Questions and answers for exam Preparation

MCQs

Total Questions : 473 | Page 44 of 48 pages

Question 431. 7. If the load on a squirrel-cage induction motor, operating on a constant voltage, constant frequency ac supply is increased, then

power factor angle will increase and slip will decrease
power factor angle will decrease and slip will increase
both power factor angle and slip will decrease
both power factor angle and slip will increase

Discuss Question

Answer: Option B. -> power factor angle will decrease and slip will increase

Question 432. 3. A 3-phase induction motor draws 1000 kVA at a pf of 0.8 lag. A synchronous condenser is connected in parallel to draw an additional 750 kVA at 0.6 pf lead. The pf of the total load supplied by the mains is

unity
0.707 lag
0.707 lead
zero

Discuss Question

Answer: Option A. -> unity

Question 433. 1. A 3-phase induction motor operates on a variable frequency, variable voltage supply such that V/f is constant. The breakdown torque of the drive will

decrease with decrease in frequency
remain constant irrespective of the frequency
be slightly lower at very low frequencies
increase with decrease in frequency

Discuss Question

Answer: Option B. -> remain constant irrespective of the frequency

Question 434. 2. A 3-phase stator and rotor winding of a slip-ring induction motor is fed by 3-phase ac supply at a frequency f. If the rotor speed is Nr and synchronous speed is Ns, then the absolute speed of resultant flux in space is

N. irrespective of rotor speed Nr
(Ns + Nr) if the flux rotates in the same direction as the rotor
(Ns _Nr) if the flux rotates in the same direction as the rotor
Nr, irrespective of Ns

Discuss Question

Answer: Option B. -> (Ns + Nr) if the flux rotates in the same direction as the rotor

Question 435. 0. Consider the following steps 1CSE 19941 (I) Raversing connections to the terminals of the capacitor (2) Changing the position of the capacitor from auxiliary winding circuit to main winding circuit (3) Reversing supply connection to the main winding (4) Reversing supply connection to the auxiliary circuit While installing a new ceiling fan, if the fan motor is found to be rotating in the wrong direction, then the direction of rotation of the motor can be corrected by

1,2 and 3
1, 2 and 4
1,3 and 4
2, 3 and 4

Discuss Question

Answer: Option C. -> 1,3 and 4

Question 436. 4. An induction generator

takes reactive power from the mains and supplies active power to the mains
takes active power from the mains and gives reactive power to the mains
supplies both active and reactive powers to the mains
consumes both active and reactive powers from the mains

Discuss Question

Answer: Option B. -> takes active power from the mains and gives reactive power to the mains

Question 437. 8. A polyphase slip-ring induction machine has the stator and rotor windings wound for

the same number of phase but different number of pairs of poles
the same number of pairs of poles but different number of phases
the same number of phases with number of pairs of poles either equal or different
the same number of pairs of poles with number of phases either equal or different

Discuss Question

Answer: Option D. -> the same number of pairs of poles with number of phases either equal or different

Question 438. 5. The approximate value of efficiency of a three-phase induction motor running at a slip 's' is given by

1 + s
1 + s
1 ? s 1 + s
1 ? s

Discuss Question

Answer: Option C. -> 1 ? s 1 + s

Question 439. 7. A single-phase induction motor with only the main winding excited would produce zero torque at

synchronous speed only
starting and synchronous speed
starting only
starting and a speed slightly below synchronous speed

Discuss Question

Answer: Option B. -> starting and synchronous speed

Question 440. 6. Consider the following statements : The emf induced in the rotor of a 3-phase induction motor is proportional to the (I) relative velocity between stator field and rotor conductors. (2) voltage applied to the stator. (3) slip Of these statements :