What is a motor controller?

The device which can coordinate the performance of an electric motor in a predetermined manner is called a motor controller. A motor controller is a device that is used to start or stop a motor manually or automatically. Also, it helps in selecting forward or reversed rotation/motion and selecting or regulating speed. The regulation and selection of torque and protection against faults are also provided by the motor controller. To regulate the speed and direction of a motor they may use electrochemical switching or power electronic devices.

Use of Motors

Motor controllers are used with both types of current- direct current (DC) and alternating current (AC). It connects the motor with the power supply. It also includes overload protection of the motor and wiring. It also helps in detecting conditions such as low voltage supply, incorrect phase sequence, incorrect polarity, or motors high temperature. This also helps in accelerating the motor by limiting the inrush current. Motor controllers may be fully manual or fully automatic using current sensors or internal timers.

The speed of some electric motors is adjusted using motor controllers. The motor controller may adjust the voltage applied to the motor for DC motors or adjust the current flowing in its windings. There is little or no speed response to adjusting terminal voltage for AC motors. The controllers for AC motors change the rotor resistance or change the frequency of the AC applied to the electric motor to control the speed by using power electronic devices /electromechanical frequency changers. In industries, motors may have more complex controllers connected to the systems of automation. There may be a large number of motor controllers in a factory grouped in a motor control center. The pumped-storage hydroelectric plants use the largest motor controllers and have ratings of tens of thousands of horsepower.

Types of motor controller

The motor controllers can be operated manually, automatically, or remotely. They may have other functions but also they help in stopping or starting a motor.

The types of electric motors are permanent magnet motors, servo motors, series motors, separately excited and alternating current motors. In the form of analog and digital input signals, a motor controller is connected to a battery bank or power supply and control circuitries.

Motor Starters

By connecting the small motor to power it can be started simply. The large motor requires a switching unit which is special in nature called a motor starter or contactor. A direct online starter (DOL) connects the motor immediately to supply when it is energized. Power circuit breakers may be used by very large motors as switching elements.

The full line voltage is applied to the motor terminals when a direct online starter is used. This is one of the simplest starters. This starter may sometimes contain protection devices and condition monitoring in some cases. If the high inrush current does not cause a high amount of voltage drop in the supply circuit then the direct online starter can be used. These starters are sometimes used to start small water pumps, fans, compressors, and conveyor belts. The motor will draw a high amount of current until it has run-up to its full speed in the case of a three-phase squirrel-cage motor. The full load current is less than 6-7 times the starting current. To minimize voltage dip the large motors will have reduced-voltage starters.

Reduced-voltage starters

The motor is connected to supply through a voltage reduction device using a reduced-voltage starter and the voltage is increased gradually or in steps. To provide reduced-voltage starting of a motor, two or more contractors may be provided. At the motor terminals, a lower potential is present by using an auto-transformer, reducing inrush current and starting torque. At the motor terminals, the starter gets switched to its full potential when the motor has come up to a fraction of its full-load speed. Within the motor, the power may be reduced in large 3 phase induction motors.  The full potential is supplied to the motor's outer part field coils when the motor is started direct online. The magnetic field generator by the stator is in contact with the rotor and has a current induced in it due to the field. The magnetic field of the rotor can be varied by connecting the rotor part with external resistance.

Another type of reduced voltage starter is star-delta starters. The star-delta starter has star-connected stator winding that will start a motor. It will start to run in a delta-connected stator winding when it reaches 80% of full load speed.

Servo motor controllers

Common features of servo controllers are that is they have fast acceleration rates, closed-loop, precise position control. The precise speed control servo motors are made from several motor types which are brushless DC motor, brushed DC motor, AC servo-motors. To close the control loop they use position feedback. To directly measure the rotor's position they are implemented with position encoders, hall-effect sensors, and resolvers. Using the pulse-width modulation (PWM) technique these motors can be controlled. The pulse and direction is another control method.

Stepper motor controllers

A stepper motor is a brushless, synchronous, high pole count, and poly-phase motor. Control is done by assuming that the rotor follows a controlled rotating field. Then closed-loop controls, the positioning with steppers is simpler and cheaper. Through chopping, this controller drives the motor with much higher potential. To send a direction pulse to a separate high potential drive circuit a positioning controller called indexer is used which is responsible for current limiting and commutation.

Loss of voltage protection

The starters that use magnetic contactors derive the supply for the coil from the same source as the motor electric supply. To keep the contactor coil energized, an auxiliary contact is used from the contactor after the start command for the motor has been given. Until a new start command is given the contactor will open and not get closed again if a momentary loss of supply potential occurs. After a power failure, this prevents starting of a motor. This connection provides protection against loss of phase and low supply potential.

Context and Application

This topic is significant in the professional exams for Under-Graduate, Graduate, and Post-Graduate courses.

• Bachelors in Electrical Engineering
• Bachelors in Electronics and Communication
• Masters in Electrical Engineering
• Masters in Electronics and Communication

Practice Problems

1. How the PWM control of the DC motor varies with speed?

1. Linearly with speed.
2. Inversely with speed.
3. Parabolically with speed.
4. Exponentially with speed.

Answer: Option a

Explanation:  Duty ratio is a linear function with respect to speed in the PWM technique.

2. Which speed control is the best speed control for small DC motors?

1. Armature voltage control
2. Field resistance control
3. Both
4. None

Answer: Option a

Explanation:  Using field control it is very difficult to control the speed for small DC motors.

3. Which AC motor is used in servo-applications?

1. Single-phase induction motor
2. Synchronous motor
3. 3-phase induction motor
4. 2-phase induction motor

Answer: Option d

Explanation: In 2 phase motors two field windings at 90 degrees are used.

4. Which of the following is a disadvantage of a hybrid stepper motor as compared to a variable reluctance stepper motor?

1. Greater torque per unit volume
2.  High inertia
3. Less tendency to resonate
4. Small step length

Answer: Option b

Explanation:  As compared to hybrid one a variable reluctance motor requires a less excitation current because of the PM excited rotor.

5. How initial inrush current is controlled?

1. Reduced voltage is applied across the stator during the starting period
2. Reduced voltage is applied to the stator when the motor is running at a rated speed
3. Either 1 or 2
4. None

Answer: Option a

Explanation: Initial inrush current is controlled by applying a reduced voltage across the stator during the starting period.

Related concepts

• Motor controller center (MCC)