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What Is a Motor Controller?

By Paul Scott
Updated May 17, 2024
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The term motor controller describes a group of electronic and electromechanical devices used to start and run electric motors in a pre-defined fashion. They may be used to gradually increase the speed of the motor at start up, increase starting torque, or reverse the rotational direction of the motor. These control phases are typically necessary due to the demands of the operational environment, specific installation requirements, or to increase motor efficiency. The motor controller may be little more than a fairly simple operator interface such as a direct on-line starter, or highly sophisticated and automated motor soft starter or frequency drive systems.

Electric motors form the backbone of most industrial and manufacturing environments and may be found in large numbers in most similar installations. When supplied with a suitably-rated electric current, a motor will start, accelerate to its maximum design speed, and continue to run until the supply of power is cut. It will then decelerate until inertial forces overcome the impetus of the rotor and the motor comes to a standstill. In the most simple of installations, the introduction and cessation of power is all of the control needed, and, if the correct size of motor is used, the system will run correctly with no more than a simple start and stop station. Unfortunately, most large, complex installations require far more sophisticated control of many of their machinery drives.

Many machines require gradual increases in rotational speed during start ups as part of the system's operational regimen. Other high load machines such underground ventilation fans, which feature fan blades weighing many tonnes, also require gradual start up speeds to overcome the considerable inertia of the static blade set to prevent motor trips and mechanical damage. Other installations require a single motor to reverse rotational direction at will or be capable of range of speeds during normal operation.

All of these operational requirements may be achieved with one of the wide range of motor controller models available. These devices fall into two basic categories: electronic and electromechanical controllers. Electronic motor controller units are generally highly sophisticated and include device categories such as soft start and variable frequency drives which can gradually increase start up speeds and control running speeds. They can also be programmed to respond to a wide variety of system inputs or pre-set running conditions.

The electromechanical motor controller is the more simple of the two groups and generally makes use of electromagnetic contactors or relays to stop, start, and reverse the motor's direction. In the case of three-phase star/delta starters, they can also be used to overcome high starting loads by starting the motor in a high-torque star configuration. Once the initial inertial load is overcome, they then switch to a more economical and efficient low-torque delta configuration. In some older speed control systems, a mechanical variable resistance is used to control the speed of the motor.

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