Magical Motor Power Trick!

Share

VSD Motor Power, Speed & Torque – How and Why

We came across a situation recently on site where an induction motor being driven by a VSD at a reduced speed had the operators asking "Why is the motor running at the rated current but not at the rated power?"
It was actually a very good question and one that we thought we would share with you all.

First of all there are a few things to remember about a motor;

• Torque is a measurement of the amount of work the motor is doing.
  
• Power is the rate at which the work (torque) is being done. I.E. How fast the motor can deliver the torque or do the work.
• The motor’s power rating of is the maximum power that motor can deliver when it is run at the full rated speed (i.e. 50Hz).
  

The most basic method of control for an induction motor on a VSD is the Volts per Hertz (V/Hz) method, also known as open-loop or scalar control. This is where the VSD controls the motor speed by maintaining a constant ratio between motor voltage and frequency, as the frequency of the motor is reduced so is the voltage by the same ratio to maintain a constant magnetic flux.

If we have a look at the motor power formula, we see that the speed of a motor (shown as rpm) directly effects the power of a motor;

Read More