Overview
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Induction motors are widely used because they have a simple construction
that offers a motor that are compact, lightweight, affordable, and robust.
The induced current flowing in the cage largely effect the performance
of the motor because the motor rotates with the interaction between the
magnetic field of the stator windings and the induced current that flows
through the cage.
Evaluating the relationship between the induced current and performance
of the motor using a magnetic field analysis is advantageous.
This example presents the use of a magnetic field analysis to obtain the
current density distribution and the speed versus torque characteristics
of a single-phase induction motor. |
Current Density Distribution
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The current density distribution for the maximum torque at a rotation speed
of 300 rpm and 2,100 rpm is indicated in Fig. 1. The difference in speed
between the rotor and rotating magnetic field becomes smaller as the speed
of the rotor reaches high speed. For this reason, the variations in the
magnetic field acting on the cage become smaller, making the amount of
eddy currents flowing in the bars smaller. The eddy current distribution
in the bars largely affects the torque characteristics. |
Speed vs. Torque
| The speed versus torque is indicated in Fig. 2. The torque of the induction
motor is 0 when the induction motor rotates slower than synchronous speed,
and the synchronous speed becomes breaking torque at synchronous speed.
The maximum torque is reached at around 2000 rpm. A motor matching the
goals of the designers can be evaluated by changing the materials and geometry
of the cage because the rotation regions where the maximum torque and the
high torque are obtained vary depending on the electric resistance of the
cage. |
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