Speed Regulation and Speed Control

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.,

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

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,

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

Where,s is the spp 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

Spp (s).

In practice, the change in supply frequency is generally not possible because the commercial electric supppes 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 spp (s). We shall now discuss the speed control of squirrel-cage and spp-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 suppped 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 comppcated.

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

Speed Control of Spp-Ring Induction Motors

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

By the changing the stator pne 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

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

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