Page 708 - Basic Electrical Engineering

Efficiency of an induction motor can be determined by loading the motor and
measuring the mechanical output and the electrical input. By converting the
input and output in either electrical unit or in mechanical unit, the efficiency

of the motor can be determined by taking the ratio of output and input. Small
induction motors can be tested by this method where it is possible to load the

machine in the laboratory.
In testing of large motors, the indirect method is adopted where the losses

of the motor are determined from some tests and the efficiency is calculated
as

Two types to losses take place in a motor. One is called constant loss and the
other is called variable loss. Iron loss and friction and rotational losses are

constant at all loads as long as supply voltage and frequency are constant and
the speed of rotation does not change much as the load on the motor varies.

Copper loss in the stator and rotor varies with load. As the load on the motor
changes, the stator and rotor currents vary and the copper losses in the

windings also vary.
Two tests are performed on the motor to determine the constant losses and
variable losses so as to determine the efficiency of the motor. These tests are

described as follows.

8.16.1 No-load Test

The motor is run on no load with full voltage applied across its stator

terminals as shown in Fig. 8.19. Two single-phase wattmeters are connected
to measure the three-phase power input. Since with no load connected on the

motor shaft, the input power is completely wasted as loss, the sum of the
wattmeter reading can be considered equal to the various losses. Therefore,

2
W + W = I R loss in the windings at no load
2
1
+ Iron loss in the core