Electrical Machines Questions and Answers_pag339

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b) x^2 *Tfl 
c) 1/x *Tfl 
d) no change in the starting current 
View Answer 
Answer: a 
Explanation: Tst/Tfl = (x^2)(Ist/Ifl)^2*slip. 
15. For the application requiring high starting torque and minimum starting current, the most 
suitable starting method will be 
a) Auto transformer starting method 
b) Reactance starting method 
c) DOL method 
d) Star-Delta method 
View Answer 
Answer: a 
Explanation: Auto transformer provides the maximum starting torque. 
16. Auto transformer method of starting has most acceptable method for the starting of the 
induction motor due to 
a) maximum torque 
b) maximum reduction of inrush current 
c) variable tapping 
d) all of he mentioned 
View Answer 
Answer: a 
Explanation: Maximum torque, reduction of inrush current and variable turns ratio makes it 
popular method of starting of IM. 
This set of Electrical Machines test focuses on “Starting of Polyphase Induction Motors-II”. 
1. A 3-phase induction motor is fed at the stator at a frequency f. If the rotor speed is Nr and 
synchronous speed is Ns, 
then the absolute speed of the resultant flux in space is 
a) Ns 
b) (Ns+Nr) if the flux rotates in the same same direction as the rotor 
c) (Ns-Nr) if the flux rotates in the same same direction as the rotor 
d) Nr 
View Answer 
Answer: a 
Explanation: The flux speed in space will be Ns irrespective of any rotor speed. 
2. I. Leakage reactance is dependent on supply current. 
II. Magnetizing reactance is dependent on the air gap flux. 
Which of the following is correct? 
a) I,II 
b) II 
c) I 
d) None of the mentioned