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Variable Frequency Drives

Many of our equipment often is driven by an electric motor. The motor used to drive the equipment is thus called a driver motor. We often know that a full capacity is not required all the time as in case of a ceiling fan.

A normal 3 phase induction motor has a fixed speed at a constant frequency. The speed of the motor is given by an equation 120 x frequency /(No of poles). We have 2 Pole, 4 Pole and 6 Pole Motors commonly used in our utilities. The speed of these motors at 50 Hz are as below:

2 Pole Motor – 3000 RPM

4 Pole Motor – 1500 RPM

6 Pole Motor – 1000 RPM.

The actual speed will be slightly less by about 5% than what is given above, which is termed as slip. Ignoring the slip for the time being in the discussion.

Now to change the speed of a 1500 rpm motor to say 1200 rpm, what can be done is to change the frequency of the power supply, which is done by a Variable Frequency Drive.

Variable Frequency Drives often permits manual setting of frequency or through a signal from a control device.

The power consumption of equipment like fans, pumps etc. are considerably lowered while reducing the RPM. Thus if you have any device which is often desirable to operate at lower capacity and is driven by a 3 phase induction motor, consider the option of using a variable frequency drive.