Electrical Machines Test Questions - Set 4

1)   A series wound motor is also called as universal motor because

a. It will run equally well using either an ac or a dc voltage source
b. It will run well below and above rated speed
c. It can be used in all kinds of applications
d. None of these
Answer  Explanation 

ANSWER: It will run equally well using either an ac or a dc voltage source

Explanation:
No explanation is available for this question!


2)   In the flux control method for controlling the speed of d.c. shunt motor, the speed control is not possible

a. Below normal rated speed
b. Above normal rated speed
c. Both (a) and (b)
d. None of these
Answer  Explanation 

ANSWER: Below normal rated speed

Explanation:
No explanation is available for this question!


3)   In armature voltage control method or rheostatic control method for speed control in d.c. shunt motor, a variable resistance is

a. Added in series with the armature
b. Added in series with the field
c. Connected across armature
d. Both in series with armature and field
Answer  Explanation 

ANSWER: Added in series with the armature

Explanation:
No explanation is available for this question!


4)   Which method is suitable for the speed control, below and above the normal rated speed of d.c. shunt motor?

a. Flux control method
b. Rheostatic control method
c. Voltage control method
d. All of these
Answer  Explanation 

ANSWER: Voltage control method

Explanation:
No explanation is available for this question!


5)   If a resistance is added in series with the field winding of d.c. shunt motor, then its

a. Both speed and torque decreases
b. Both speed and torque increases
c. Speed decreases, torque increases
d. Speed increases, torque decreases
Answer  Explanation 

ANSWER: Speed increases, torque decreases

Explanation:
No explanation is available for this question!


6)   Ward-Leonard system is used for

a. Wide range of speed control
b. Very sensitive speed control
c. Both (a) & (b)
d. None of these
Answer  Explanation 

ANSWER: Both (a) & (b)

Explanation:
No explanation is available for this question!


7)   For frequent starting, stopping and reversals, which motor is commonly used?

a. Permanent d.c. motor
b. Ward-Leonard system
c. Brushless d.c. motor
d. All of these
Answer  Explanation 

ANSWER: Ward-Leonard system

Explanation:
No explanation is available for this question!


8)   The basic difference between three point starter and four point starter is

a. No volt coil is connected in series with the field winding in three point starter while connected independently across the supply in four point starte
b. No volt coil is connected across supply in three point starter while in series with field winding in four point starter.
c. No volt coil is connected across field winding in three point starter while connected across supply in four point starter.
d. None of these
Answer  Explanation 

ANSWER: No volt coil is connected in series with the field winding in three point starter while connected independently across the supply in four point starte

Explanation:
No explanation is available for this question!


9)   Which starter does not provide high speed protection to the d.c. shunt motor?

a. Three point starter
b. Four point starter
c. Two point starter
d. None of these
Answer  Explanation 

ANSWER: Four point starter

Explanation:
No explanation is available for this question!


10)   For the protection of d.c. series motor, which starter is commonly used?

a. Two point starter
b. Three point starter
c. Four point starter
d. None of these
Answer  Explanation 

ANSWER: Two point starter

Explanation:
No explanation is available for this question!


11)   The speed of permanent magnet d.c. motor cannot be controlled by

a. Flux control method
b. Rheostatic control method
c. Electronic circuits
d. None of these
Answer  Explanation 

ANSWER: Flux control method

Explanation:
No explanation is available for this question!


12)   The number of windings on the field of permanent magnet d.c. motor is

a. More compared to shunt motor
b. Less compared to shunt motor
c. Absent
d. None of these
Answer  Explanation 

ANSWER: Absent

Explanation:
No explanation is available for this question!


13)   Permanent magnet d.c. motors are extensively used in

a. Automobiles
b. Heaters
c. Air conditioners
d. All of these
Answer  Explanation 

ANSWER: All of these

Explanation:
No explanation is available for this question!


14)   In the biomedical instruments like artificial heart pumps, the commonly used motor is

a. Permanent magnet d.c. motor
b. Brushless d.c. motor
c. Ward-Leonard system
d. Series motor
Answer  Explanation 

ANSWER: Brushless d.c. motor

Explanation:
No explanation is available for this question!


15)   A machine operating as motor may go into regenerative braking mode if its speed becomes

a. Sufficiently high to make back emf greater than supply voltage
b. Sufficiently low to make back emf greater than supply voltage
c. Sufficiently high to make back emf half of the supply voltage
d. Sufficiently low to make back emf half of the supply voltage
Answer  Explanation 

ANSWER: Sufficiently high to make back emf greater than supply voltage

Explanation:
No explanation is available for this question!


16)   In a three phase induction motor, the resultant of three alternating fluxes, separated from each other by 120 degree has constant amplitude

a. Equal to the maximum amplitude of an individual flux
b. Twice of the maximum amplitude of an individual flux
c. 1.5 times of the maximum amplitude of an individual flux
d. All of these
Answer  Explanation 

ANSWER: 1.5 times of the maximum amplitude of an individual flux

Explanation:
No explanation is available for this question!


17)   If the frequency of a three phase induction motor is doubled and number of poles is halved, then its synchronous speed is given by

a. Remains same
b. Becomes double of the original synchronous speed
c. Becomes four times of the original synchronous speed
d. Becomes half of the original synchronous speed
Answer  Explanation 

ANSWER: Becomes four times of the original synchronous speed

Explanation:
No explanation is available for this question!


18)   A three phase 4 pole, induction motor is supplied from 50 Hz supply. Then its synchronous speed will be

a. 1000 rpm
b. 1500 rpm
c. 1600 rpm
d. 2000 rpm
Answer  Explanation 

ANSWER: 1500 rpm

Explanation:
No explanation is available for this question!


19)   In a three phase induction motor, the direction of rotation of rotating magnetic field can be reversed by

a. Interchanging any two terminals of three phase winding while connecting it to ac supply
b. by reversing the direction of rotating magnetic field
c. Cannot be changed
d. None of these
Answer  Explanation 

ANSWER: Interchanging any two terminals of three phase winding while connecting it to ac supply

Explanation:
No explanation is available for this question!


20)   For connecting the rotating member of machine to the external stationary circuit,

a. Slip rings and brush assembly is used
b. Only slip ring is used
c. Only brushes are used
d. Directly connected to the external stationary circuit
Answer  Explanation 

ANSWER: Slip rings and brush assembly is used

Explanation:
No explanation is available for this question!


21)   If the air gap of the induction motor increases then its power factor

a. Increase
b. Decreases
c. Remains same
d. Either increases or decreases
Answer  Explanation 

ANSWER: Decreases

Explanation:
No explanation is available for this question!


22)   When the air gap is smaller in induction motor then its

a. magnetizing current reduces
b. starting torque increases
c. both a and b
d. neither a nor b
Answer  Explanation 

ANSWER: magnetizing current reduces

Explanation:
No explanation is available for this question!


23)   In squirrel cage rotor, bars are usually brazed to the end rings. The end rings are used to

a. Prevent rotor leakage current
b. Give reduced starting torque
c. Provide good mechanical strength
d. All of these
Answer  Explanation 

ANSWER: Provide good mechanical strength

Explanation:
No explanation is available for this question!


24)   Slots of squirrel cage rotor are generally skewed to

a. Reduce magnetic hum
b. Reduce magnetic locking
c. Make motor operation smooth
d. All of these
Answer  Explanation 

ANSWER: All of these

Explanation:
No explanation is available for this question!


25)   The wound rotor induction motor gives

a. high stating torque
b. moderate starting torque
c. reduced starting torque
d. all of these
Answer  Explanation 

ANSWER: high stating torque

Explanation:
No explanation is available for this question!


26)   The slip ring or wound rotor induction motor find its applications in

a. lifts, cranes
b. lathes
c. drilling machines
d. printing machines
Answer  Explanation 

ANSWER: lifts, cranes

Explanation:
No explanation is available for this question!


27)   For the speed control of squirrel cage induction motor, the most suitable method is

a. Rotor resistance control
b. Rotor reactance control
c. Stator resistance control
d. Stator reactance control
Answer  Explanation 

ANSWER: Rotor resistance control

Explanation:
No explanation is available for this question!


28)   “The direction of induced current in the rotor is so as to oppose the cause producing it”. This is given by

a. Faraday’s law
b. Lenz’s law
c. Electromagnetic law
d. Ampere’s law
Answer  Explanation 

ANSWER: Lenz’s law

Explanation:
No explanation is available for this question!


29)   If the rotor of the machine catches the speed of the rotating magnetic field, then the motor will

a. Rotate at the speed of rotating magnetic field
b. Rotate at double the speed of the rotating magnetic field
c. Eventually stop
d. None of these
Answer  Explanation 

ANSWER: Eventually stop

Explanation:
No explanation is available for this question!


30)   At start, the slip of the induction motor is

a. 1
b. 0
c. 0.5
d. None of these
Answer  Explanation 

ANSWER: 1

Explanation:
No explanation is available for this question!