Motor controllers are electronic and electromechanical devices used to start and operate electric motors in a predefined manner. They can gradually increase motor speed, increase starting torque, or reverse the direction of motor rotation. They can be simple or sophisticated and fall into two categories: electronic and electromechanical controllers. They are necessary for larger and more complex installations that require more sophisticated control of machinery drives.
The term motor controller describes a group of electronic and electromechanical devices used to start and operate electric motors in a predefined manner. They can be used to gradually increase motor speed when starting, increase starting torque or reverse the direction of motor rotation. These check steps are usually required due to operating environment demands, specific installation requirements, or to increase motor efficiency. The motor controller can 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 can be found in large numbers in most similar installations. When supplied with a suitably rated electrical current, a motor starts, accelerates to its maximum design speed, and continues to run until the power is removed. It will then decelerate until the inertial forces overcome the rotor impulse and the motor stops. In the simplest installations, the introduction and removal of power is all the control needed, and if the correct motor size is used, the system will operate correctly with no more than a simple start and stop station. Unfortunately, larger and more complex installations require much more sophisticated control of many of their machinery drives.
Many machines require gradual increases in rotational speed during startups as part of the system’s operating regime. Other heavily loaded machines such as underground fans, which have fan blades weighing many tons, also require soft start speeds to overcome the large inertia of the static blade assembly to avoid motor tripping and mechanical damage. Other installations require a single motor to reverse the direction of rotation at will or be able to reach a range of speeds in normal operation.
All of these operational requirements can be met with one of the wide range of motor controller models available. These devices fall into two basic categories: electronic and electromechanical controllers. Electronic engine control units are generally very sophisticated and include categories of devices such as soft starters and variable frequency drives that can ramp starting speeds and control operating speeds. They can also be programmed to respond to a variety of system inputs or pre-set operating conditions.
The electromechanical motor controller is the simpler of the two groups and generally uses electromagnetic contactors or relays to stop, start and reverse the direction of the motor. In the case of star/delta three-phase starters, they can also be used to overcome high starting loads by starting the motor in star configuration at high torque. Once the initial inertial load has been overcome, they switch to a more economical and efficient low-torque delta configuration. In some older speed control systems, variable mechanical resistance is used to control the speed of the motor.
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