Page 277 - Industrial Power Engineering and Applications Handbook

11/256 Industrial Power Engineering and Applications Handbook
11.4 Load test terminals at the conclusion of the no-load test, calculate
the stator winding resistance R,.
0 Calculate the no-load stator copper loss: i2. R, watts.
Tests on load are conducted to determine the performance Subtract the no-load stator copper loss from the stator
of the machine, such as its efficiency, power factor, speed input power P,, at no-load, i.e. (Pnt - i2. RI). This is
and temperature rise etc. For all tests, a machine with the net no-load loss made up of core, friction, windage
load should be properly aligned and securely fastened. and stray losses.
Load characteristics are obtained by taking readings at Calculate the total stator winding resistance:
high loads, followed by reading at lower loads. This is I?,!,,, 1.5 (1.5 for A-connected, and 0.5 for Y-connected
,=
usually carried out at 125%, loo%, 75%, 50% and 25% windings, Figure 11.2) x cold values of the resistance,
of the full load values. measured across the stator line terminals, corrected to
the required temperature. The required temperature
Methods of loading values will be:
1 Brake and pulley method (usually for very small For class A 75°C
For class E
motors). Considerable care needs be taken in the For class B 90°C
construction and use of the brake and pulley. When For class F 95°C
conducting this test conditions should be such that a For class H 11 5°C
scale pointer remains practically stationary at any given 130°C
load. Proper cooling, preferably water cooling, should 0 Calculate the stator copper loss on load: I,?. Rlhot.
0 Add the net no-load loss and stator copper loss on
be provided for the pulley.
2 Dynamometer method (for medium-sized motors, say load and subtract their sum from the stator input power
up to 500 hp). The output of an induction motor measured on load. The remainder is the power input
may be calculated by to the rotor.
0 Calculate the rotor copper loss (input to the rotor x
percentage slip on load)
Subtract this rotor copper loss from the rotor input.
Subtract the stray loss to give the motor output for
3 Calibrated machine. When brake and pulley or nominal full load. The amount of stray loss is normally
dynamometer methods are not possible. the test motor taken as 0.5% of the nominal power output of the
may be loaded onto a calibrated generator. The machine. The stray losses at other values of loads are
efficiency curve of the generator must be available. obtained from:
When it is not possible to conduct any of the above
three methods, the test motor may be loaded onto an (required stator current)2
uncalibrated generator or any other loading device. x stray loss at full load
(full load stator current)'
11.4.1 To determine efficiency After deducting the stray loss, the resultant kW gives
the machine output.
By the input-output method kW output
Efficiency, 1) = kW input X 100%
As noted above, efficiency may be detemiined by adopting
any of the following three methods: As a check, add together the net no-load loss, the
stator copper loss on load, the rotor copper loss on
Brake and pulley load and stray loss to give the total losses (total fixed
Dynamometer and loss plus load loss):
Calibrated machine.
kW input
By summation of losses
The two must tally.
Calculation of efficiency is based on the readings obtained
after the heat run test when the machine has achieved 11.4.2 Slip measurement
thermal stability. The losses will fall into following four
groups: For the range of load for which the efficiency is deter-
mined, the measurement of slip is very important. To
1 Core friction and windage loss (obtained from the determine slip by subtracting from the synchronous speed
no-load test) the value of speed, obtained through a tachometer is not
2 Stator copper loss (primary loss) recommended. The slip must be directly measured by
3 Rotor copper loss (secondary loss) one of the following methods:
4 Stray load loss (hysteresis and eddy current core loss).
1 Stroboscopic
The procedure to be followed is as follows: 2 Slip-coil
3 Magnetic needle
From the resistance measured across the stator line 4 Any other suitable method.