Motor controller


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.[1] 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.[2]

Contents

Applications

Every electric motor has to have some sort of controller. The motor controller will have differing features and complexity depending on the task that the motor will be performing.

The simplest case is a switch to connect a motor to a power source, such as in small appliances or power tools. The switch may be manually operated or may be a relay or contactor connected to some form of sensor to automatically start and stop the motor. The switch may have several positions to select different connections of the motor. This may allow reduced-voltage starting of the motor, reversing control or selection of multiple speeds. Overload and overcurrent protection may be omitted in very small motor controllers, which rely on the supplying circuit to have overcurrent protection. Small motors may have built-in overload devices to automatically open the circuit on overload. Larger motors have a protective overload relay or temperature sensing relay included in the controller and fuses or circuit breakers for overcurrent protection. An automatic motor controller may also include limit switches or other devices to protect the driven machinery.

More complex motor controllers may be used to accurately control the speed and torque of the connected motor (or motors) and may be part of closed loop control systems for precise positioning of a driven machine. For example, a numerically controlled lathe will accurately position the cutting tool according to a preprogrammed profile and compensate for varying load conditions and perturbing forces to maintain tool position.

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.[2][3][4]

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

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. Reduced-voltage, star-delta or soft starters connects the motor to the power supply through a voltage reduction device and increases the applied voltage gradually or in steps.[2][3][4] In smaller sizes a motor starter is a manually-operated switch; larger motors, or those requiring remote or automatic control, use magnetic contactors. Very large motors running on medium voltage power supplies (thousdands of volts) may use power circuit breakers as switching elements.

A direct on line (DOL) or across the line starter applies the full line voltage to the motor terminals. This is the simplest type of motor starter. A DOL motor starter also contain protection devices, and in some cases, condition monitoring. Smaller sizes of direct on-line starters are manually operated; larger sizes use an electromechanical contactor (relay) to switch the motor circuit. Solid-state direct on line starters also exist.

A direct on line starter can be used if the high inrush current of the motor does not cause excessive voltage drop in the supply circuit. The maximum size of a motor allowed on a direct on line starter may be limited by the supply utility for this reason. For example, a utility may require rural customers to use reduced-voltage starters for motors larger than 10 kW.[5]

DOL starting is sometimes used to start small water pumps, compressors, fans and conveyor belts. In the case of an asynchronous motor, such as the 3-phase squirrel-cage motor, the motor will draw a high starting current until it has run up to full speed. This starting current is typically 6-7 times greater than the full load current. To reduce the inrush current, larger motors will have reduced-voltage starters or variable speed drives in order to minimise voltage dips to the power supply.

A reversing starter can connect the motor for rotation in either direction. Such a starter contains two DOL circuits—one for clockwise operation and the other for counter-clockwise operation, with mechanical and electrical interlocks to prevent simultaneous closure.[5] For three phase motors, this is achieved by transposing any two phases. Single phase AC motors and direct-current motors require additional devices for reversing rotation.

Reduced voltage starters

Two or more contactors may be used to provide reduced voltage starting of a motor. By using an autotransformer or a series inductance, a lower voltage is present at the motor terminals, reducing starting torque and inrush current. Once the motor has come up to some fraction of its full-load speed, the starter switches to full voltage at the motor terminals. Since the autotransformer or series reactor only carries the heavy motor starting current for a few seconds, the devices can be much smaller compared to continuously-rated equipment. The transition between reduced and full voltage may be based on elapsed time, or triggered when a current sensor shows the motor current has begun to reduce. An autotransformer starter was patented in 1908.

Adjustable-speed drives

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.[3][4]

Intelligent controllers

An Intelligent Motor Controls (IMC) uses a microprocessor to control power electronic devices used for motor control. IMCs monitor the load on a motor and accordingly match motor torque to motor load. This is accomplished by reducing the voltage to the AC terminals and at the same time lowering current and kvar. This can provide a measure of energy efficiency improvement for motors that run under light load for a large part of the time, resulting in less heat, noise, and vibrations generated by the motor.

Overload relays

A starter will contain protective devices for the motor. At a minimum this would include a thermal overload relay. The thermal overload is designed to open the starting circuit and thus cut the power to the motor in the event of the motor drawing too much current from the supply for an extended time. The overload relay has a normally closed contact which opens due to heat generated by excessive current flowing through the circuit. Thermal overloads have a small heating device that increases in temperature as the motor running current increases.

There are two types of thermal overload relay. In one type, a bi-metallic strip located close to a heater deflects as the heater temperature rises until it mechanically causes the device to trip and open the circuit, cutting power to the motor should it become overloaded. A thermal overload will accommodate the brief high starting current of a motor while accurately protecting it from a running current overload. The heater coil and the action of the bi-metallic strip introduce a time delay that affords the motor time to start and settle into normal running current without the thermal overload tripping. Thermal overloads can be manually or automatically resettable depending on their application and have an adjuster that allows them to be accurately set to the motor run current.

A second type of thermal overload relay uses a eutectic alloy, like a solder, to retain a spring-loaded contact. When too much current passes through the heating element for too long a time, the alloy melts and the spring releases the contact, opening the control circuit and shutting down the motor. Since eutectic alloy elements are not adjustable, they are resistant to casual tampering but require changing the heater coil element to match the motor rated current.[5]

Electronic digital overload relays containing a microprocessor may also be used, especially for high-value motors. These devices model the heating of the motor windings by monitoring the motor current. They can also include metering and communication functions.

Loss of voltage protection

Starters using magnetic contactors usually derive the power supply for the contactor coil from the same source as the motor supply. An auxiliary contact from the contactor is used to maintain the contactor coil energized after the start command for the motor has been released. If a momentary loss of supply voltage occurs, the contactor will open and not close again until a new start command is given. this prevents restarting of the motor after a power failure. This connection also provides a small degree of protection against low power supply voltage and loss of a phase. However since contactor coils will hold the circuit closed with as little as 80% of normal voltage applied to the coil, this is not a primary means of protecting motors from low voltage operation.[5]

Motor control centers

A small, early 1960's-vintage motor control center for 480 volt motors.

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.[1] Motor control centers are in modern practice a factory assembly of several motor starters. A motor control center can 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 208 V to 600 V. 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.[6]

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 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.

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.[4]

Variable frequency drives

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.

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.

Brushed DC motor speed or torque controls

An industrial grade first quadrant PWM DC-motor controller

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 convert AC power to direct current, with adjustable voltage. Small DC drives are common in industry, running from line voltages, with motors rated at 90 V for 120 V line, and 180 V for a 240 V line. Larger drives, up to thousands of horsepower, are powered by three phase supplies and are used in such applications as rolling mills, paper machines, excavators, and ship propulsion. DC drivers are available in reversing and non-reversing models. The waveform of the current through the motor by a single-phase drive will have strong ripple components due to the switching at line frequency. This can be reduced by use of a poly phase supply or smoothing inductors in the motor circuit; otherwise the ripple currents produce motor heating, excess noise, and loss of motor torque.

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, solid state relays, or 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.

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.

Other position feedback methods measure the back EMF in the undriven coils to infer the rotor position, or detect the Kick-Back voltage transient (spike) that is generated whenever the power to a coil is instantaneously switched off. These are therefore often called "sensorless" control methods.

Sensorless control methods

Ripple Counting

Ripple counting works on the 'law of induction', or more specifically Lenz's law, which says that the magnetic field of any induced current opposes the charge that induces it. This so-called back EMF (sometimes called the counter electromotive force) can be detected by measuring the current flowing through each coil as the motor rotates.[7]

In a fully encapsulated motor and particularly a multi-pole motor this is difficult to measure. Therefore ripple counting usually relies on measuring the voltage variations over a small resistor inserted in one of the power supply wires to the motor. The result is a voltage curve representing the accumulated currents running through the coils of the motor assembly as the motor rotates.

The current ripple waveform characteristics are highly dependent of a number of factors such as the supply voltage and the actual load, speed, direction and temperature of the motor. Other factors such as the in-rush current, aging of motor parts and electromagnetic interference can also influence the ripple waveform. The amplitude and the shape of the waveform can vary significantly due to these factors. In other applications noise transients superposed onto the ripple current waveform can generate false counting pulses.

It has proven difficult to design a detection system based on ripple counting that can be used to precisely and reliably count the number of commutations of a rotating motor from start to stop. Numerous attempts to seek to improve the reliability of ripple counting have been described in the literature, examples in.[8][9]

Transient Counting

Transient counting works on the basic principle of Ohm's law and the behavior of a collapsing magnetic field in which a Back-Fire or Kick-Back transient (spike) is generated whenever the power to a coil is instantaneously switched off.[10]

At each commutation point, when the brush breaks contact with a commutation segment, the energy stored in the motor winding as a magnetic field causes an arc or voltage spike between the brush and the commutator segment. This occurs not only during normal commutation but also in situations where the brushes bounce on the rotating commutator.

A dedicated transient detector circuit (in effect a high pass filter) detects the Kick-Back spike from the collapsing magnetic field in the coil when the power to the coil is turned off. The Kick-Back transients trigger the modulation of an electronic encoder signal for each of the motor commutations. Thus an N pole motor will encode N signals per rotation. The Kick-Back spikes can be measured anywhere on the power supply wires to the motor. The encoded signal can be used as position feedback in the servo controller.

The performance of transient counting is by and large unaffected by the parameters which are influencing ripple counting. Whether the motor is powered or is coasting in generator mode driven by the inertia of a load has no influence on the counting reliability.[11] The amplitude of the Kick-Back transients is mainly influenced by the conductivity of the surrounding air. This is because the intensity of the arc generated between a brush and a commutator depends on the air conductivity.[2]

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, i.e. 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.

See also

References

  1. ^ a b National Fire Protection Association (2008). "Article 100 Definitions". NFPA 70 National Electrical Code. 1 Batterymarch Park, Quincy, MA 02169: NFPA. pp. 24. http://www.nfpa.org/aboutthecodes/AboutTheCodes.asp?DocNum=70&cookie%5Ftest=1. Retrieved January 2008. 
  2. ^ a b c Siskind, Charles S. (1963). Electrical Control Systems in Industry. New York: McGraw-Hill, Inc.. ISBN 0070577463. 
  3. ^ a b c National Fire Protection Association (2008). "Article 430 Motors, Motor Circuits and Controllers". NFPA 70 National Electrical Code. 1 Batterymarch Park, Quincy, MA 02169: NFPA. pp. 298. http://www.nfpa.org/aboutthecodes/AboutTheCodes.asp?DocNum=70&cookie%5Ftest=1. Retrieved January 2008. 
  4. ^ a b c d Campbell, Sylvester J. (1987). Solid-State AC Motor Controls. New York: Marcel Dekker, Inc.. ISBN 0-8247-7728-X. 
  5. ^ a b c d Terrell Croft and Wilford Summers (ed), American Electricans' Handbook, Eleventh Edition, McGraw Hill, New York (1987) ISBN 0-07013932-6 pages 78-150 through 7-159
  6. ^ Robert W. Smeaton (ed) Switchgear and Control Handbook 3rd Ed., Mc Graw Hill, New York 1997 ISBN 0-07-058451-6, chapter 26
  7. ^ Rajaram, S.; Murugesan, S.; "A New Method for Speed Measurement/Control of DC Motors", IEEE Trans. on Instrumentation and Measurement, Vol. IM-27, No. 1, March 1978
  8. ^ Afjei, E.; Ghomsheh, A.N.; Karami, A.; "Sensorless Speed/Position Control of Brushed DC Motor", IEEE Int. Aegean Conf. on Electrical Machines and Power Electronics, Sept. 2007, pp 730-732
  9. ^ U.S. patent no 6,144,179, "Method for establishing the rotational speed of mechanically commutated D.C. motors", Temic Telefunken Microelectronic GmbH
  10. ^ (Pend.) Patent No. WO2010/040349 A1, "A power supply system and method for controlling a mechanically commutated electric motor", IDEAssociates (IoM) Ltd
  11. ^ [1]

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