19-10-2012, 01:25 PM
ELECTRICAL MACHINES LAB. MANUAL
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Introduction
• The power pack contains a three-phase transformer, rectifier, and variable voltage transformer connected
so that fixed and variable DC and AC voltages can be supplied.
• The torque meter can be used to drive or brake an electric machine in order to measure its characteristics.
It consists of a DC machine, the stator of which is mounted in bearings so that its torque reaction is
transmitted to a pressure sensor. The range of the torque meter is ± 25 Nm. The reaction between the
shaft torque, electromagnetic torque and losses torque, for both motoring and generating, is given in Fig.
2. The dynamometer reading is the net coupling torque (shaft torque).
Shunt Generator
1. Connect the DC generator with shunt excitation as shown in Fig. 2.
2. Start the torque meter as mentioned before in Part 1 (3-5) with S open. Adjust the speed to 1400 rev/min.
3. Adjust Rmy of the generator so that the voltage becomes 220V. The setting of Rmy must not be changed
during this experiment.
4. Turn on S with RL set to maximum resistance (minimum current). Using RL vary IL in steps of 1A up to
the rated current.
5. Record for each step IL and voltage Vt and fill the results in Table 2. Make sure that the speed is kept
constant at 1400 rev/min before each reading.
6. Stop the machine.
Series Motor
1. S1 should be in position (1) and S2 should be in position (OFF).
2. Switch ON the 0-240V DC supply. Increase the voltage up to 60V and keep it constant throughout this
test.
3. Switch ON the 200V DC supply. Vary the dynamometer field resistance to apply more torque to lower
speed successively in steps of about 100 rev/min. Each time adjust the voltage to be 60V.
4. Take the readings of armature current Ia, torque Te, and speed n at each setting until the series motor
armature current reaches 10A. Fill in Table 3.
Overview
A single - phase transformer can be represented by an equivalent circuit. An equivalent circuit can
be either referred to primary side or secondary side. Fig. a shows a single - phase two winding
transformer. The exact equivalent circuit of the transformer referred to primary side is shown in
Fig. b while the approximate equivalent circuit referred to primary side is shown in Fig. c.