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Electric drives concepts and Applications – Vedam Subrahmanyam Multiple choice questions 1.1 An unsaturated dc shunt mot

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Electric drives concepts and Applications – Vedam Subrahmanyam Multiple choice questions 1.1 An unsaturated dc shunt motor runs at its rated speed when rated voltage is applied to it. If the supply voltage to the motor is reduced by 25 % the speed of the motor a) Increases by 25% b) Remains the same c) Decreases by 25% d) Increases only slightly by an amount less than 25% 1.2 Speed control of dc shunt motors by means of field weakening is suitable for a) Constant power operation b) Constant torque operation c) Constant torque and Constant power operation d) Variable torque and variable power operation 1.3 The polarity of the supply voltage to a dc motor running at rated speed is reversed. Then a) the motor reverses its direction of rotation b) the motor comes to a dead stop c) the motor speed increases slightly d) the motor continues to run in the same direction 1.4 The series field of a cumulatively compounded motor is short circuited when it runs at rated speed. The speed of the motor a) remains the same b) increases c) decreases d) becomes zero 1.5 In an unsaturated dc series motor a) torque is proportional to armature current b) torque is proportional to field current c) torque is proportional to square root of the armature current d) torque is proportional to the square of the armature current 1.6 The staring torque of a three – phase induction motor a) Increases with rotor resistance b) Decreases with rotor resistance c) Increases with rotor resistance up to a certain value and decreases as the rotor resistance increases further d) Does not depend on the rotor resistance 1.7 In a three phase induction motor the maximum torque is a) Directly proportional to voltage b) Inversely proportional to the standstill rotor reactance

c) Directly proportional to the standstill rotor reactance d) Inversely proportional to the rotor resistance 1.8 A three phase induction motor is started by means of a star/delta switch. The starting current is a) Three times the current with DOL b) 1/3 times the current with DOL c) √ 3 times the current with DOL d) 1/ √3 times the current with DOL 1.9. A three phase induction motor has negligible stator impedance. When the motor has constant torque load, its speed is controlled by rotor resistance. The slip of the motor a) varies directly with the rotor resistance b) varies inversely with rotor resistance c) does not depend on the rotor resistance d) varies directly with rotor standstill reactance

1.10. A variable speed three – phase induction motor drive obtained by varying the applied voltage is suitable for a) driving constant torque loads b) driving fan type loads where T ∝ N2 c) driving loads where T ∝ 1/N2 d) driving all types of loads

1.11. An ideal synchronous motor has no starting torque because a) the rotor is made up of salient poles b) the relative velocity between the stator and rotor mmfs is zero c) the relative velocity between the stator and rotor mmfs is not zero

d) the rotor contains dc winding

1.12. The locus of constant power factor points on a set of V- curves is called a) power circle b) excitation circle c) compounding curves d) open circuit characteristics

2.1 A motor driving a passive load is said to be steady- state stable if a)

dT 1 dT M − =0 dω dω

b)

dT 1 dT M − 0 dω dω

d) All 2.2 Equal area criterion gives information regarding the a) transient stability of a synchronous motor b) steady state stability of a synchronous motor c) steady state stability of an induction motor d) transient stability of an induction motor e) 2.3 From the characteristics of load and motor torques given below, a typical example of unstable system is a)

b)

c)

d)

2.4 In the drive system all the mechanical quantities are referred to a single rotation the principal of a) power invariance b) torque invariance c) conservation of momentum

shaft using

d) All a,b,c 2.5 Retardation test is employed a) to determine the losses of the motor b) to determine the moment of inertia of the rotating parts c) to determine the speed torque curve of the motor d) to determine the overload capacity of the motor 2.6 Active loads a) have the capacity to accelerate as well as decelerate the drive motor b) have the capacity only to oppose the motion trying to retard the motor c) have the capacity to provide accelerating torque only d) all the above 2.7 A typical active load is a) Hoist b) Blower c) Pump d) Lathe

3.1. A fully controlled line commutated converter operates as an inverter a) In the range of firing angles 0 ≤ α ≤ 90o b) In the range of firing angles 90o ≤ α ≤ 180o c) In the range of firing angles 90o ≤ α ≤ 180o d) When it supplies RLE load

with a suitable dc source in the load

3.2 . The inverter limit of a line commutated inverter is due to a) Overlap alone b) turn off time of the thyristor c) turn on time of the thyristor d) Due to both overlap of the converter and turnoff time of the thyristor.

3.3 The overlap introduces a) Additional reactive power requirement b) Additional losses in the load c) Additional harmonics in the load current d) A heavy short circuit current 3.4. In the thyristor power converters during discontinuous conduction a) The load current is zero even though the load voltage is present b) Load current and load voltage are both simultaneously zero c) The load current is present even though the load voltage is zero d) The current is ripple free. 3.5. A free wheeling diode in a phase controlled rectifier a) Enables the inverter operation b) Smoothens the load current consequently the smoothing inductance required is small c) Makes the converter draw additional reactive power d) Impairs the line power factor 3.6. A phase controlled converter is designed for 220 V, 50 Hz supply .If this converter operates on 170 V, and 50 Hz supply there is a possibility for a) Commutation failure during rectification b) Commutation failure during inversion c) Increased voltage drop d) Increased losses in the inverter 3.7. In a 12 – pulse bridge converter with freewheeling diode, the width of the diode current pulse is a) π b) α c) π −α d) π + α 3.8. Midpoint converters in general require input transformers. The input transformer is of special construction because a) Harmonic currents are present in the load b) DC magnetization of the core c) To give strength to the system d) To improve the converter behavior 3.9. A voltage source inverter supplying an inductive load requires a gate pulse of long duration to turn the thyristor ON a) To ensure the initiation of conduction of the thyristor immediately when its voltage is positive b) because a wider pulse is required invariably to urn on the thyristor c) To reduce the thyristor losses d) Because a wider pulse effectively provide an output with least harmonics

Content 3.10 The phase control employed for a cycloconverter imparts a) A very good power factor b) A very bad power factor c) Undesirable harmonic behavior d) Poor voltage regulation 3.11. The conduction angle of an ac voltage controller depends on a) The variation in the input voltage b) The impedance angle of the load c) The supply frequency d) Both supply frequency and voltage 3.12. A single phase ac voltage controller feeds a pure inductive load. Its control angle range a) 0 ≤ α ≤180 o b) 0 ≤ α ≤ 90o c) 90o ≤ α ≤ 180o d) 180o ≤ α ≤270 o 3.13. In a two quadrant chopper the load voltage is varied from positive maximum to negative maximum by varying a) Time ratio of the chopper from 0 to 1. b) Time ratio of the chopper from 1 to 0. c) Time ratio of the chopper from 0 to 0.5. d) Time ratio of the chopper from 0.5 to 0 3.14. A dual converter with circulating current mode a) has no possibility of discontinuous conduction b) Requires a most sophisticated control c) has a very sluggish behavior d) Offers difficulties during transition from motor to generators.

4.1. A three phase ac voltage controller feeding a three phase induction motor has an output of a) b) c) d)

Constant voltage of variable frequency Variable voltage of variable frequency Variable voltage of Constant frequency Constant voltage of Constant frequency

4.2. The method of speed control using a three phase voltage controller is suitable for loads a) b) c) d)

whose torque is proportional to speed having constant torque whose torque is proportional to square root of speed having torque proportional to square of speed

4.3. A three phase ac voltage controller feeds an induction motor. The motor has a) b) c) d)

Very good efficiency and power factor at all speeds. Very good efficiency but poor power factor at all speeds. poor efficiency but good power factor at all speeds poor efficiency and poor power factor at all speeds

4.4. A three phase induction motor having speed control using chopper controlled resistance is characterized by a) b) c) d)

poor power factor and good efficiency poor efficiency and poor power factor good efficiency and poor power factor good power factor and poor efficiency

4.5. A three phase induction motor with chopper controlled resistance has its torque proportional to a) b) c) d)

rotor current square of rotor current stator resistance square root of rotor current

4.6. A three phase induction motor having a combination of diode rectifier and line commutated inverter in the rotor circuit can give a) b) c) d)

speeds below synchronous speed only speeds above synchronous speed only both sub and super synchronous speeds speeds varying 0 to 50% of synchronous speeds

4.7. The three phase induction motor having sub synchronous converter cascade in the rotor circuit is characterized by a) b) c) d)

good power factor and poor efficiency good power factor and good efficiency poor power factor and poor efficiency poor power factor and good efficiency

4.8. The motor having slip energy recovery scheme can be braked by means of a) b) c) d)

regenerative breaking plugging dc dynamic breaking all the methods of a,b,c

4.9. The speed of an induction motor can be varied by means of variable frequency supply from a static power converter. A simultaneous voltage variation is also effected in order to a) avoid saturation and provide optimum torque capability b) the torque pulsations decrease if supplied from variable voltage supply. c) to limit the peak value of stator current d) to minimize the additional losses. 4.10. A three phase induction motor is supplied from a variable voltage variable frequency supply such that air gap flux is constant (E/f is constant). The armature current of the motor now a) is independent of stator frequency but depends on rotor frequency b) is dependent on stator frequency but does not depend on rotor frequency c) is depend on both stator and rotor frequency d) is independent on both stator and rotor frequency 4.11. A current source inverter fed induction motor is inherently unstable when ts operates on

a) open loop b) closed loop c) a variable frequency supply keeping air gap flux constant d) a variable frequency supply keeping stator flux constant 4.12. A three phase induction motor operates at constant rotor frequency when the stator frequency is varied from zero to rated value. The torque developed by the motor is a) constant from zero to rated speed b) proportional to speed c) proportional to square of the speed d) inversely proportional to speed 4.13. A variable frequency variable voltage induction motor a) can be accelerated at constant torque or constant current b) suffers from poor starting characteristics as in the case of mains fed motor c) has only stepped variation of speed d) suffers from stability considerations 4.14. When operated with variable frequency a synchronous motor has an advantage over an induction motor a) that is free from torque oscillations b) that it has very good efficiency c) that the line power factor can be improved by varying excitation d) that in certain cases the inverter can be simpler configuration due to the possible load commutation. 4.15. In a three phase synchronous motor operating on variable frequency supply the armature resistance is neglected. The voltage to the motor is varied keeping V/f constant and If constant. The stator current would now a) increase as the frequency increases b) decreases as the frequency increases c) increase with frequency up to certain value of frequency and decreases as the frequency is increases beyond this value. d) is constant as long as the load torque is constant. 4.16 By self control of a synchronous motor we mean that a) elimination of torque ripple b) the speed of the motor is varied insteps c) the speed of the motor is a function on input frequency d) the input frequency is controlled from the speed of the motor. 4.17. The disadvantage of load commutation is a) harmonics torques b) loss of efficiency due to losses c) the speed from 0 to 10% of base speed are not possible d) the speed control range is limited to 0 – 10% of base speed.

4.18. The operation of ac motors fed from Current source inverters characterized by a) b) c) d)

Sinusoidal line voltage Peaky armature voltage Good harmonic torques Sinusoidal armature voltage with spikes

4.19. While operating on a phase controlled converter the commutation capability of the motor deteriorates because of a) increase in RMS current b) harmonic content of armature current c) peak value of armature current d) peak value of armature current , harmonic content of armature current and possible discontinuous conduction 4.20. When only one quadrant operation is required the converter normally preferred is a) fully controlled converter b) fully controlled converter with FWD c) half controlled converter d) sequence control of two series connected converters.

4.21. A separately excited DC motor designed for operation on phase controlled converter has laminated interpoles a) to improve the commutation of the motor b) to facilitate easy construction c) to decrease the losses in the interpole d) to improve the power factor

4.22. For low speed high power reversible drives a) Cycloconverter fed ac drives are suitable. b) Current source inverter fed ac drives are suitable c) Voltage source inverter fed ac drives are suitable d) Induction motor fed from ac voltage controller are suitable

5.1 The heating time constant of an electrical machine gives an indication of its a) cooling b) rating c) overload capacity d) short time rating 5.2 Short time rating of an electrical machine a) is equal to name plate rating b) is less than the name plate rating c) is greater than the name plate rating d) has no bearing to its name plate rating 5.3 All the physical dimensions of two electric machines are in the ratio K. The iron losses of the machine are in the ratio (assumimg constant flux density in both the cases ) a) K b) K2 c) K3 d) K4 5.4 Class B insulation can withstand a maximum temperature of a) 145oC b)105oC c) 135oC d) 120oC 5.5 The rating of a motor for a given industrial load cycle should have a) b) c) d)

Sufficient thermal capacity sufficient overload capacity both of the above sufficient starting torue.

5.6 A machine driving pulsed torque load is equipped with flywheel in order to a) equalize the current demand during the operation b) equalize the torque requirement c) reduce the mechanical overload d) make the motor thermally suitable to drive the load.

5.7 Two motors of the same name plate details have different thermal time constants. a) the short time ratings of the two motors are the same. b) the short time rating of the motor with large time constant is large c) the short time rating of the motor with large time constant is small d) overload capacity of the motor with large time constant is large.

6.1 A system has a transfer function given by G(s) = 1/(s 2+3s+6). When it is excited by a step input of 4u(t), the steady-state output of the system is a) 0.5 b) 2/3 c) 1

d) 3/2 6.2 The response of a system in s – domain to a uint impulse input gives a) gain of the system b) transfer function of the system c) time response of the system d) error under steady – state conditions 6.3 A system has a transfer function given by G(s) = 25/(s 2 + 6s +9). When it is excited by a step input, it reaches its final steady-state response a) immediately after the input is applied b) after going through a set of oscillations c) after a very long time without going through any oscillations d) after a very short interval of time however without oscillations 6.4 The imaginary part of the conjugate poles of a second order system represents a) natural frequency of the system b) overshoot of the system c) damped frequency of oscillation d) damping factor of the system 6.5 The transfer function having least steady-state error is a) 5/(s2 +3s + 5) b) 10/(s2 + 3s + 10) c) 25/ (s2 +3s + 25) d) 15/(s2 +3s +15) 6.6 Integral control of a system a) improves the transient response of the system b) improves the steady-state stability of the system c) increases the order of the system d) decreases the order of the system 6.7 The derivative action of a PID controller a) increases the rise time b) decreases the rise time c) decreases the steady-state error d) has no effect on the rise time 6.8 Routh’s stability criterion gives a) the degree of stability of a given system b) the indication of stability condition of a linear system without actually finding the roots of the characteristic equation c) the indication to improve the stability of a system d) The indication of stability condition only if the roots of the characteristic equation are known.

6.9 Bode plots of a certain transfer function show that gain cross – over frequency is greater than phase cross –over frequency. The system is a) stable b) unstable c) marginally stable d) the information is not sufficient to indicate stability condition 6.10 A third order system is compensated by a PI controller. The ratio of the controller time constant to the uncompensated time constant of the system is K 0. When the value of K0 is increased. a) phase margin of the overall system increases b) phase margin of the overall system decreases c) phase margin remains the same d) the damping of the system is unaffected

7.1 A spinning motor in a textile mill should have a) a very good torque capability b) moderate starting torque and uniform acceleration c) high starting torque and peak torques d) a very good starting torque. 7.2 To have uniform acceleration the motor should have a) Flat speed torque curve b) A speed torque curve with a small difference between peak torque and starting torque c) Very high starting torque d) Very high peak torque and low starting torque. 7.3 DC drives in the steel rolling mills are slowly being replaced by

a) b) c) d)

Variable frequency induction motors with torque and speed control Variable frequency synchronous motor fed from a load commutated CSI. Variable frequency ac motors with regeneration capabilities Variable speed induction motor with chopper controlled resistance in the rotor circuit.

7.4 A drive motor for continuous mills a) Must be capable of two quadrant operation b) Must be capable of four quadrant operation c) Operates only in one direction, reverse rotation is not necessary d) Must be a very high speed motor 7.5 For applications in cranes a) differentially compounded motors are suitable b) cumulatively compounded motors are suitable c) dc shunt motors are suitable d) dc series motors are suitable. 7.6 A kiln motor in a cement mill a) Must be a constant speed motor b) Must be able to have creeping speeds c) Need not have very large overload capacity d) Has very low power requirement 7.7 For fans and blowers in cement mills a) Variable frequency induction motor may be employed b) Three phase induction motor with chopper controlled resistance is suitable c) DC motors with field control are suitable d) The three phase slip ring induction motors with slip energy recovery schemes are very well employed with advantages. 7.8 To drive a centrifuge in sugar mills the most suited system is a) A solid rotor induction motor with stator voltage control b) A normal commercially available induction motor fed from a voltage controller c) A slip ring induction motor with slip energy recovery scheme d) A slip ring induction motor with rotor resistance control. 7.9 When an oversized motor is used the energy saving can be accomplished on light load portion duty cycles when a) the motor is controlled from a variable frequency variable voltage converters b) the motor is controlled from a constant frequency variable voltage converter such as three phase voltage controller c) the motor is fed from conventional speed control systems d) the motor is controlled on the rotor side. 7.10 For a mine winder the suitable choice is a) An induction motor with stator voltage variation

b) A self controlled CSI fed synchronous motor c) A dc motor with Ward Leonard control d) A squirrel cage motor with ac voltage controller 7.10 a) b) c) d)

For centrifugal pumps the most suitable drive system is Wound rotor induction motor with slip energy recovery schemes Wound rotor induction motor with rotor resistance control using a chopper Vector controlled three phase induction motor Self controlled CSI fed synchronous motor.

8.1 The centre of operations and control in a microcomputer is a) Memories b) Microprocessor unit (MPU) c) Address decoders d) interfaces

8.2 The OR operation in a microprocessor is classified as a) Arithmetic instruction b) data transfer instruction c) logical instruction d) decision making instruction

8.3 A subroutine in a microcomputer program is

a) a set of branching instructions b) a set of logical instructions c) a set of transfer operations d) a special group of instructions that perform a commonly used specific task in a program

8.4 The CPU of a microcomputer typically contains a variety of storage devices called a) ROM b) RAM c) instructions decoders d) registers

8.5 A microprocessor responds basically to a listing of operations called a) high level language b) machine language c) assembly language d) to all the languages above

8.6 An assembler a) is a special computer program for translating from assembly language to machine language b) is a special computer program for translating from high – level language to machine language c) a uint used to connect several parts of Microprocessor. d) is a computer program which translates machine language to assembly language.

8.7 Interconnection of parts within a microprocessor based system is called a) interrupt

b) assembly c) interfacing d) programming

8.8 Keyboard is an a) output peripheral device b) input peripheral device c) output interface adapter d) input interface adapter

8.9 The 8085 microprocessor uses a a) +5 V power supply b) – 5 V power supply c) +10 V power supply d) +15 V power supply

8.10 A dual converter is operating in non circulating current mode. The microprocessor software must capable of a) detecting discontinuous conduction b) sensing zero crossing of current c) identifying actual zero crossing of the current and it should not confuse with the discontinuous conduction d) identifying the discontinuous conduction and it should not confuse with the natural zero of the current.