Session: 2018-2019 (Spring) Class: B. Tech Branch: ECE Semester: 4th Exam: Minor
Course Code: ELE-405 Course Name: Electrical Machines Total Time: 1.5 hours Max Marks: 30
Course Objectives:
CO1: To study transformer construction, operation, various tests, find efficiency & voltage regulation.
CO2: Study about DC machines, operation, performance, applications, recent advancements.
Note: Attempt All Questions.
1. The resistances and leakage reactances of a 30-kVA, 60-Hz, 2400-V : 240-V distribution
transformer are
R1 = 0.68 Ω; R2 = 0.0068 Ω; Xl1 = 7.8 Ω; Xl2 = 0.0780 Ω
where subscript 1 denotes the 2400-V winding and subscript 2 denotes the 240-V winding. Each
quantity is referred to its own side of the transformer.
a) Draw the equivalent circuit referred to (i) the high-voltage and (ii) the low-voltage sides. Label
the impedances numerically.
b) Consider the transformer to deliver its rated kVA to a load on the low-voltage side with 230 V
across the load. (i) Find the high-side terminal voltage for a load power factor of 0.85 lagging.
(ii) Find the high-side terminal voltage for a load power factor of 0.85 leading.
(8M) [CO1]
2. A 15-kVA, 2400:240-V, 60 Hz transformer has the following equivalent circuit parameters:
R1=2.5 Ω; R2=0.025 Ω; X1=7.0 Ω; X2=0.070 Ω; Rc=32 kΩ; Xm=11.5 kΩ
If the transformer is supplying a 10-kW, 0.8 power factor lagging load at rated voltage, calculate:
(a) Input voltage (b)Input current, and (c) Input power factor (8M) [CO1]
3. A 120 V dc shunt motor having an armature circuit resistance of 0.2 ohm and a field circuit
resistance of 60 ohms, draws a line current of 40 A at full load. The brush volt drop is 3 V and rated,
full load speed is 1800 rpm. Calculate:
(a) The speed at half load.
(b) The speed at an overload of 125 percent. (8M) [CO2]
4. Discuss classification of DC motors with necessary equations. Also derive the equations for
generated emf and torque developed in DC machine.
(6M) [CO2]
[THE END]
National Institute of Technology-Srinagar
Hazratbal Srinagar-190006, Kashmir (J&K)
Department of Electrical Engineering
