Variable speed drives provide effective speed control of AC motors by manipulating voltage and frequency. Controlling the speed of a motor provides users with improved process control, reduced wear on machines, increased power factor and large energy savings.

The most significant energy savings can be achieved in applications with a variable torque load. Reducing a fan’s speed in a variable torque load application by 20% can achieve energy savings of 50%. Therefore, for most motion control applications, reducing motor speed is often the easiest way to get large energy savings.

AC drives significantly reduce energy consumption by varying the speed of the motor to precisely match the effort required for the application. To vary the speed of the motor dynamically, a closed-loop regulator that considers the measured output of a process is required. Common applications where this is used include pressure, level and temperature control. The most common method of regulation is the PID (Proportional-Integral-Derivative) control loop.

The input stage of an AC drive is usually an uncontrolled diode rectifier whereby power cannot be fed back onto the AC mains supply. Excess energy is converted to heat.

Active front end (AFE) systems use a controlled rectifier bridge to enable energy to flow bi-directionally between the mechanical system and the AC supply. This provides far greater efficiency levels than standard drives in applications where electrical braking of the mechanical system would otherwise result in significant amounts of energy being wasted as heat