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Have I Got This Right?


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1) At the instance an induction motor is connected to a suitable supply a relatively high current is drawn because the conductors in the power windings have a low resistance. (V =IZ)


2) As the rotor commences to spin, its windings are being cut by the flux lines of the magnetic field generated by the field windings. The relative movement between the rotor and the magnetic field causes a voltage to be induced in the conductors in the rotor. (Faraday's Law)


3) This induced voltage is called a back emf.


4) The back emf generated in the conductors of the rotor opposes the supply voltage (Lenz's Law) and thus the current being drawn reduces.


5) The back emf is at a maximum when the rotor is at its maximum speed. This usually corresponds to the unloaded state.


6) When the motor is put under load, the rotor speed reduces and this results in the back emf also reducing. The reduction in back emf now allows the supply voltage to push more current into the motor and this strenghtens the magnetic field. This stronger magnetic field results in the motor delivering more torque to counter the load.


7) When a motor is overload it is slowed down to the extent that the back emf becomes insufficient to counter the force of the supply voltage. This results in an over high current being pushed through the field winding and as a consequence the insulation heats beyonds its designed temperature and the winding get burnt out.


How much of that have I got right?

Is my understanding suspect or weak in any particular area?






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Hello solas


This is essentially true for the DC motor. The induction motor does not have field windings etc.

The circuit is an AC circuit and is modelled as a short circuited tranformer.

The current curve is related to the reflected impedances at different slip frequencies rather than "back emfs"

In analyising the induction motor, you need to consider the AC characteristics which means looking at magnitudes and angles etc.

http://www.lmphotonics.com/m_control.htm describes the results and characteristics, but not the detailed mechanism.

Search for papers with full analysis based on the equivilent circuit and you will get a better understanding.


Best regards,

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