Performance curves or operating characteristics of an Induction Motor


Performance curves means when motor performance is shown as a graph from starting of the motor to running.

Generally variation of following parameters are noted at various loading of the motor and than these parameters are plotted on the graph from no load to full load.

Speed, stator current , slip , power factor , efficiency. Lets discuss one by one.

1. Speed characteristic: Generally induction motor runs at constant speed but speed slightly falls from No load( zero loading) to full load (100 percent loading ).

2. Stator current characteristic: At no load , induction motor draws only 30 to 40 percent current of the rated current , only to set up the magentic field and when it is loaded the current increases to supply the load torque. Same is shown is the figure.

3. Power factor characteristic: At no load(zero load), induction motor draws only current to set up magentic field so the power factor is very poor as the motor is loaded the power factor increases towards 1 but does not cross 0.9 as magentizing current is a major part of the total current.

4. Efficiency characteristic: at light loads , efficiency is low due to high fixed losses compared to variable losses than efficiency increase as load increases and efficiency becomes maximum at 80 to 90 percent of the rated load and than it decreases.


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