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Posted By: Sree.... Member Level: Diamond Posted Date: 14 Mar 2008
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2007 Andhra Pradesh State Jawaharlal Nehru Technological University B.Tech ELECTRO MECHANICS-II(RR) Set No. 2 Question paper
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Code No: RR220205 Set No. 2
II B.Tech II Semester Supplimentary Examinations, Apr/May 2007
ELECTRO MECHANICS-II
(Electrical & Electronic Engineering)
Time: 3 hours Max Marks: 80
Answer any FIVE Questions
All Questions carry equal marks
? ? ? ? ?
1. (a) Explain the principle of operation of transformer. Deduce its equivalent cir-
cuit. [8]
(b) The equivalent circuit of a single phase transformer is shown. Figure relates
to primary side. The ratio of secondary to primary turns is 10 and load is
inductive. (figure 1) [8]
Find
i. Secondary terminal voltage
ii. Primary current
iii. Efficiency
Figure 1
2. (a) Derive the condition for zero voltage regulation. Also show that the magnitude
of maximum voltage regulation equals to per unit value of leakage impedance.
[4+4]
(b) A 20 KVA 2000/200 V single phase transformer has the following parameters.
H. V. Winding: R1 = 3
, X1 = 5.3
, L. V. Winding: R2 = 0.05
, X2 = 0.1
Find the voltage regulation at
i. At a power factor of 0.8 lagging
ii. UPF
iii. 0.707 power factor leading [3+2+3]
3. (a) Explain the procedure for conducting OC and SC tests with neat diagrams.
[4+4]
1 of 2
Code No: RR220205 Set No. 2
(b) A 20 KVA, 2500/250V, 50Hz, Single phase transformer gave the following test
results: OC test (LV side): 250V, 1.6A, 110W; SC test (HV side): 90V, 7A,
300W. Compute the parameters of the approximate equivalent circuit referred
to LV side. [8]
4. (a) What are the conditions required for the parallel operation of two transformers.
(b) Derive the equations for the currents supplied by each transformer when two
transformers are operating in parallel with equal voltage ratios. [6+10]
5. (a) Explain why the rotor of polyphase induction motor can never attain synchro-
nous speed
(b) The rotor of a slip ring induction motor is connected to an AC source, where
as its stator winding is short circuited. If rotating magnetic field produced by
rotor winding’ rotates clock wise, Explain the direction in which rotor must
revolve. [8+8]
6. (a) Sketch and discuss the typical torque-speed characteristics of an induction
motor.
(b) A 3-phase wound rotor induction motor develops a maximum torque of 4 times
the full load torque at slip of 0.20. The per phase rotor resistance is 0.04
.
The stator resistance and rotational losses are negligible. Calculate the value
of external resistance that must be inserted in the rotor circuit in order to
minimize torque at starting. [8+8]
7. (a) Draw and explain the phasor diagram of 3-phase induction motor.
(b) Discuss the phenomenon of crawling and cogging in an induction motor. [8+8]
8. The rotor of 3-phase slip ring induction motor has an induced voltage of 100V and
impedance of 0.2 + j1 ohm at stand still. The induction motor has full load slip of
0.04 driving constant torque load and running at 1440 rpm. Calculate the voltage
to be injected if the motor is to be driven at
(a) 800 rpm
(b) 1000 rpm. [8+8]
? ? ? ? ?
2 of 2
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