Important Short Questions and Answers: Design of Controllers For Drives

1.  What is meant by frequency control of IM?

The speed of IM can be controlled by changing the supply freq because the speed is directly proportional to supply frequency. This method of speed ctrl is called freq control.

2.  What is meant by V/F control l?

When the freq is reduced the i/p 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 IM behavior is similar to the working of dc series motor.

3.    What are the advantages of V/F control?

·        Smooth speed ctrl

·        Small i/p current and improved power factor at low freq. start

·        Higher starting torque for low case resistance

3.  What is meant by stator current control?

The 3 phase IM speed can be controlled by stator current control. The stator current can be varied by using current source inverter.

5.    What are the 3 modes of region in the adjustable-freq IM drives characteristics?

·        Constant torque region

·        Constant power region

·        High speed series motoring region

6.    What are the two modes of operation in the motor?

The two modes of operation in the motor are, motoring and braking. In motoring, it converts electrical energy to mechanical energy, which supports its motion. In braking, it works as a generator converting mechanical energy to electrical energy and thus opposes the motion.

7.  How will you select the motor rating for a specific application?

When operating for a specific application motor rating should be carefully chosen that the insulation temperature never exceed the prescribed limit. Otherwise either it will lead to its immediate thermal breakdown causing short circuit and damage to winding, or it will lead to deterioration of its quality resulting into thermal breakdown in near future.

8. What is braking? Mention its types.

The motor works as a generator developing a negative torque which opposes the motion is called barking.

It is of three types. They are,

a.     Regenerative braking.

b.     Dynamic or rheostat braking.

c.      Plugging or reverse voltage braking.

9.    What are the three types of speed control?

The three types of speed control as,

a.     Armature voltage control

b.     Field flux control

c.      Armature resistance control.

10.    What are the advantages of armature voltage control?

The advantages of armature voltage control are,

a.     High efficiency

b.     Good transient response

c.      Good speed regulation.

11.      What are the methods involved in armature voltage control? When the supply in A.C.

a. Ward-Leonard schemes

b. Transformer with taps and an uncontrolled rectifier bridge.

c.  Static ward Leonard scheme or controlled rectifiers when the supply in D.C.

d. Chopper control.

12.            Give some drawbacks and uses of Ward-Leonard drive.

The drawbacks of Ward . Leonard drive are.

a.    High initial cost b. Low efficiency

The Ward-Leonard drive is used in rolling mills, mine winders, paper mills, elevators, machine tools etc.

13.            Give some advantages of Ward-Leonard drive.

The advantages of Ward-Leonard drive are,

a.       Inherent regenerative barking capability

b.       Power factor improvement.

14. What is the use of controlled rectifiers?

Controlled rectifiers are used to get variable D.C. Voltage form an A.C. Source of fixed voltage.

15. What is known as half-controlled rectifier and fully controlled rectifier?

The rectifiers provide control of D.C. voltage in either direction and therefore, allow motor control in quadrants I and IV. They are known as fully-controlled rectifiers.

The rectifiers allow D.C. Voltage control only in one direction and motor control in quadrant I only. They are known as half-controlled rectifiers.

16. What is called continuous and discontinuous conduction?

A D.C. motor is fed from a phase controlled converter the current in the armature may flow in discrete pulses in called continuous conduction.

A D.C. motor is fed from a phase controlled converter the current in the armature may flow continuously with an average value superimposed on by a ripple is called discontinuous conduction.

17. What are the three intervals present in discontinuous conduction mode of single phase half and fully controlled rectifier?

The three intervals present in half controlled rectifier are,

a.                 Duty interval

b. Free, wheeling interval c. Zero current intervals.

The two intervals present in fully controlled rectifier are

a.                 Duty interval

b.Zero current intervals.

18. What is called inversion?

Rectifier takes power from D.C. terminals and transfers it to A.C. mains is called inversion.

19. What are the limitations of series motor? Why series motor is not used in traction applications now a days?

1. The field of series cannot be easily controlled. If field control is not employed, the series motor must be designed with its base speed equal to the highest desired speed of the drive.

2. Further, there are a number of problems with regenerative braking of a series motor. Because of the limitations of series motors, separately excited motors are now preferred even for traction applications.

20. What are the advantages of induction motors over D.C. motors?

The main drawback of D.C. motors is the presence of commutate and brushes, which require frequent maintenance and make them unsuitable for explosive and dirty environments. On the other hand, induction motors, particularly squirrel-cage are rugged, cheaper, lighter, smaller, more efficient, require lower maintenance and can operate in dirty and explosive environments.

21. Give the applications of induction motors drives.

Although variable speed induction motor drives are generally expensive than D.C. drives, they are used in a number of applications such as fans, blowers, mill run-out tables, cranes, conveyors, traction etc., because of the advantages of induction motors. Other applications involved are underground and underwater installations, and explosive and dirty environments.

22. How is the speed controlled in induction motor?

The induction motor speed can be controlled by supplying the stator a variable voltage, variable frequency supply using static frequency converters. Speed control is also possible by feeding the slip power to the supply system using converters in the rotor circuit, basically one distinguishes two different methods of speed control.

a.     Speed control by varying the slip frequency when the stator is fed from a constant voltage, constant frequency mains.

b.    Speed control of the motor using a variable frequency variable voltage motor operating a constant rotor frequency.

23.            How is the speed control by variation of slip frequency obtained?

Speed control by variation of slip frequency is obtained by the following ways.

a.Stator voltage control using a three-phase voltage controller.

b.Rotor resistance control using a chopper controlled resistance in the rotor circuit.

c. Using a converter cascade in the rotor circuit to recover slip energy.

d.Using a cyclconverter in the rotor circuit.

24. Mention the effects of variable voltage supply in a cage induction motor.

When a cage induction motor is fed from a variable voltage for speed control the following observations may be made.

a.The torque curve beyond the maximum torque point has a negative shape. A stable operating point in this region is not possible for constant torque load.

b.The voltage controlled must be capable of withstanding high starting currents. The range of speed control is rather limited.

c.                        The motor power factor is poor.

25.            Classify the type of loads driven by the motor.

The type of load driven by the motor influences the current drawn and losses of the motor as the slip various. The normally occurring loads are

a.                 Constant torque loads.

b.                 Torque varying proportional to speed.

c.                  Torque varying preoperational to the square of the speed.

26. What are the disadvantages of constant torque loads?

The constant torque loads are not favored due to increase in the losses linearly with slip and becoming maximum at s= 1.0. This is obvious form the variation of flux as the voltage is varied for speed control. To maintain constant torque the motor draws heavy current resulting in poor torque/ampere, poor efficiency ad poor power factor at low speeds.

27. In which cases, torque versus speed method is suitable.

Torque versus speed method is suitable only for the following cases.

a.For short time operations where the duration of speed controls ids defined.

b.For speed control of blowers or pumps having parabolic or cubic variations of torque with speed. This is not suitable for constant torque loads due to increases and heating.

28. How is the speed of a squirrel cage induction motor controlled?

The speed of a squirrel cage induction motor can be controlled very effectively by varying the stator frequency. Further the operation of the motor is economical and efficient, if it operates at very small slips. The speed of the motor is therefore, varied by varying the supply frequency and maintaining the rotor frequency at the rated value or a value corresponding to the required torque on the linear portion of the torque-speed curve.

29. Why the control of a three-phase indication motor is more difficult than D.C. motors.

The control of a three-phase induction motor, particularly when the dynamic performance involved is more difficult than D.C. motors. This is due to

a. Relatively large internal resistance of the converter causes voltage fluctuations following load fluctuations because the capacitor cannot be ideally large.

b.In a D.C. motor there is a decoupling between the flux producing magnetizing current and torque producing armature current. They can be independently controlled. This is not the case with induction motors.

c.An induction motor is very poorly damped compared to a D.C. motor.

30. Where is the V/f control used?

The V/f control would be sufficient in some applications requiring variable torque, such as centrifugal pumps, compressors and fans. In these, the torque varies as the square of the speed. Therefore at small speeds the required torque is also small and V/f control would be sufficient to drive these leads with no compensation required for resistance drop. This is true also for the case of the liquid being pumped with minimal solids.