Motors & Variable Speed Drives

In many commercial, industrial, and utility applications electric motors are used to transform electrical energy into mechanical energy. Those electric motors may be part of a pump or fan or they may be connected to some other form of mechanical equipment such as a conveyor or mixer. In many of these applications the speed of the system is determined primarily by its mechanical design and loading. For an increasing number of these applications, however, it is necessary to control the speed of the system by controlling the speed of the motor.

Introduction to AC drive

All AC drives convert AC to DC, and then through various switching techniques invert the DC into a variable voltage, variable frequency output.

An AC drive can vary the speed and torque of the motor according to changing machine requirements. In other words after start up, the motor does not have to run at 100% speed and torque but these elements can be varied to suit the application.

Reasons for growing popularity of AC drive

• Energy savings, particularly for fans and pumps
• Extended equipment life through reduced mechanical stress (belts, bearings)
• Elimination of excessive motor inrush current, which in turn, extends useful motor life
• Standard AC motors can be used. This means that off-the-shelf motors, which are easier to repair, purchase and maintain
• With an AC drive, retrofits from a DC or wound-rotor motor to a NEMA B squirrel cage motor are relatively easy.
• Solid-state device, which has no moving, parts or contacts to wear out.