the coils are arranged such that by changing the polarity of some of the coils, you will change the number of magnetic poles developed in the motor. Depending upon how the series/parallel connections are changed at the same time as the polarity, you will also change the magnetic flux density or voltage of the machine.

Hope this helps.

-Jon

 

[Onitram]

Hi [quo low-speed parallel-wye configuration, and is then switched to a series-delta configuration for high-speed operation.te]

Don't know about poles you wrote about, but this motor is a wyen start delta run, I wired one some years ago on a refrigerant compressor, it uses the wyen for reduced voltage to start .then after it starts it runs on delta. it has 12 leads, an it is wired to contactors that do the switching wyen to delta, it was my first one! [/quote]

 

[winnie]

Re: Consequent Pole Motor

I understand that it starts in a low-speed parallel-wye configuration, and is then switched to a series-delta configuration for high-speed operation.

My main question is this: Are the two coils that make up the series connection of a phase separated by mechanical degrees? I'm hoping I'm on the right track here. Wouldn't this make them act as a single pole when in series, but two separate poles when in parallel?

To add to my above comments, you are most certainly on the right track with the above. The coil sets that make up each phase are physically separated at different angles on the stator.

But changing from series to parallel configuration is not what changes the pole count. If you look closely, you will see that the system is arranged so that when changing from series to parallel configuration, you are also inverting the polarity of half of each phase. It is the change of polarity which changes the current arrangement (described in my last post), thus changing the pole count.

-Jon