Electrical Engineering ⇒ Topic : Losses and Efficiency
LOSSES AND EFFICIENCY:-
(1) LOSSES :-
In an induction motor following losses occur :
There are copper losses and core losses in the stator, and copper losses and frictional losses in the rotor. Actually there is some core loss in thè rotor. Under operating conditions, however, the rotor frequency is so low that it may logically be assumed that all core losses occur in the stator only.
The efficiency of induction motor can be determined (as with any rotating equipment) by loading the motor and measuring the input and output diirectly. This method, however, is not always desirable, because of the inherent inaccuracy, or may not even be feasible, because of the inability to simulate an actual load. The latter is especially true in the case of large machines As with D.C. dynamos an.d the transformer, therefore, a method of measuring the losses with electrical instruments only is used. Since the circuit of the induction motor closely approximates that of the transformer, losses may be measured in a manner virtually identical with that used for transformers.
It is seen that the efficiency calculation based on equivalent circuit of induction motor actually requires three tests
The following procedure is adopted for determining the efficiency of an induction motor by the use of the equivalent circuit :
Although calculation of efficiency from the equivalent circuit is considered more accurate than the previous method, it does have drawbacks. A load run must be performed, and slip must be accurately measured. The second factor is not a serious drawback since a technique of direct simple measurement can be easily used. Nevertheless, the efficiency test using the no-load run and blocked-rotor test is sometimes used, despite the fact the efficiency measured in this manner is slightly lower
LOSSES AND EFFICIENCY
The different kinds of losses are given below
Fixed loss = Power input at no load - Stator Cu loss at no load
This loss can be obtained by performing no-load test of the induction motor.
Total ohmic losses under variable losses can be obtained using blocked rotor test of induction motor. It should be noted that the brush contact loss for WRIM = slip ring current X 1 volt.
Stray load loss occurs in iron and conductors. It is very difficult to measure stray load loss.
To account this, efficiency is taken as 0.5% less than the calculated value on full-load and prodata for other loads.
The efficiency of a three-phase induction motor is given by
where Psh is shaft power, Pfl is fixed loss, and Pcu is stator and rotor ohmic losses plus brush contact loss
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