Induction Motor Efficiency, Power Factor and Mechanical Load

1. Explain why the efficiency of the induction motor increases when the mechanical load increases.
2. Explain why the input power factor of the induction motor increases when the mechanical load increases.

Explain:
1. The efficiency of the induction increases when the mechanical load increases because as the motors load increases, its slip increases, and the rotor speed falls. Since the rotor speed is slower, there is more relative motion between the rotor and the stator magnetic fields in the machine. Greater relative motion produces a stronger rotor voltage which in turn produces a larger rotor current. With a larger rotor current, the rotor magnetic field also increases. The increase in the rotor magnetic field tends to increase the overall torque induced in the motor. Ultimately induction motors are more efficient when they are running closer to synchronous speed i.e. the pullout torque is near synchronous speed.

2. The input power factor of the induction increases when the mechanical load increases because in general, the higher the resistance (a load), the higher the power factor. A higher power factor means that there is a tapping of electrical energy in terms of active power. And more active power means higher mechanical output to compensate for the increasing rotor resistance.

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Comments

12/19/2017 10:32 AM
I have a 3 Ph inductio motor in one of my CNC machines.  I am currently logging current draw, voltage, KVA and power factor on all three phases and have noticed that under load one of the phases shows a very different PF the the other two in that it is not only highly variable but much lower than the other phases.  I was wondering if you guys had any thoughts as to the reasons for this?