Adjustable-speed drive
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Adjustable speed drive (ASD) is one of the most general terms applied to equipment used to control the speed of machinery. Adjustable speed drives are also known as variable speed drives (VSD).
Industrial machinery is often driven by electric motors that have provisions for speed adjustment. Such motors are simply larger, more powerful versions of those driving familiar appliances such as food blenders or electric drills. These motors normally operate at a fixed speed. If speed control is required that controller is called an adjustable speed drive. Adjustable speed drives are used in a wide variety of industrial applications. They are used in the larger industrial versions of the food blenders and power tools mentioned. Adjustable speed drives are often used with fans to provide adjustable airflow in large heating and air conditioning systems. The flow of water and chemicals in industrial processes is often controlled by adjusting the speed of pumps.
An adjustable speed drive might consist of an electric motor and controller that is used to adjust the motor's operating speed. The combination of a constant-speed motor and a steplessly adjustable mechanical speed-changing device might also be called an adjustable speed drive.
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[edit] Reasons for using adjustable speed drives
Process control and energy conservation are the two primary reasons for using an adjustable speed drive. Historically, adjustable speed drives were developed for process control, but energy conservation has emerged as an equally important objective.
[edit] Adjusting speed as a means of controlling a process
Consider the process of driving to work. If you drove to work at the highest possible speed, you would probably cause an accident. If you drove at a single speed that would be safe for every part of the route, it would take too long to get to your destination. Adjusting your speed to suit the route minimizes the time to achieve the objective of the process within the limits of reliable operation.
The following are process control benefits that might be provided by an adjustable speed drive:
- Smoother operation
- Acceleration control
- Different operating speed for each process recipe
- Compensate for changing process variables
- Allow slow operation for setup purposes
- Adjust the rate of production
- Allow accurate positioning
- Control torque or tension
[edit] Example
An adjustable speed drive can often provide smoother operation compared to an alternative fixed speed mode of operation. For example, in a sewage lift station sewage usually flows through sewer pipes under the force of gravity to a wet well location. From there it is pumped up to a treatment process. When fixed speed pumps are used, the pumps are set to start when the level of the liquid in the wet well reaches some high point and stop when the level has been reduced to a low point. Cycling the pumps on and off results in frequent high surges of electrical current to start the motors resulting in electromagnetic and thermal stresses in the motors and power control equipment. The pumps and pipes are subjected to mechanical and hydraulic stresses. The sewage treatment process is forced to accommodate surges in the flow of sewage through the process. When adjustable speed drives are used, the pumps operate continuously at a speed that increases as the wet well level increases. This matches the outflow to the average inflow and provides a much smoother operation of the process.
[edit] Saving energy by using adjustable speed drives
An adjustable speed drive often uses less energy than an alternative fixed speed mode of operation. Fans and pumps are the most common energy saving applications. When a fan is driven by a fixed speed motor, the airflow may sometimes be higher than it needs to be. Airflow can be regulated by using a damper to restrict the flow, but it is more efficient to regulate the airflow by regulating the speed of the motor.
Saving energy with adjustable speed pump operation is similar to saving energy with adjustable speed fan operation. It is more efficient to regulate the flow of fluid by regulating the speed of the motor rather than by restricting the flow using a control valve.
[edit] Types of adjustable speed drives
Speed adjustment techniques have been used in transmitting mechanical power to machinery since the earliest use of powered machinery. Before electric motors were invented, mechanical speed changers were used to control the mechanical power provided by water wheels and steam engines. When electric motors came into use, means of controlling their speed were developed almost immediately. Today, various types of mechanical drives, hydraulic drives and electric drives compete with one another in the industrial drives market.
[edit] Mechanical adjustable speed drives
There are two types of mechanical drives, variable pitch drives and traction drives.
Variable pitch drives are pulley and belt drives in which the pitch diameter of one or both pulleys can be adjusted.
Traction drives transmit power through metal rollers running against mating metal rollers. The input/output speed ratio is adjusted by moving the rollers to change the diameters of the contact path. Many different roller shapes and mechanical designs have been used.
[edit] Hydraulic adjustable speed drives
There are three types of hydraulic drives, hydrostatic drives, hydrodynamic drives and hydroviscous drives.
A hydrostatic drive consists of a hydraulic pump and a hydraulic motor. Since positive displacement pumps and motors are used, one revolution of the pump or motor corresponds to a set volume of fluid flow that is determined by the displacement regardless of speed or torque. Speed is regulated by regulating the fluid flow with a valve or by changing the displacement of the pump or motor. Many different design variations have been used. A swash plate drive employs an axial piston pump and/or motor in which the swash plate angle can be changed to adjust the displacement and thus adjust the speed.
Hydrodynamic drives or fluid couplings use oil to transmit torque between an impeller on the constant-speed input shaft and a rotor on the adjustable-speed output shaft. The torque converter in the automatic transmission of a car is a hydrodynamic drive.
A hydroviscous drive consists of one or more discs or connected to the input shaft pressed against a similar disc or discs connected to the output shaft. Torque is transmitted from the input shaft to the output shaft through an oil film between the discs. The transmitted torque is proportional to the pressure exerted by a hydraulic cylinder that presses the discs together.
[edit] Continuously variable transmission (CVT)
Mechanical and hydraulic adjustable speed drives are usually called transmissions or continuously variable transmissions when they are used in vehicles, farm equipment and some other types of equipment.
[edit] Electric adjustable speed drives
There are three general categories of electric drives, DC motor drives, eddy current drives and AC motor drives. Each of these general types can be further divided into numerous variations. Electric drives generally include both an electric motor and a speed control unit or system. 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. Later, electronic controllers were designed using various types of vacuum tubes. As suitable solid state electronic components became available, new controller designs incorporated the latest electronic technology.
[edit] DC drives
DC drives are DC motor speed control systems. Since the speed of a DC motor is directly proportional to armature voltage and inversely proportional to field current, either armature voltage or field current can be used to control speed. Several types of DC motors are described in the electric motor article. The electric motor article also describes electronic speed controls used with various types of DC motors.
[edit] Eddy current drives
An eddy current drive consists of a fixed speed motor and an eddy current clutch. The clutch contains a fixed speed rotor and an adjustable speed rotor separated by a small air gap. A direct current in a field coil produces a magnetic field that determines the torque transmitted from the input rotor to the output rotor. The controller provides closed loop speed regulation by varying clutch current, only allowing the clutch to transmit enough torque to operate at the desired speed. Speed feedback is typically provided via an integral AC tachometer.
Eddy current drives are a type of slip controlled drive. Slip controlled drives are generally less efficient than other types of 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 turns at a slower speed. Since power is proportional to torque multiplied by speed, the input power is proportional to motor speed times operating torque while the output power is output speed times operating torque. The difference between the motor speed and the output speed is called the slip speed. Power proportional to the slip speed times operating torque is dissipated as heat in the clutch. Because of efficiency concerns, slip controlled drives have lost popularity and have recently been used only in special situations.
[edit] AC drives
AC drives are AC motor speed control systems.
Slip controlled drives control the speed of an induction motor by increasing a motor's slip. This is accomplished by reducing the voltage applied to the motor or increasing the resistance of the rotor windings. Because they are generally less efficient than other types of drives, slip controlled drives have lost popularity and have recently been used only in special situations. See eddy current drives above.
Adjustable-frequency drives (AFD) control the speed of either an induction motor or a synchronous motor by adjusting the frequency of the power supplied to the motor. Adjustable frequency drives are also known as variable-frequency drives (VFD).
When changing the frequency of the power supplied to an AC motor, the ratio of the applied voltage to the applied frequency (V/Hz) is generally maintained at a constant value between the minimum and maximum operating frequencies. Operation at a constant voltage (reduced V/Hz) above a given frequency provides reduced torque capability and constant power capability above that frequency. The frequency or speed at which constant-voltage operation begins is called the base frequency or speed. Whether to applied voltage is regulated directly or indirectly, the V/Hz tends to follow the general pattern described for the performance described. The variable-frequency drive article provides additional information on electronic speed controls used with various types of AC motors.
[edit] References
- Cowie, Charles J. (2001). Adjustable Frequency Drive Application Training. Powerpoint presentation. Excerpts donated to Wikipedia by the author.
- Phipps, Clarance A. (1997). Variable Speed Drive Fundamentals. The Fairmont Press, Inc. ISBN 0-88173-258-3.
- Spitzer, David W. (1990). Variable Speed Drives. Instrument Society of America. ISBN 1-55617-242-7.
- Campbell, Sylvester J. (1987). Solid-State AC Motor Controls. New York: Marcel Dekker, Inc.. ISBN 0-8247-7728-X.
- Jaeschke, Ralph L. (1978). Controlling Power Transmission Systems. Cleveland, OH: Penton/IPC.
- Siskind, Charles S. (1963). Electrical Control Systems in Industry. New York: McGraw-Hill, Inc.. ISBN 0-07-057746-3.
