Motor Controllers

Every electric motor, regardless of its size or purpose, requires some sort of controlling mechanism. Motors come in a large variety of sizes and applications. Simple motors are found in home appliances such as hairdryers in the bathroom, juicers in the kitchen, and drills in the garage. These appliances require simple controllers, often just an on/off switch.  Much more complex motor controllers are used in applications such as 3D printers, CNC machines, and robotic welders.

The simplest motor controller is an ordinary on-off switch that connects the motor to the power source. This switch can be manual or a relay connected to an automatic sensor to provide logic to start or stop the motor. On the other scale, there can be very complex motor controllers,  controlling many motors coupled with a wide variety of sensors to provide logic for the operation of these motors.

Motor controllers may perform many functions such as automatically or manually starting or stopping an electric motor, set its speed, direction, or control torque if required. It provides the essential protection functions to the motor against any adverse ambient or operating conditions. Some motor controllers are embedded in a complex closed-loop control system.

Classification of Motor Controller Systems

Manual Controller

Manual controllers are generally very simple devices that connect the motor directly to the power line. A manual controller may simply be a switch connected in series with a motor. In manual controllers, the operators must physically go to the controller to operate it. Overload protection or low-voltage disconnection may or may not be provided in a manual controller.

Semi-automatic controller

A semi-automatic controller is a much more complex device and may have several pushbuttons, switches, and sensors to control the operation of a motor starter. The starter is connected to the motor, which is connected to a control panel located away from the starter and the motor. However, the operator must initiate all the actions such as starting and stopping the motor, but it can be done remotely.

Automatic controller

An automatic controller is a sophisticated device so that once the operator sets up the operating parameters, the controller automatically controls the operation of the motor.

A simple example is the control system of a domestic air conditioner. The user sets the temperature, fan speed, and airflow direction and the controller then ensures that the unit maintains the correct temperature and airflow by automatically operating the air conditioner fan and condenser motors.

The controllers in even such common appliances have safety features such as current overload and voltage surge protection, which automatically shut down the device in case of unsafe operating conditions. 

Different Types of Motor Controllers

Motor Starters

Small motors can be started by just a simple electric on-off switch which is connected to a power outlet. However, the larger motor requires starters,  which are specialized switching units required to power up the electric motor. Once switched on, their power terminals are directly connected to the power source. Very large motors that require medium to high voltage have power-circuit breakers as starter devices.

Reduced Voltage Starters

These are special types of motor controllers in which two or more starters are used for starting a motor at a lower voltage. This reduces the starting torque and lowers the high starting current. After a predetermined speed is obtained, full voltage is applied to the motor.

Adjustable-Speed Drives

These are also called the variable-speed drive. This type of motor controller the operator can adjust the speed drive depending on the load on the motor. These are complex devices that have several components such as a speed controller, power convertor, and other auxiliary devices to control the electric motor.

Intelligent Controllers

These are microprocessor-based intelligent systems that control the power of devices that use electric motors.  These controllers sense the load on the motor and match the torque to the corresponding load.  This is done by complex intelligent management of voltage and current to control the motor efficiently.

Servo Controllers

Servo controllers are quite sophisticated devices that are used in a wide variety of motor control applications. They have precise closed-loop positional and speed control and are used to control servo AC and DC motors. These controllers have complex feedback control loops and accurate sensors to control the precise movement of motors.

Functions of Motor Controllers


Starting the motor is one of the principal functions of a motor controller. The simplest method is across-the-line starting. This is done by connecting the motor directly to the power line. In certain cases, the motor may be required to start at a low speed and then accelerate to full speed after some time. In some cases, current or torque needs to be controlled during starting, requiring a more complex motor controller.


Once started, a motor has to be stopped at some time. The simplest way is to disconnect the motor from the power source, which will stop it. In some cases, the motors may be required to stop quickly or it may be required to hold the load. These functions are accomplished by more complex controllers or mechanical devices.

Jogging or Inching

Even more complex function is jogging and inching a motor with short bursts of power. This is generally required when positioning a load to some specific position. The difference between jogging and inching is that in jogging momentarily full line voltage is applied to the motor but in inching momentarily connection to power is at reduced voltage.

Speed Control

If a variable speed of the motor is required by a system, then a common way is to use a variable frequency controller using AC motors. It may also be achieved in a DC motor by varying the applied voltage. In some cases a DC clutch is preferable.

Motor and Circuit Protection

A very important function of control systems is to protect the motor and its circuit components. Fuses, circuit breakers, and overload relays are used to provide this protection. Even the simplest of motor control circuits, provide some motor protection functions such as overcurrent, overheating, and over and under-voltage cutoff.

Motor Control Accuracy

In some applications accuracy of motor control is paramount. For example, an industrial robot performing welding needs very high motor control accuracy.  Highly accurate motor control systems require both accurate and reliable sensors and robust pathways as critical data has to pass through cables on long distances.

Surge Protection

High voltage spikes or surges are produced when the magnetic fields collapse as power is turned off. These high voltage spikes can potentially damage the circuit components. The problem is quite severe where the control system is microprocessor-based, which is very sensitive to voltage spikes and surges. Metal oxide varistors (MOVs) which are voltage-sensitive resistors are used in these circuits to provide surge protection.