Electrical Drives Anna University Previous Year Question Papers with Answers: Set - 6
Anna University Previous Year Question Papers with Answers on Electrical Drives
 What is meant by V/F control?
When the frequency is reduced, the input voltage must be reduced proportionally so as to maintain constant flux.
Otherwise the core will get saturated resulting in excessive iron loss and magnetizing current.
This type of induction motor behaviour is similar to the working of dc series motors.
 What are the advantages of V/F control?
1. Smooth speed control
2. Small input current and improved power factor at low-frequency start
3. Higher starting torque for low case resistance
 What is meant by stator current control?
The three-phase induction motor speed can be controlled by stator current control.
The stator current can be varied by using current source inverter.
 What is meant by regenerative braking?
Regenerative braking occurs when the motor speed exceeds the synchronous speed.
In this case, the induction motor would run as the induction machine is converting the mechanical power into electrical power, which is delivered back to the electrical system.
This method of braking is known as regenerative braking.
 What is meant by dynamic braking?
Dynamic braking of electric motors occurs when the energy stored in the rotating mass is dissipated in an electrical resistance.
This requires the motor to operate as a generator to convert this stored energy into electrical.
 What is meant by plugging?
It is one method of braking of induction motor.
When phase sequence of supply of the motor running at a speed is reversed, by interchanging connections of any two phases of stator with respect to supply terminals, operations shifts from motoring to plugging region.
 What is meant by rotor resistance control?
In a slip ring induction motor, a variable resistances are connected in the rotor circuit.
By controlling the rotor resistance, the motor speed can be controlled as well as increase the starting torque and reduce the starting current.
This method of speed control is called rotor resistance control method.
 What are the advantages and disadvantages of rotor resistance control?
- Absence of in-rush starting current
- Availability of full rated torque at starting
- High line power factor
- Absence of line current harmonics
- Smooth and wide range of speed control
- Reduced efficiency because of the slip energy is wasted in the rotor circuit resistance
- Speed changes very widely with load variation
- Unbalance in voltages and current of rotor circuit resistances are not equal.
 What is meant by slip power?
The portion of air gap power, which is not converted into mechanical power, is called slip power.
Slip power is nothing but multiplication of slip(s) and airgap power (PA-G)
Slip power = s. PA-G
 What are the advantages of slip power recovery system?
In chopper method of speed control for SRM, the slip power is wasted in the external resistance and the efficiency also reduced.
However, instead of wasting the slip power can be recovered by various schemes for the speed control of slip ring induction motor.
This system is called as slip power recovery system.
The slip power can be recovered and fed back to the supply. So the overall efficiency also improved.
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