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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.

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 centres in buildings

Motor control centres usually sit on floors, which are often required to have a fire-resistance rating. MCCs have cable running in and out of their cabinets through that concrete slab below, as well as above, where those are often routed through walls that often have a fire-resistance rating as well. This necessitates the use of firestops. The floor openings are usually the same size as the footprint of each cabinet. To meet the requirements of the local building code and fire code, it is necessary to seal these through penetrations with firestops that are acceptable to the Authority Having Jurisdiction through listing and approval use and compliance. This applies both to the initial construction, as well as to cable changes that may occurr throughout the life of the building.

Types of motor controllers

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.

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 present an advantage in robustness of design, cost, and reduced maintenance.

Variable Frequency Drives

Phase Vector Drives

Phase vector drives (or simply vector drives) are an improvement over vanilla VFD's 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.

Direct Torque Control Drives

Brushed DC Motor Speed or Torque Controls

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

SCR controls for DC motors derive 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 power transistors.

Servo Controllers

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

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

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.

References

• Operation Principle of Stepper Motor
• "Dallas Personal Robotics Group". Brief H-Bridge Theory of Operation. Retrieved July 7. {{cite web}}: Check date values in: |accessdate= (help); Unknown parameter |accessyear= ignored (|access-date= suggested) (help)
• "Frogfot Electronics". H-bridge (DC motor controller). Retrieved July 7. {{cite web}}: Check date values in: |accessdate= (help); Unknown parameter |accessyear= ignored (|access-date= suggested) (help)
• Links to manufacturers, associations, and other resources

Software

• Parallel Port PWM/Encoder Linux Driver - a poor-mans motor controller software driver

See also

• Motor Soft Starter
• Direct on line starter
• Adjustable-speed drive
• Electronic speed control
• Variable-frequency drive
• Thyristor drive
• DC motor starter section of Electric motor
• Firestop
• Cable
• Passive fire protection

External links

• Electric Motors Complete Guide
• Closed Loop Speed and Position Control of DC motors

• Automation
• BEV components
• Electrical engineering
• Electric motors
• Electromagnetic components
• Power components
• Power electronics
• Passive fire protection