An induction motor is basically a constant speed motor, that means, for the entire loading range, change in speed of the motor is quite small. Speed of a DC shunt motor can be varied very easily with good efficiency, but in case of Induction motors, speed reduction is accompanied by a corresponding loss of efficiency and poor power factor. As induction motors are widely being used, their speed control may be required in many applications.
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Following are the May two type of controlling arrangement which need to consider by controlling Induction motors.
Speed control with respect to stator side of the induction motor.
Speed control with respect to rotor side of the induction motor.
The speed control of induction motor by considering to control on stator side can also subdivide into several speed controlling methods such as.
1. By changing the supply voltage of the motor.
2. Changing the stator pole numbers of the motor.
3. Changing the frequency of the supply side of the motor.
More about learn Speed Control of DC Motor.
1. By Changing The Supply Voltage Of The Motor:-
As can be seen from Equation, the torque developed by an induction motor varies as square of the voltage applied to its stator terminals. Thus by varying the applied voltage, the electromagnetic torque developed by the motor can be varied. This method is generally used for small squirrel-cage motors where cost is an important criterion and efficiency is not. However, this method has rather limited range of speed control.
2. Changing The Stator Pole Numbers Of The Motor:-
The primary factor in determining the speed of an induction motor is the number of poles, given by the below formula;
Ns = Synchronous speed, in revolutions per minute.
f = AC power frequency.
p = Number of poles per phase winding.
From the above formula, Pole changing in induction machine can be done using a Dahlander motor also known as a pole changing motor. Pole changing can be used to achieve different speeds in induction machine by switching the configuration of the electrical stator windings in the ratio of 2:1, indirectly adding or removing poles and thus varying the rotor speed. The number of stator poles can be changed by, multiple stator winding, method of consequent poles, pole amplitude modulation.
3. Changing The Frequency Of the Supply Side Of The Motor:-
By changing the supply frequency, the motor synchronous speed can be altered and thus the torque-speed of a three- phase induction motor can be controlled. The synchronous speed of the motor at rated conditions is known as the base speed. By using variable frequency control, it is possible to adjust the speed of the motor either above or below the base speed. Increase in frequency increases the torque-speed relation and a decrease in frequency decreases the torque-speed relation of the motor.
The speed control of induction motor by considering to control on rotor side can subdivide into several speed controlling methods such as-
1. Adding an external resistant into rotor side circuit.
2. Applying cascade connection.
3. Injecting emf into rotor side circuit of the motor.
This method is applicable only the wound rotor induction motors. Speed variation can be obtained by inserting external resistance in the rotor circuit. External resistances are placed in series with the rotor windings during starting to limit the starting current. Without the external resistances, the starting currents are many times the rated currents. Depending on the size of the machine, it can draw 300% to over 900% of the full-load current. Once the motor is started, the external resistance can be cut out to obtain high torque throughout the accelerating range.
In this cascade operation method of induction motor speed controlling there are two motors are mount on the same shaft of the motor and run at same speed.
One motor is connected to three-phase supply and other is work from the feed induced emf of the first motor through slip rings.By using this method different speed can apply for induction motor and speed can control.
3. Injecting EMF Into Rotor Side Circuit Of The Motor:-
In this induction, motor speed control method external voltages add into the rotor circuit of the motor and the frequency must be same as slip frequency. So injecting emf in opposite phase of the rotor increase the rotor resistance. By that, this method can use to speed controlling of the induction motor.