Motor drives modes
Posted 13 June 2004 - 10:05 PM
Posted 14 June 2004 - 10:16 AM
Welcome to the forum.
If you are using standard induction motors, there is really only one option for varying the speed. That is the VVVF inverter based system. If you try to vary the speed with variable voltage only, you will cause the motor to overheat.
Special high resistance rotor motors are available, commonly used as an integral part of fans, and these can be speed controlled by varying the voltaged only.
The standard induction motor is designed to operate at almost synchronous speed and delivers maximum torque at a very low value of slip. When the slip is higher than that where maximum torque is delivered, the rotor losses increase considerably and the motor impedance becomes very low. This results in a high stator current and considerable heat being dissipate in the motor.
In the case of the high resistance rotor, the maximum torque is available at close to zero speed. These motors are relatively inefficient compared to standard induction motors.
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