SPEED CONTROL FOR ELECTRIC MOTOR TUTORIAL GUIDE
How to control the speed of an electric motor?

The relationship between rotational speed, supply frequency, number of poles and slip for induction motors is usually written:

n = (2/p) x f x 60 x (1-s)

Where: n = speed, r/min
f = frequency, hertz
p = number of poles
s = slip

In principle the speed can be controlled by changing the number of poles, the slip or the frequency. All three possibilities are used.

Changing the number of poles
There are three ways to change the number of poles in an induction motor. The stator can be given:
1. Two or more separate windings
2. A pole changing winding
3. Combinations of the above

Two-speed motors with separate windings
Having separate windings makes it possible to combine different numbers of poles with considerable freedom, but the method does not utilize the motor well, since only half the stator winding is in use at each speed, in principle the possible rated power at each speed will thus only be half that of a single-speed motor of the same size. In addition, the stator and rotor cores are normally dimensioned for a given number of poles. This may also impose certain limitations on ways in which different numbers of poles can be combined in a given core design.

Two-speed motors with pole changing winding
There are several ways of achieving a pole changing winding in order to utilize a motor better than with two separate windings, but some limits are set by the need for the switchgear to be kept simple. The most widely used systems are the Lindstrom-Dahlander connection, often simply called the Dahlander connection, and PAM, Pole Amplitude Modulation. The Dahlander connection gives a pole number ratio of 1:2.The winding
of each phase is in two parts connected in series. These are two common applications that use Dahlander pole-changing:
•  "Constant torque", where the rated torque of the motor is approximately the same at both speeds. The ratio between the rated outputs is about 3:2. This is achieved by connecting the windings in double star for the higher speed and in delta for the lower speed. This is usually represented in catalogues by the symbols YY/Δ.
• "Fan torque", where the torque varies as the square of the speed. "Falling torque" and "square-law torque" are other terms used for this. The ratio between the rated outputs at the two speeds is about 1:5. This is achieved by connecting the windings in double star for the higher speed and in star for the lower speed. This is usually represented in catalogues by the symbols YY/Y.
PAM
PAM (pole amplitude modulation) makes it possible to design a twospeed motor with a stator winding for pole number ratios other than 2:1. Pole number changing is achieved by changing the direction of the current in part of the winding, thus varying the excitation in the same way as with the Dahlander arrangement. Among the advantages of PAM is that a given motor size can be better utilized and a higher rated power can be extracted from it. Just as with the Dahlander connection, constanttorque or fan-torque versions are possible.

source: 2010 IIEE Technical Manual