How does 2-speed fan motor work

Thread Starter

MrChips

Joined Oct 2, 2009
30,819
I tried to repair a 2-speed table-top house fan that has a 120VAC motor that looks a bit like this.
(I no longer have the fan since it was returned to the owner.)

1708221229230.png

I have two questions that I genuinely cannot understand.

1) There are three connectors to the coil to select low speed and high speed. What is the physics behind the 2-speed operation?

2) When I try to power the fan (no changes made to the wiring from the switch to the motor) the motor just vibrates and does not rotate.
Trying to get it to spin by hand does not work.
 

nsaspook

Joined Aug 27, 2009
13,297
https://trace.tennessee.edu/cgi/viewcontent.cgi?article=7494&context=utk_gradthes
http://www.servomotor.co/shaded-pole-motor-851503.html
Unlike other types of single phase motors, shaded-pole motors have only one main winding and no start winding or switch. As in other induction motors, the rotating part is a squirrel-cage rotor. Starting is by means of a design that uses a continuous copper loop around a small portion of each motor pole. Currents in this copper loop delay the phase of magnetic flux in that part of the pole enough to provide a rotating field. This rotating field effect produces a very low starting torque compared to other classes of single-phase motors. Although direction of rotation is not normally reversible, some shaded-pole motors are wound with two main windings that reverse the direction of the field. Slip in the shaded-pole motor is not a problem, as the current in the stator is not controlled by a counter-voltage determined by rotor speed, as in other types of single-phase motors. Speed can therefore be controlled merely by varying voltage, or through a multitap winding.
 

Dodgydave

Joined Jun 22, 2012
11,303
Last edited:

MaxHeadRoom

Joined Jul 18, 2013
28,692
They are also the most inefficient of all induction style motors and display this in the amount of heat generated, mainly due to the shorted turn, (shading pole).
The shaded pole also is used in other typical applications such as AC contactor coils in order to keep the coil armature retained when AC crosses zero.
 

Tonyr1084

Joined Sep 24, 2015
7,903
One time, messing with a box fan motor, I noticed that the RPM on Low, Medium and High remained the same no matter what. So wanting to know why "Low" produced a gentle breeze from the fan, "Medium" produces a more significant breeze and "High" produced the strongest amount of air movement, it dawned on me that it's not the motor it's the load and the amount of power going into moving the air. On "Low" there was limited power to the motor. As the fan spun the air resistance held the fan from reaching the maximum RPM as when there was no fan on the motor. "Medium" moved more air because it had larger coils and used more power. The result was a stronger air flow. "High" - well, the same thing. More power resulted in more air movement. That was a lesson I learned back in the early 70's.

As for selecting different speeds on a record player, this is the type of motor I'm familiar with. Notice the shaft diameters are different. The large diameter played 78's, the mid diameter played 45's and the tip, the smallest diameter, played 33 1/3 RPM LP Albums. Speed selection was by an idler wheel that could adjust position among the three shaft diameters.
1708273923298.png
 

Tonyr1084

Joined Sep 24, 2015
7,903
That was the most popular style of cheap record player back in the day. But they also came out with variable speed machines that had a strobe and three lines with black squares around the platen. You adjusted the speed so that the desired speed made those squares stand still. When they stood still you were at the right playing speed. This was seen in higher end record players, and instead of being driven by a shade pole motor and idler wheel they were belt driven by a different motor. I can't speak to what kind of motor that was, I never messed with one of those.
 

Thread Starter

MrChips

Joined Oct 2, 2009
30,819
My final question is, why does the motor not run, it just chatters.
The motor shaft is well lubricated and turns freely with no power applied.
The fan was observed to run once after doing the repair attempt.
 

nsaspook

Joined Aug 27, 2009
13,297
My final question is, why does the motor not run, it just chatters.
The motor shaft is well lubricated and turns freely with no power applied.
The fan was observed to run once after doing the repair attempt.
Shorts in the main coil, shades are open, bent shaft, etc ...
 

Tonyr1084

Joined Sep 24, 2015
7,903
My final question is, why does the motor not run, it just chatters.
The motor shaft is well lubricated and turns freely with no power applied.
The fan was observed to run once after doing the repair attempt.
I'm having a good "GUESSING GAME" morning, and given your experience I'd imagine you probably thought of this, but if two wires run low and the other two run high, perhaps if you try to run low and high in series they may be interfering with their ability to create electromotive force. I'm sure you have tried this, but switch up the wiring and see what happens.
 

Thread Starter

MrChips

Joined Oct 2, 2009
30,819
I'm having a good "GUESSING GAME" morning, and given your experience I'd imagine you probably thought of this, but if two wires run low and the other two run high, perhaps if you try to run low and high in series they may be interfering with their ability to create electromotive force. I'm sure you have tried this, but switch up the wiring and see what happens.
The motor is installled in a house fan. No wires were moved. The only thing the repair person did was to clean the OFF-LOW-HIGH switches with contact cleaner. He said the fan worked before reassembling.
 
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