Устройство регулирования работы электродвигателя

Device for regulating the performance of a motor
A motor controller is a device or group of devices that can coordinate in a 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 electrical faults. Motor controllers may use electromechanical switching, or may use power electronics devices to regulate the speed and direction of a motor.

Motor controllers are used with both direct current and alternating current motors. A controller includes means to connect the motor to the electrical power supply, and may also include overload protection for the motor, and over current protection for the motor and wiring. A motor controller may also supervise the motor's field circuit, or detect conditions such as low supply voltage, incorrect polarity or incorrect phase sequence, or high motor temperature. Some motor controllers limit the inrush starting current, allowing the motor to accelerate itself and connected mechanical load more slowly than a direct connection. Motor controllers may be manual, requiring an operator to sequence a starting switch through steps to accelerate the load, or may be fully automatic, using internal timers or current sensors to accelerate the motor. Some types of motor controllers also allow adjustment of the speed of the electric motor. For direct current motors, the controller may adjust the voltage applied to the motor, or adjust the current flowing in the motor's field winding. Alternating current motors may have little or no speed response to adjusting terminal voltage, so controllers for alternating current instead adjust rotor circuit resistance (for wound rotor motors) or change the frequency of the AC applied to the motor for speed control using power electronic devices or electromechanical frequency changers. The physical design and packaging of motor controllers is about as varied as that of electric motors themselves. A wall mounted toggle switch with suitable ratings may be all that is needed for a household ventilation fan. Power tools and household appliances may have a trigger switch that only turns the motor on and off. Industrial motors may be more complex controllers connected to automation systems; a factory may have a large number of motor controllers grouped in a motor control center. Controllers for electric travelling cranes or electric vehicles may be mounted on the mobile equipment. The largest motor controllers are used with the pumping motors of pumped storage hydroelectric plants, and may carry ratings of tens of thousands of horsepower (kilowatts).

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.

A small motor can be started by simply connecting it to power. 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. 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 (thousands 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 often contains protection devices (see below), and in some cases, condition monitoring. Smaller sizes of direct on line starters are manually operated; larger sizes use an electromechanical contactor 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. 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 adjustable 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. The above function achieved by using a power contactor and timer in automatic star delta starter. The automatic star delta starter is manufactured from three contactors, a timer and a thermal overload. The contactors are smaller than the single contactor used in a direct on line starter as they are controlling winding currents only. The currents through the winding are 1/root 3 (58%) of the current in the line. There are two contactors that are close during run, often referred to as the main contractor and the delta contactor. These are AC3 rated at 58% of the current rating of the motor. The third contactor is the star contactor and that only carries star current while the motor is connected in star. The current in star is one third of the current in delta, so this contactor can be AC3 rated at one third (33%) of the motor rating. The transition from star to delta can be an open transition or a closed transition. During open transition, the motor starter momentarily disconnects from the motor and reconnects in a delta configuration. In closed transition, the transition from the star to delta configuration is achieved without disconnecting the motor. In order to achieve that, an additional three pole contactor and three resistors are required.

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.

An Intelligent Motor Controller (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.

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 bimetallic 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.

After the electrical power has been restored (typically after a time delay of 30 to 60 seconds), then the time sequences of the automatic restarts of multiple motors are set to automatically begin. Without a time sequenced schedule, any attempt to restart many motors simultaneously could lead to partial or total site wide power failure.

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.