Q1) The method which can be used for the speed control of an induction motor from the stator side is _______.?
a) V/R control
b) V/F control
c) adding rheostats in rotor circuit
d) V/Z control
Solution
Speed Control Of Induction Motor.
Methods of speed control of an induction motor
The speed of the induction motor can be controlled by any of the following methods:
1.) Cascade control method
- In this method of speed control, two motors are used. Both are mounted on the same shaft so that both run at the same speed.
- One motor is fed from a 3-phase supply and the other motor is fed from the induced emf in the first motor via slip-rings.
2.) V/f control or frequency control
- This method is used to control the speed of the induction motor from the stator side.
- The basic idea of a V/f control is to maintain the stator flux constant. To operate the machine under nominal conditions, the stator flux must be nominal.
- This control method is commonly applied to blower fans and centrifugal pumps.
3.) Pole changing method
- Pole Changing Method is one of the main methods of speed control of an induction motor.
- This method of controlling the speed by pole changing is used mainly for the cage motor only because the cage rotor automatically develops many poles, which are equal to the poles of the stator winding.
4.) Supply voltage method
- The speed control of a three-phase induction motor is obtained by changing the supply voltage until the torque required by the load is developed at the desired speed.
- The torque developed by the induction motor is directly proportional to the square of the supply voltage and the current is proportional to the voltage.
- Therefore, the stator voltage control method is suitable for applications where the load torque decreases with the speed, as in the case of a fan load.
Q2) Which of the following statements related to the speed control of DC shunt and series motors is correct?
a) In series-parallel control, the motors are joined in parallel at lower speeds and series at higher speeds.
b) In the Rheostatic control methods of shunt motors, the series resistor must be connected between the line and the motor.
c) Field diverters and tapped field control methods are mostly used in DC shunt motors.
d) In a tapped field control when all the field turns are present in the circuit, the motor runs at the lowest speed.
Solution
Methods of speed control DC shunt and series motors
1.) DC shunt motor
- Flux control method
- Armature control method
- Voltage Control Method: a) Multiple voltage control & b) Ward-Leonard System
2.) DC series motor
- Series-parallel control
- Variable resistance in series with armature
- Flux control method: a) Field diverter b) Armature diverter c) Tapped field control
Explanation
- Series-Parallel Control: In series-parallel control, motors are usually joined in series for lower speeds and parallel for higher speeds, not vice versa. This is because, in series, the current through each motor is the same, resulting in lower speeds due to the combined voltage drop across both motors. Therefore, statement 1 is false.
- Rheostatic Control of Shunt Motors: In the rheostatic control method, the series resistor is connected in series with the armature, not between the line and the motor. It controls the speed by varying the voltage across the armature. Therefore, statement 2 is false.
- Field Diverters and Tapped Field Control: Field diverters and tapped field control methods are used, but they are more commonly associated with series motors for controlling the speed by varying the field strength. Therefore, statement 3 is false.
- Tapped Field Control: In tapped field control, the speed of the motor is inversely proportional to the field current. When all field turns are present, the field is strongest, which results in the lowest speed. Reducing the number of field turns decreases the field strength, increasing the speed. Therefore, statement 4 is true.
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