Electrical Machines Questions and Answers_pag340

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View Answer 
Answer: a 
Explanation:Leakage flux depends on the load current and the magnetizing reactance depends 
on the air gap flux. 
3. I. Rotor resistance is dependent on the speed. 
II. Core loss does not depend on the supply frequency and voltage. 
a) I 
b) II 
c) I,II 
d) None of the mentioned 
View Answer 
Answer: a 
Explanation: Core loss does depend on the supply ‘V’ and ‘f’. 
4. The stator of a 3-phase, six pole induction motor’s stator is connected to 50 Hz supply while 
its rotor is fed from a 20 Hz supply. 
The possible speeds at which the machine will run will be 
(i) 1600rpm (ii) 600rpm (iii) 1400rpm (iv) 400 rpm 
a) (ii),(iii) 
b) (ii),(iv) 
c) (i),(iii) 
d) (i),(iv) 
View Answer 
Answer: a 
Explanation: Stator speed = 120*50/6 = 1000 rpm 
Rotor speed = 120*20/6 = 400 rpm 
 Hence the possible speeds could be 1400rpm or 600rpm. 
5. For a 3-phase 4-pole induction motor has open circuited slip rings. The frequency across the 
slip rings is 75 Hz. Then we can conclude that the motor is run at a speed of 
(i) 750rpm (ii) 1500rpm (iii) 2250rpm (iv) 3750 rpm 
a) (i),(iv) 
b) (i),(iii) 
c) (ii),(iii) 
d) (ii),(iv) 
View Answer 
Answer: a 
Explanation: Stator speed = 120*50/4 = 1500 rpm 
Rotor speed = 120*50/4 = 2250 rpm 
 Hence the possible speeds could be 750rpm or 3750rpm. 
6. A 3-phase slip ring IM is has its copper bars replaced by the aluminium bars in the machine. 
With the above change, 
a) starting torque increases, maximum torque remains same, smT increase 
b) starting torque increases, maximum torque remains same, smT remains same 
c) starting torque increases, maximum torque remains increases, smT increase