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Speed Control of Induction Motor by Pole Changing Method
The rotor speed (Nr) of an induction motor is given by,
$$\mathrm{𝑁_𝑟 = (1 − 𝑠)𝑁_𝑠}$$
And the synchronous speed is given by,
$$\mathrm{𝑁_𝑠 =\frac{120𝑓}{𝑃}}$$
Therefore,
$$\mathrm{𝑁_𝑟 = (1 − 𝑠) (\frac{120𝑓}{𝑃}) … (1)}$$
It is clear from eqn. (1) that the speed of the induction motor can be changed by varying the frequency (f), number of poles (P) or slip (s).
Speed Control of Induction Motor by Pole Changing Method
By changing the stator poles, the speed of the induction motor can be changed. The number of stator poles can be changed by,
- Multiple Stator Windings
- Method of consequent poles, and
- Pole-Amplitude Modulation (PAM)
The pole changing method of speed control is suitable for squirrel cage induction motors because the squirrel cage induction motors automatically develops rotor poles equal to the poles of the stator winding.
Multiple Stator Winding
In the multiple stator winding method of speed control, the stator is provided with two separate windings which are wound for two different number of poles. One stator winding is excited at a time. For example, suppose that a motor has two stator windings for 4 and 8 poles. For 60 Hz supply the synchronous speeds will be 1800 RPM and 900 RPM respectively. If the full-load slip is 4% in each case, then the operating speed will be 1728 RPM and 864 RPM respectively.
This method of speed control of induction motor is less efficient and expensive, therefore, it is used only when absolutely necessary.
Due to the complications in design and switching of the interconnections of the stator windings, this method can provide a maximum of four different synchronous speeds for any one motor.
Method of Consequent Poles
In the method of consequent poles, a single stator winding is divided into two coil groups. The terminals of both the groups are taken out. The number of poles of the machine can be changed with only simple changes in coil connections. The number of the poles can be changed in the ratio of 2:1.
The one phase of stator winding is shown in the figure. Here, the stator winding consists of 4 coils which are divided into two groups a-b and c-d. The group a-b consists of odd numbered coils, i.e., 1 and 3 which are connected in series. The group c-d consists of even numbered coils, i.e., 2 and 4, which are connected in series. The terminals a, b, c and d of these coils are brought out as shown in Figure-1.
With this arrangement, there will be 4-poles in the motor giving a synchronous speed 1500 RPM for 50 Hz supply system. Now, if the current in the coils of the group a-b is reversed, then north poles will be produced by all the coils. Therefore, to complete the magnetic path, the magnetic flux of the north poles must pass through the spaces between the groups, hence inducing the south poles (poles of opposite polarity) in the spaces between the groups. These induced poles are known as consequent poles (see Figure-2). Hence, the motor now has twice as many poles as before (i.e., 8-poles) and the synchronous speed is half of the previous speed (i.e., 750 RPM). The two sets of coil groups a-b and c-d can be connected either in series for one speed or in parallel for the other speed of the motor.
The principle discussed above can be extended to all the 3-phases of the induction motor.
By choosing a suitable combination of series or parallel connections between the coil groups of each phase and star or delta connection between the phases of the 3-phase induction motor, the speed control can be obtained with constant-torque operation, constant-power operation or variable-torque operation.
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