Is three phase induction motor is a fixed speed motor?

Answer 1

This is a more complicated question then first meets the eye.

First, the direct answer. Synchronous or Asynchronous motors can work.

However, varying speed in an AC motor can only be done by changing the frequency. This requires the use of a Variable Frequency Drive (Like the Altivar series of drives). A variable frequency drive can create noise on the output, thus you may need filter, and the motor should have inverter duty windings.

If the motor does not have inverter duty windings, then you may not be able to reduce the speed much. Also, when running a motor at a slower speed, you will need to determine the minimum speed that will allow proper torque to the load, as well as still maintain cooling of the motor coils.

Putting a drive onto a motor may require the assistance of a licensed engineer.

Answer 2

Without any modification to its number of operating poles or alteration to the supply frequency, all three phase induction motors are fixed speed motors.

However their speeds can be varied in two ways.

1. A manufacturer design the motor such that its number of operational poles at a time can be varied i.e increased or decreased consequently decreasing or increasing the motor speed respectively. These types of motors are usually called two-speed motors owing to the fact that the motor only have two set operational speeds.

2. The second method make use of a VSD, a Variable Speed/ Frequency Drive. This device increases or decreases the frequency of the supply voltage to the motor. Frequency is directly proportional to the motor's speed. Note that you have to enter the frequency or the speed itself at which you want the motor to run.