Adjustable speed drivers (ASD) or variable speed drives (VSD) describe the equipment used to control engine speed. Many industrial processes such as assembly lines must operate at different speeds for different products. Where the process conditions demand flow adjustment from the pump or fan, varying the drive speed can save energy compared to other techniques for flow control.
Where speed can be chosen from several different pre-set ranges, usually the drive is said to be adjustable speed. If the output speed can be changed without the steps in the range, the drive is usually referred to as variable speed .
The adjustable speed drivers and variables may be purely mechanical (called variators ), electromechanical, hydraulic, or electronic.
Video Adjustable-speed drive
Electric motors
AC electric motors can be run in fixed speed operation determined by the number of stator pole pairs in the motor and the frequency of alternating current supply. AC motors can be made for "pole change" operations, reconnecting stator windings to vary the number of poles so that two, sometimes three, velocities are obtained. For example a machine with 8 pairs of physical poles, can be connected to allow to run with either 4 or 8 pairs of poles, giving two speeds - at 60 Hz, this would be 1800 RPM and 900 RPM. If speed changes are rare, motors may initially be connected for one speed then reconnected to other speeds due to changing process conditions, or, magnetic contactors can be used to switch between two speeds as the process requirements fluctuate. Connection for more than three speeds is not economical.
The number of such fixed speed operating speeds is limited by the cost when the number of pole pairs increases. If many different speeds or continuous variable speeds are required, other methods are required.
Direct current motor allows speed changes by adjusting shunt field currents. Another way to change the speed of a direct current motor is to change the applied voltage to the armature.
The adjustable speed drive may consist of an electric motor and the controller used to adjust the motor's operating speed. The combination of constant speed motors and constantly adjustable mechanical speed changer devices can also be referred to as customizable speed drivers. The electronic variable-based variable hard disk quickly makes the old technology redundant.
Maps Adjustable-speed drive
Reasons to use customizable speed drivers
Process control and energy conservation are two main reasons for using customizable speed drivers. Historically, customizable speed drivers have been developed for process control, but energy conservation has emerged as an equally important goal.
Accelerated control
The adjustable speed drive can often provide smoother operation compared to the alternative fixed speed operating mode. For example, in the sewage sewer the exhaust station usually flows through the sewer pipe under the gravity to the wet well location. From there it is pumped into the treatment process. When a fixed speed pump is used, the pump is set to begin when the liquid level in the wet well reaches some high point and stops when the level has been reduced to a low point. Cycling pumps on and off results in frequent high current electric waves to start motors that produce electromagnetic and thermal voltages in motors and power control equipment, pumps and pipes undergoing mechanical and hydraulic pressures, and waste treatment processes are forced to accommodate spikes of waste flows through process. When adjustable speed drivers are used, the pump operates continuously with increasing speed as the wet well level increases. This matches the outflow to the average inlet and provides a smoother process operation.
Save energy by using customizable speed drivers
Fans and pumps consume most of the energy used by industrial electric motors. If fans and pumps serve varying process loads, a simple way to vary the amount of liquid delivered is by a damper or valve in the fan or pump outlet, which due to the increased pressure drop, reduces the flow in the process. However, this additional pressure drop indicates a loss of energy. Sometimes it is economically practical to include some devices that recover this lost energy. With variable speed drivers at the pump or fan, the supply can be adjusted to match demand and no additional losses are introduced.
For example, when the fan is driven directly by a fixed-speed motor, the airflow is designed for maximum system demand, and will usually be higher than it should be. Airflow can be set using a silencer but more efficient to directly adjust the speed of the fan motor. Following affinity laws, for 50% of airflow, variable speed motors consume about 20% of input power (Amps). The fixed speed of the motor still consumes about 85% of the input power at half flow.
Type of adjustable speed drive
Some of the main drivers (internal combustion engines, reciprocating steam engines or turbines, water wheels, and others) have a range of operating speeds that can be varied continuously (by adjusting fuel rates or similar way). However, the efficiency may be low in the extreme speed range, and there may be a reason why the main driving speed can not be maintained at very low or very high speeds.
Before an electric motor is invented, a mechanical speed modifier is used to control the mechanical power provided by the water wheel and steam engine. When electric motors begin to be used, the way to control their speed is developed immediately. Currently, various types of mechanical drive, hydraulic drive, and electric drive compete with each other in the industrial drive market.
Mechanically adjustable speed drivers
There are two types of mechanical drive, variable pitch drive, and traction drive.
The variable pitch drive is a pulley and belt drive where the pitch diameter of one or both pulleys can be adjusted.
Traction power drives transmission power through metal rollers that run against mating metal rollers. The input/output speed ratio is adjusted by moving the rollers to change the diameter of the contact path. Many different forms of rollers and mechanical designs have been used.
Drive adjustable hydraulic speed
There are three types of hydraulic actuators, namely: hydrostatic actuators, hydrodynamic propulsion and hydroviskous propulsion.
The hydrostatic drive consists of a hydraulic pump and a hydraulic motor. Because positive displacement pumps and motors are used, one pump or motor rotation corresponds to a set of fluid flow volumes determined by displacement regardless of speed or torque. Speed ​​is regulated by adjusting the fluid flow with the valve or by changing the pump or motor displacement. Many different design variations have been used. A swash plate drive uses axial piston pump and/or motor where the swash plate angle can be changed to adjust the displacement and thus adjust the speed.
Hydrodynamic drives or liquid couplings use oil to transmit torque between the impeller at the constant input speed shaft and the rotor on the output speed shaft that can be adjusted. Torque converter in automatic car transmission is a hydrodynamic drive.
The hydroviscous drive consists of one or more discs connected to the input shaft pressed to the same disc or disc connected to the output shaft. The torque is transmitted from the input shaft to the output shaft through the oil film between the discs. The transmitted torque is proportional to the pressure provided by the hydraulic cylinder that suppresses the shared disc.
Continuously variable transmission (CVT)
The mechanically and hydraulically adjustable speed converters are usually called continuous variable transmissions or transmissions when used in vehicles, farm equipment and some other types of equipment.
Electrically adjustable speed drives
Control type
Control can mean it can be manually adjusted - via a potentiometer or linear hall gauge, (which is more resistant to dust and grease) or can be controlled automatically eg by using a rotation detector such as Gray Gray optical encoder.
Drive type
There are three general categories of electric drive: DC motor drive, eddy drive drive and AC motor drive. Each of these common types can be subdivided into many variations. Electric motors generally include electric motors and units or speed control systems. The term drive is often applied to the controller without the motor. In the early days of electric drive technology, electromechanical control systems were used. Then, the electronic controller is designed using various types of vacuum tubes. When suitable solid state electronic components become available, the new controller design incorporates the latest electronic technology.
DC drive
DC drive is a DC motor speed control system. Since the speed of a DC motor is directly proportional to the anchor voltage and inversely proportional to the motor flux (which is a function of the field current), the anchor voltage or field current can be used to control the speed. Some types of DC motors are described in electric motor articles. The electric motor article also explains the electronic speed control that is used with different types of DC motors.
Drive Eddy currently
The eddy current drive consists of a fixed speed motor and a navel current clutch. The coupling contains a fixed speed rotor and an adjustable speed rotor separated by a small air gap. Direct current in the field coil produces a magnetic field that determines the torque transmitted from the input rotor to the output rotor. The controller provides a closed loop rate setting by varying the coupling current, allowing only the clutch to transmit sufficient torque to operate at the desired speed. Speed ​​feedback is usually provided through an integral AC tachometer.
Eddy's current drive is a slip-controlled system, an energy slip that is always dissipated as heat. Therefore, such drives are generally less efficient than AC/DC-AC conversion drives. The motor develops the torque required by the load and operates at full speed. The output shaft transmits the same torque to the load, but rotates at a slower speed. Because the power is proportional to the torque multiplied by the speed, the input power is proportional to the timing speed of the motor that operates the torque while the output power is the output velocity of the torque operation time. The difference between motor speed and output speed is called slip speed . Power is proportional to the slip rate of torsion operation lost as heat in the clutch.
AC drive
AC drive is an AC motor speed control system.
A slip-controlled rotor wound-induction motor (WRIM) drive controls the speed by varying the motor slip through the rotor slip ring either by recovery of the electronic power slip which is fed back to the stator bus or by varying the resistance of the external resistor at rotor circuit. Along with eddy current drives, WRIM-based endurance drives have lost popularity because they are less efficient than WRIM drives based on AC/DC-AC and are only used in special situations.
Slipping the energy recovery system returns energy to the WRIM stator bus, changing the slip energy and returning it to the stator supply. Such recovered energy will be wasted as heat on a WRIM-driven resistance drive. Slip energy recovery variable-speed drives are used in applications such as large pumps and fans, wind turbines, ship propulsion systems, large hydro-pumps/generators and utility energy-style energy wheels. Early slip energy recovery systems use electromechanical components for AC/DC-AC conversion (ie, comprising rectifiers, DC motors and AC generators) called Kramer drives , newer systems use variable frequency drives (VFDs) ) called the static Kramer drive .
In general, the VFD in the most basic configuration controls the speed of an induced or synchronous motor by adjusting the power frequency supplied to the motor.
When changing the VFD frequency in a standard low-performance torque app using the Volt-per-Hertz (V/Hz) control, the AC-frequency-to-frequency ratio of the AC motor can be maintained constant, and its power may vary, between the minimum and maximum operating frequency to frequency basic. Operation voltage is constant above the base frequency, and hence with reduced V/Hz ratio, providing reduced torque and constant power capability.
A regenerative AC drive is a type of AC drive that has the capacity to recover a load braking energy that travels faster than motor speed (overhaul load) and returns it to the power system.
The VFD article provides additional information about electronic speed controls used with different types of AC motors.
See also
- Continuous variable transmission
- Regenerative variable frequency drive
- Variable frequency drive
- DC injection braking
- Provision of recurring electric machines
References
- Spitzer, David W. (1990). Variable Drive Speed ​​. Society of American Instruments. ISBN: 1-55617-242-7.
- Campbell, Sylvester J. (1987). Solid-State AC Control Motor . New York: Marcel Dekker, Inc. ISBNÃ, 0-8247-7728-X.
- Jaeschke, Ralph L. (1978). Power Transmission Control System . Cleveland, OH: Penton/IPC.
- Siskind, Charles S. (1963). Industrial Electricity Control System . New York: McGraw-Hill, Inc. ISBN: 0-07-057746-3. Ã,
Source of the article : Wikipedia
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