What about efficiency of ac induction motor when load is applied?
will it increase or decrease?
Answer:
Motor manufacturers generally have efficiency data for 1/4, 1/2, 3/4, full load and 25% overloaded. I found a 2 Hp motor with efficiency of 81.5%, 86.9%, 87.8%, 87.5% and 85.8%. for those load points. Note than maximum efficiency occurs at 3/4 load for that motor. That is not unusual. Since efficiency is zero with zero load, efficiency must increase as load increases from zero, but there is often a maximum point at less than full load. The curve of efficiency vs. load is generally smooth with only one maximum point, but the curve is often fairly flat near the maximum efficiency point.
You can only calculate efficiency AFTER a load has been applied to any motor, induction or otherwise. Otherwise there is no output and thus there is zero efficiency of the motor.
All motors have an efficiency curve.
At zero torque, the efficiency is zero, and all of the input energy is spent just spinning the motor.
As torque (load) is increased, the efficiency rises for a while, but then drops off as heating losses/slip/etc mount up. Of course, if you apply enough togue, you burn out the motor.
Mfrs usually design motors to reach peak efficiency at 75% of rated horsepower, or whatever the most typical expected load is likely to be.
You need to be clear about your definition of efficiency.
Do you mean the ratio of
(mechanical power out / electrical power in)?
If there is no load applied, the motor is not doing any useful work (in the physics sense of 'work'), so its efficiency is zero.
If there is so much load applied that the motor cannot move, then using the definition above, the efficiency is again zero.
Somewhere in between these two extremes, the efficiency will be at a maximum.
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