Speed Regulation of Induction Motors

The speed regulation of induction motor is defined as the change in the motor speed with change in load. It is expressed as a fraction or percentage of full-load speed, i.e.,

$$mathrm{mathrm{Speed:regulation}:=:mathit{frac{N_{nl}-N_{fl}}{N_{fl}}}times 100%}$$

Where,$mathit{N_{nl}}$ is the no-load speed of the motor and $mathit{N_{fl}}$ is the full-load speed of the motor.

The speed regulation of an induction motor is about 3% to 5%. Due to this small speed regulation, the induction motors are classified as the constant speed motors.

Speed Control of Three-Phase Induction Motors

The speed of a three-phase induction motor is given by,

$$mathrm{mathit{N_{r}}:=:left ( 1-mathit{s} right )mathit{N_{s}}:cdot cdot cdot (1)}$$

Where,s is the slip and $mathit{N_{s}}$ is the synchronous speed in RPM.

$$mathrm{mathit{N_{s}}:=:frac{120mathit{f}}{mathit{P}}:cdot cdot cdot (2)}$$

From equations (1) & (2), it is clear that the speed of a three-phase induction motor can be varied by changing the following −

• Frequency (f) of AC supply,

• Number of stator poles (P), and

• Slip (s).

In practice, the change in supply frequency is generally not possible because the commercial electric supplies have a constant frequency. Hence, the speed of a three-phase induction motor can be changed either by changing the number of stator poles (P) or the slip (s). We shall now discuss the speed control of squirrel-cage and slip-ring induction motors.

Speed Control of Squirrel-Cage Induction Motors

The speed control of squirrel-cage induction motors is changed by changing the number of stator poles. By the pole changing method, there are only two or four speeds are possible.

In a two-speed induction motor, one stator winding is provided, which may be switched through a suitable control equipment to provide the two speeds. Where, one speed is half of the other. For example, the stator winding may be connected for either 4 or 8 stator poles, giving synchronous speeds of 1500 RPM and 750 RPM, when the motor is supplied from a source of 50 Hz AC supply.

In a four speed induction motor, two separate stator windings are provided each of which provides two speeds.

Following are the major disadvantages of pole changing method of speed control −

• This method cannot be used to obtain gradual continuous speed control.

• It makes the motor design and switching of the interconnection of stator windings more complicated.

• This method can provide a maximum of four different speeds for any one motor due to design and interconnection complications.

Speed Control of Slip-Ring Induction Motors

The speed of a slip-ring induction motor can be varied by changing the motor slip. The following methods are employed for changing the slip and hence the speed −

• By the changing the stator line voltage.

• By changing the resistance of the rotor circuit.

• By adding and changing a foreign voltage in the rotor circuit.

Numerical Example

For a three-phase induction motor, the no-load speed of the motor is 900 RPM and its full-load speed is 880 RPM. Find the speed regulation of the motor.

Solution

Given data,

• $mathit{N_{nl}}$ = 900 RPM

• $mathit{N_{fl}}$ = 880 RPM

$$mathrm{thereforemathrm{Speed:regulation}:=:mathit{frac{N_{nl}-N_{fl}}{N_{fl}}}times 100%}$$

$$mathrm{Rightarrow mathrm{Speed:regulation}:=:frac{900-880}{880}times 100%:=:2.273%}$$