EEE4117 Electrical Machines and Drives
Assignment 1 (Answer all questions, full marks is 50)
1. A 20 kVA, 11k / 380 V, 50 Hz, 92% efficiency single-phase transformer has a resistance of 0.5 and a leakage reactance of 0.24 on the primary side, and a resistance of 0.005 and a leakage reactance of 0.003 on the secondary side respectively. Evaluate:
(a) the total impedance referred to the secondary side;
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(b) the load current at rated voltage on the secondary side if the output of the transformer is connected to a load of 8 kW at
0.8 p.f. lagging; and
(c) the induced voltage on the low voltage side.
(2 marks) (2 marks)
2. Table Q2 shows the open circuit and short circuit tests results of a 5 kVA, single-phase step down transformer with a turn ratio of 5.
Open Circuit Test (Measure on L.V. side) Short Circuit Test (Measure on H.V. side)
Voltage 220 V (rated)
39 V Table Q2
Current 1 A
Power 100 W
(a) the equivalent core loss resistance, magnetizing reactance
and the corresponding current flowing in the parameters
referred to the high voltage side;
(b) the equivalent impedance on short circuit test referred to the
high voltage side; and
(c) the loading at which maximum efficiency is attained, and
hence find the value of the maximum efficiency at unity power factor.
(3 marks) (6 marks)
Assignment 1
Page 1 of 2
EEE4117 Electrical Machines and Drives
3. Table Q3 shows the open circuit characteristics of a D.C. shunt motor at a constant speed.
If(A) 0 0.1 0.25 0.5 0.75 1 1.25 1.5 1.75 Ea(V) 12184080130150160170170
(a) Plot the O.C.C. graph;
(b) Determine the generated e.m.f. with the shunt field resistance
(c) Determine the critical resistance of the shunt circuit
(3 marks) (2 marks)
4. A 220 V D.C. shunt motor has an armature resistance of 0.3 and field resistance of 120 . If the motor draws an armature current of 120 A at 75% rated load and has the mechanical loss of 400 W at 1300 r.p.m., evaluate:
(a) the back e.m.f of the motor at 75% rated load;
(b) the output torque of the motor at 75% rated load;
(c) the motor speed in r.p.m. at full load, assume that the flux per
pole is proportional to the field current; and
(d) the power drawn by the motor if a 20 resistor is connected
in series with the field resistor at full load condition.
(2 marks) (2 marks) (3 marks)
5. A 220 V D.C. series motor draws a current of 20 A from the supply at 1000 r.p.m. The back e.m.f. and armature resistance are 200 V and 0.5 respectively.
(a) Evaluate the field resistance connected to armature;
(b) Evaluate the mechanical output torque; and
(c) A resistance of 0.8 is connected in parallel with the field
resistor, and the torque is reduced to 75 % of the original value. Assume the flux per pole is directly proportional to the field current, evaluate the current flowing into the field winding.
– End of Assignment 1 –
Assignment 1
Page 2 of 2
(2 marks) (1 marks) (8 marks)
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