3. What is a variable speed drive?

Recall that the speed of an induction motor is directly proportional to the frequency of the AC supply. If it were possible to adjust the supply frequency then it would be possible to control the speed. This exactly what a VSD does!

A VSD is an electronic device that converts constant frequency AC power input into a variable frequency output. This variable frequency output is used to control motor speed which, as said before, is proportional to the frequency of the VSD’s output.

4. Starting motors

A starter motoris designed to supply a motor with sufficient current to establish a starting torque to get the motor to its rated load speed.

There are a number of starting methods that are commonly used in industry. Each method has benefits and advantages. The most energy efficient is the variable speed drive.

• Direct-on-line starting: stator windings directly connected to supply via contactors
  

• star delta starting: star and delta connection of stator windings are used
  

• autotransformer starting: stator windings connected to the supply through an auto transformer
  

• soft starters: variable speed drive

A quick comparison of each of the starting methods reveals that the starting current (inrush current) is considerably higher in some cases than the final current at full loaded speed: