Motor Controller

A motor controller is a device or group of devices that serves to govern in some predetermined manner the performance of an electric motor. A motor controller might include a manual or automatic means for starting and stopping the motor, selecting forward or reverse rotation, selecting and regulating the speed, regulating or limiting the torque, and protecting against overloads and faults.

Scope of motor controller applications

The scope of motor control technology must be very wide to accommodate the wide variety of motor applications in various fields.

Domestic applications

Electric motors are used domestically in personal care products, small and large appliances, and residential heating and cooling equipment. In most domestic applications, the motor controller functions are built into the product. In some cases, such as bathroom ventilation fans, the motor is controlled by a switch on the wall. Some appliances have provisions for controlling the speed of the motor. Built-in circuit breakers protect some appliance motors, but most are unprotected except that the household fuse or circuit breaker panel disconnects the motor if it fails.

Office equipment, medical equipment etc.

There is a wide variety of motorized office equipment such as personal computers, computer peripherals, copy machines and fax machines as well as smaller items such as electric pencil sharpeners. Motor controllers for these types of equipment are built into the equipment. Some quite sophisticated motor controllers are used to control the motors in computer disc drives. Medical equipment may include very sophisticated motor controllers.

Commercial applications

Commercial buildings have larger heating ventilation and air conditioning (HVAC) equipment than that found in individual residences. In addition, motors are used for elevators, escalators and other applications. In commercial applications, the motor control functions are sometimes built into the motor-driven equipment and sometimes installed separately.

Industrial applications

Many industrial applications are dependent upon motors (or machines), which range from the size of one’s thumb to the size of a railroad locomotive. The motor controllers can be built into the driven equipment, installed separately, installed in an enclosure along with other machine control equipment or installed in motor control centers. Motor control centers are multi-compartment steel enclosures designed to enclose many motor controllers. It is also common for more than one motor controller to operate a number of motors in the same application. In this case the controllers communicate with each other so they can work the motors together as a team.

Vehicle applications

All types of engine-driven vehicles from automobiles, airplanes, aircraft carriers and agricultural equipment to zambonis may have electric motors to perform a variety of functions. In electric vehicles, diesel-electric vehicles, and hybrid vehicles, electric motors are used to propel the vehicle. The motor controllers in vehicle applications are integrated into the vehicle.

Power tools

Power tools such as drills, saws and sanders are widely used by home owners, hobbyists, construction and repair trades people, and industrial workers. Both portable and stationary power tools usually have built in motor controllers and often include an adjustable speed feature.

Hobby equipment

A variety of hobbies make use of specialized motorized equipment that is similar to domestic appliances or portable tools.

Radio controlled (R/C) models may include fairly sophisticated motor controllers. The motor controllers are ultimately built into the equipment, but the hobbyist may purchase the controller separately or construct it.

Robotics is another area in which the hobbyist may purchase a motor controller as a separate item or construct it.

Motor control centers

A motor control center (MCC) is an assembly of one or more enclosed sections having a common power bus and principally containing motor control units. Motor control centers are in modern practice a factory assembly of several motor starters. A motor control center can also include variable frequency drives, programmable controllers, and metering and may also be the electrical service entrance for the building. Motor control centers are usually used for low voltage three-phase alternating current motors from 230 volts to 600 volts. Medium-voltage motor control centers are made for large motors running at 2300 V to around 15000 V, using vacuum contactors for switching and with separate compartments for power switching and control.

Motor control centers have been used since 1950 by the automobile manufacturing industry which used large numbers of electric motors. Today they are used in many industrial and commercial applications. Where very dusty or corrosive processes are used, the motor control center may be installed in a separate air-conditioned room, but often an MCC will be on the factory floor adjacent to the machinery controlled.

A motor control center consists of one or more vertical metal cabinet sections with power bus and provision for plug-in mounting of individual motor controllers. Very large controllers may be bolted in place but smaller controllers can be unplugged from the cabinet for testing or maintenance. Each motor controller contains a contactor (or in more recent installations, a solid-state motor controller), overload relays to protect the motor, fuses or a circuit breaker to provide short-circuit protection, and a disconnecting switch to isolate the motor circuit. Three-phase power enters each controller through separable connectors. The motor is wired to terminals in the controller. Motor control centers provide wire ways for field control and power cables.

Each motor controller in an MCC can be specified with a range of options such as separate control transformers, pilot lamps, control switches, extra control terminal blocks, various types of bi-metal and solid-state overload protection relays, or various classes of power fuses or types of circuit breakers. A motor control center can either be supplied ready for the customer to connect all field wiring, or can be an engineered assembly with internal control and interlocking wiring to a central control terminal panel board or programmable controller.

Motor control centers (MCC) usually sit on floors, which are often required to have a fire-resistance rating. Firestops may be required for cables that penetrate fire-rated floors and walls.

Types of motor controllers

Motor controllers can be manually, remotely or automatically operated. They may include only the means for starting and stopping the motor or they may include other functions.

An electric motor controller can be classified by the type of motor it is to drive such as permanent magnet, servo, series, separately excited, and alternating current.

A motor controller is connected to a power source such as a battery pack or power supply, and control circuitry in the form of analog or digital input signals.

Motor starters

Main article: Direct on line starter
Main article: Motor soft starter

A small motor can be started by simply plugging it into an electrical receptacle or by using a switch or circuit breaker. A larger motor requires a specialized switching unit called a motor starter or motor contactor. When energized, a direct on line (DOL) starter immediately connects the motor terminals directly to the power supply. A motor soft starter connects the motor to the power supply through a voltage reduction device and increases the applied voltage gradually or in steps.

Adjustable-speed drives

Main article: Adjustable-speed drive

An adjustable-speed drive (ASD) or variable-speed drive (VSD) is an interconnected combination of equipment that provides a means of driving and adjusting the operating speed of a mechanical load. An electrical adjustable-speed drive consists of an electric motor and a speed controller or power converter plus auxiliary devices and equipment. In common usage, the term “drive” is often applied to just the controller.

Speed controls for AC induction motors

Recent developments in drive electronics have allowed efficient and convenient speed control of these motors, where this has not traditionally been the case. The newest advancements allow for torque generation down to zero speed. This allows the polyphase AC induction motor to compete in areas where DC motors have long dominated, and presents an advantage in robustness of design, cost, and reduced maintenance.

Variable frequency drives

Main article: Variable-frequency drive

Phase vector drives

Phase vector drives (or simply vector drives) are an improvement over variable frequency drives (VFDs) in that they separate the calculations of magnetizing current and torque generating current. These quantities are represented by phase vectors, and are combined to produce the driving phase vector which in turn is decomposed into the driving components of the output stage. These calculations need a fast microprocessor, typically a DSP device.

Unlike a VFD, a vector drive is a closed loop system. It takes feedback on rotor position and phase currents. Rotor position can be obtained through an encoder, but is often sensed by the reverse EMF generated on the motor leads.

In some configurations, a vector drive may be able to generate full rated motor torque at zero speed.

Main article: Vector control (motor)

Direct torque control drives

Direct torque control has better torque control dynamics than the PI-current controller based vector control. Thus it suits better to servo control applications. However, it has some advantage over other control methods in other applications as well because due to the faster control it has better capabilities to damp mechanical resonances and thus extend the life of the mechanical system.

Main article: Direct Torque Control

Brushed DC motor speed or torque controls

Main article: Torque and speed of a DC motor

These controls are applicable to brushed DC motors with either a wound or permanent magnet stator. A valuable characteristic of these motors is that they are easily controlled in torque, the torque being fairly directly proportional to the driving current. Speed control is derived by simply modulating the motor torque.

SCR or thyristor drive

Main article: Thyristor drive

SCR controls for DC motors drive power from AC power, and send rectified voltage to the motor. These controls are very common in industry, running from line voltages, with motors rated at 90V for 120V line, and 180V for a 240V line. They are available in reversing and non-reversing models. They are robust, with a minimum of electronic components. The waveform sent to the motor will have strong harmonic components due to the switching at line frequency. This results in current and torque ripple, and an audible hum.

PWM or chopper drives

PWM controls use pulse width modulation to regulate the current sent to the motor. Unlike SCR controls which switch at line frequency, PWM controls produce smoother current at higher switching frequencies, typically between 1 and 20 kHz. At 20 kHz, the switching frequency is inaudible to humans, thereby eliminating the hum which switching at lower frequency produces. However, some motor controllers for radio controlled models make use of the motor to produce audible sound, most commonly simple beeps.

A PWM controller typically contains a large reservoir capacitor and an H-bridge arrangement of switching elements (thyristors, Mosfets or transistors).

Servo controllers

Main article: Servo drive
Main article: Servomechanism

Servo controllers is a wide category of motor control. Common features are:

  • precise closed loop position control
  • fast acceleration rates
  • precise speed control

Servo motors may be made from several motor types, the most common being

  • brushed DC motor
  • brushless DC motors
  • AC servo motors

Servo controllers use position feedback to close the control loop. This is commonly implemented with encoders, resolvers, and Hall effect sensors to directly measure the rotor‘s position. Others measure the back EMF in the undriven coils to infer the rotor position, and therefore are often called “sensorless” controllers.

A servo may be controlled using pulse-width modulation (PWM). How long the pulse remains high (typically between 1 and 2 milliseconds) determines where the motor will try to position itself. Another control method is pulse and direction.

Stepper motor controllers

Main article: stepping motor

A stepper, or stepping, motor is a synchronous, brushless, high pole count, polyphase motor. Control is usually, but not exclusively, done open loop, ie. the rotor position is assumed to follow a controlled rotating field. Because of this, precise positioning with steppers is simpler and cheaper than closed loop controls.

Modern stepper controllers drive the motor with much higher voltages than the motor nameplate rated voltage, and limit current through chopping. The usual setup is to have a positioning controller, known as an indexer, sending step and direction pulses to a separate higher voltage drive circuit which is responsible for commutation and current limiting.

Relevant circuits to motor control


Main article: H-bridge

DC motors are typically controlled by using a transistor configuration called an “H-bridge“. This consists of a minimum of four mechanical or solid-state switches, such as two NPN and two PNP transistors. One NPN and one PNP transistor are activated at a time. Both NPN or PNP transistors can be activated to cause a short across the motor terminals, which can be useful for slowing down the motor from the back EMF it creates.


See also

External links


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