Magnetic Fields in a Heavy Load Induction Motor

The characteristic curve of electric torque against slip can be divided into approximately three regions.
The first is the region of low slip of the curve, in which it increases with increasing load and the mechanical speed of the rotor decreases with the load. The power factor of the rotor will be almost unitary even if the rotor current increases with slipping. The normal range of operation of the induction motor is included in this linear region of low slip.
The second region of the induction electric motor curve may be called the moderate slip region in which the rotor frequency is greater than before and the rotor reactance is of the same order as the rotor resistance. The maximum torque of the electric motor takes place at the point where, the increase in the rotor current is balanced exactly with the decrease of the power factor of the rotor.

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The third region of the induction motor curve is of high slip, in which the induced torque actually decreases with the increase of the load, because the increase in the rotor current is not remarkable due to the decrease in the power factor of the rotor.