1. (a) Discuss in detail about on-load tap changing of a transformer.
(b) A 100 KVA, 3-phase, 50 Hz, 3,300/400 V transformer is connected on the
h.v side and Y connected on the l.v side. The resistance of the h.v winding in
3.5
per phase and that of the l.v winding 0.02
per phase. Calculate the
iron losses of the transformer at normal voltage and frequency if its full-load
e ciency be 95.8% at 0.8 p.f (lag). [7+8]
2. (a) Discuss the construction details of a transformer. Mention how hysteresis and
eddy current losses are minimized.
(b) A 2200/200 V, transformer takes 1A at the H.T side on no-load at a p.f of
0.385 lagging. Calculate the iron losses, if a load of 50A at a power of 0.8
lagging is taken from the secondary of the transformer. Calculate the actual
primary current and its power factor. [7+8]
3. A 3 phase, 80kW, 200V, 50 Hz induction motor has a full load e ciency of 83%
and a power factor of 0.8. At standstill the motor drive 5 times the full load current
and develops 1.25 times full load torque. An auto transformer is installed to reduce
the starting magnetizing current, determine:
(a) voltage to be applied at starting
(b) the full load line current
(c) the current drawn by the motor at start,
(d) the current supplied to the primary of an auto transformer. [15]
4. (a) Explain with the aid of diagrams, the principle of operation of double cage
induction motor.
(b) The rotor resistance and stand still reactance per phase of a 3 phase induction
motor are 0.015 and 0.09 ohm respectively. At normal voltage full load slip
is 3%. Estimate the percentage reduction in stator voltage to develop the full
load torque at half the full load speed. Also calculate the power factor. [7+8]
5. A single phase 200/400 V, 6 KVA, 50 Hz transformer gave the following results.
OC test(lv side) : 200 V, 0.8 A, 80 W
SC test(hv side) : 25 V, 10 A, 90 W
Determine
(a) the circuit constants refered to l.v side.
(b) the e ciency at full load with 0.8 lagging p.f. [15]
6. Justify the following for 3-phase induction motor
(a) Rotor leakage impedance at starting is di erent form its value at normal run-
ning conditions
(b) Relative field between stator eld and rotor eld is zero.
(c) Stator current rises as the shaft load is increased. [15]
7. (a) Derive the condition for maximum e ciency of a transformer.
(b) A 25 KVA, 2400/120 V, 50 Hz transformer has a high voltage winding re-
sistance of 0.1 and a leakage reactance of 0.22. The low voltage winding
resistance is 0.035 and leakage reactance is 0.012. Find the equivalent
circuit parameters when refered to the high voltage side. [7+8]
8. (a) Explain briefly the di fferent methods of speed control from rotor side of 3
phase induction motor.
(b) The stand still impedances of outer and inner cages of a double cage induction
motor are (4+j2.4) ohm and (1+j7) ohm respectively. Determine the slip at
which the cages develop equal torque. [7+8]
(b) A 100 KVA, 3-phase, 50 Hz, 3,300/400 V transformer is connected on the
h.v side and Y connected on the l.v side. The resistance of the h.v winding in
3.5
per phase and that of the l.v winding 0.02
per phase. Calculate the
iron losses of the transformer at normal voltage and frequency if its full-load
e ciency be 95.8% at 0.8 p.f (lag). [7+8]
2. (a) Discuss the construction details of a transformer. Mention how hysteresis and
eddy current losses are minimized.
(b) A 2200/200 V, transformer takes 1A at the H.T side on no-load at a p.f of
0.385 lagging. Calculate the iron losses, if a load of 50A at a power of 0.8
lagging is taken from the secondary of the transformer. Calculate the actual
primary current and its power factor. [7+8]
3. A 3 phase, 80kW, 200V, 50 Hz induction motor has a full load e ciency of 83%
and a power factor of 0.8. At standstill the motor drive 5 times the full load current
and develops 1.25 times full load torque. An auto transformer is installed to reduce
the starting magnetizing current, determine:
(a) voltage to be applied at starting
(b) the full load line current
(c) the current drawn by the motor at start,
(d) the current supplied to the primary of an auto transformer. [15]
4. (a) Explain with the aid of diagrams, the principle of operation of double cage
induction motor.
(b) The rotor resistance and stand still reactance per phase of a 3 phase induction
motor are 0.015 and 0.09 ohm respectively. At normal voltage full load slip
is 3%. Estimate the percentage reduction in stator voltage to develop the full
load torque at half the full load speed. Also calculate the power factor. [7+8]
5. A single phase 200/400 V, 6 KVA, 50 Hz transformer gave the following results.
OC test(lv side) : 200 V, 0.8 A, 80 W
SC test(hv side) : 25 V, 10 A, 90 W
Determine
(a) the circuit constants refered to l.v side.
(b) the e ciency at full load with 0.8 lagging p.f. [15]
6. Justify the following for 3-phase induction motor
(a) Rotor leakage impedance at starting is di erent form its value at normal run-
ning conditions
(b) Relative field between stator eld and rotor eld is zero.
(c) Stator current rises as the shaft load is increased. [15]
7. (a) Derive the condition for maximum e ciency of a transformer.
(b) A 25 KVA, 2400/120 V, 50 Hz transformer has a high voltage winding re-
sistance of 0.1 and a leakage reactance of 0.22. The low voltage winding
resistance is 0.035 and leakage reactance is 0.012. Find the equivalent
circuit parameters when refered to the high voltage side. [7+8]
8. (a) Explain briefly the di fferent methods of speed control from rotor side of 3
phase induction motor.
(b) The stand still impedances of outer and inner cages of a double cage induction
motor are (4+j2.4) ohm and (1+j7) ohm respectively. Determine the slip at
which the cages develop equal torque. [7+8]
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