68 - Speed Versus Torque Characteristic Analysis of a Three-Phase Induction Motor
An induction motor is a motor in which a rotating magnetic field in the stator coils causes induced current to flow in an auxiliary conductor. This current and magnetic field exert force on the auxiliary conductor in the rotation direction and cause the motor's rotor to rotate. Induction motors are widely used in everything from industrial machines to home appliances because they have a simple construction and are small, light, affordable, and maintenance-free.
In an induction motor, the current induced by the auxiliary conductor exerts a large influence on its characteristics. It also causes strong magnetic saturation in the vicinity of the gap, in particular. This is why a magnetic field analysis based on the finite element method (FEM) is useful when investigating the motor's characteristics for a design study.
This Application Note explains an analysis that confirms the Speed-Torque curve and current density distribution of an induction motor.
Current Density Distribution
The current density distribution at a rotation speed of 1050 r/min is shown in fig. 1. The rotating magnetic field created by the stator coils generates induced current in the rotor cage. Torque is generated from the magnetic flux and rotating magnetic field created by this induced current, so the current has a large effect on the torque characteristics.
The Speed-Torque curve is shown in fig. 2. For this induction motor, the maximum torque can be obtained at the lower speed range. This means the induction motor has high resistance. In this case, changing the thickness of bars can reduce the electric resistance, and then the maximum torque can be obtained at the higher speed range.