Motor Controllers and Their Types
A motor controller is a group of devices that control the performance of an electric motor based on predetermined settings. Sometimes called governors, they have several functions including automatic & manual starting or stopping of a motor, setting forward or reversing the course of rotation, choosing and monitoring the speed of rotation, controlling & regulating torque, and protecting the motor from electrical overloads.
Regardless of their type, all electric motors have some version of a controller. They will differ in terms of their features and complexity, and these will be determined by the function of the motor. The most basic example of a motor control mechanism is a switch that connects a motor to its power source, but controllers can offer many other features depending on their type. There are five primary types of motor controllers: motor starters, reduced voltage starters, adjustable-speed drives, intelligent controllers, and servo controllers.
Motor Starters
While small motors can be started by simply switching the power on, large electric motors require motor starters to get started. A motor starter is a specialized switching unit used to power electric motors. When powered up, the starter connects to the motor terminals to provide an immediate power supply. Reverse starters, which contain two direct on-line circuits, can also be used to rotate the motor in both directions. Some motors, such as large motors that operate on medium voltage power supplies, use power-circuit breakers as starter elements.
Reduced Voltage Starters
In reduced voltage conditions, two or more starters are used to start an engine. Through induction or the use of an autotransformer, lower voltage is presented at the motor terminals. In turn, this helps reduce the starting torque as well as the incoming electric current. Once the motor reaches a certain amount of its top-load speed, the starter automatically transfers full voltage current to the motor terminals.
Adjustable Speed Drives
The adjustable speed drive, also known as a variable speed drive, is a combination of devices that allow operators to drive and adjust the operating speed of a mechanical load. Adjustable speed drives consist of a speed controller, or power convertor, a selection of auxiliary equipment and devices, and an electric motor.
Intelligent Controllers
An intelligent motor controller uses advanced microprocessors to control the power of electronic devices being used within an electric motor. Controllers of this type monitor the load exerted on a motor and accordingly match the torque to the recorded load. They achieve this by slashing the voltage to the AC terminal and simultaneously lower the current and KVAR (kilovolt amperes). This increases the motor’s energy efficiency while decreasing its noise, vibration, and heat generation.
Servo Controllers
Servo controllers refers to a wide range of motor control devices, known for their features including precise closed-loop positional control, fast rate of acceleration, and precise speed control servo motors made from different types of motors such as AC servo motors, brushed DC motors, and brushless DC motors. Servo controllers use position feedback to close the control loop and are frequently used in conjunction with resolvers, encoders, and Hall Effect sensors (devices used to measure the magnitude of a magnetic field) for directly measuring the position of rotors.
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