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December 6, 2013

Essay Paper on Variable Frequency Drive

Variable Frequency Drive

     Variable frequency is a device used to control the rotational speed of an alternating current electric monitor by controlling the frequency of the electrical power supplied to the motor. Also known as adjustable-speed drive, it is its specific type. Keeping in view that the voltage varies along with its frequency; this is also sometimes called variable voltage variable frequency (VVVF).
        Variable-frequency drives are largely used in ventilation systems for big buildings as its motors on fan serve to save energy by letting the volume of moving air to match the system demand. It can also be used on pumps, elevators, conveyors and machine tool drives.
        If installed to a motor-driven system, variable frequency system can provide a prospect of energy savings especially when the load varies from time to time. Motor connected to variable-frequency drive changes variably to the motor supply voltage in order to ensure continuous speed control.
      Motor-driven systems are often built (with a safety precaution) to control peak loads which sometimes leads to energy inefficiency in systems being operated for longer hours at reduced load. VFD’s ability to adjust motor speed allows for matching of motor output to load ensuring energy savings.(Carrow, 200)
     VFD uses it output devices such as transistors, thyristors, and IGBTs only as switches for tuning on and off. Transistor is linear device thus cannot be used for VFD drive. There are also some additional benefits of using VFD. It can be used to control temperature control, pressure or flow without the use of another controller. Its maintenance cost is cheaper due to its lower operating speed resulting in the longer life of motors and bearings. It gives a soft starter for the motor which is no longer required. It eliminates the throttling valves and dampers giving a controlled ramp-up speed. It has an ability to limit torque to a customized level essential to protect driven equipment which cannot resist excessive torque.(Finney, 1998)

     The variable frequency drives operate under the principle that the speed of a synchronous AC motor is determined by the frequency of the AC supply and the number of poles in the stator winding, according to the equation: where Rpm = revolutions per minute  f = frequency of alternating current (Hz) p = number of poles (an even number). 

    The constant, 120, is 60 seconds per minute multiplied by 2 poles per pole pair. Sometimes 60 is employed for the constant and p is set during the post rather than pairs of poles. 

    Synchronous motors operate at synchronous speed determined by the equation above. The speed of an induction motor is slightly less than synchronous speed. 4-pole motor which is connected directly to 60 Hz utility power (force) would have a synchronous speed of 1800 rpm: 
If the engine induction motor, the operating speed at full load is about 1750 rpm. 
If the engine is connected to a speed controller that provides power at 50 Hz, synchronous speed is 1500 rpm:
    A variable frequency drive typically consists of an AC motor, controller and operator interface. The engine used in a VFD system is usually a three-phase induction motor. Some types of single phase motors can be used, but three-phase motors are usually preferred. The various types of synchronous motors offer advantages in some situations, but the induction motors are suitable for most purposes and are generally the most economical choice. Engines that are designed for operation voltage forces fixed-speed enhancements are often used, but some standard motor designs offer higher reliability and better performance of VFD.

    Variable frequency drives are semiconductor electronic devices for power conversion. The usual design first converts the input power into AC power through DC using a rectifier bridge. The DC intermediated power is then converted into AC Quasi-sine wave with a switching inverter. The rectifier is usually a three-phase diode bridge, but controlled rectifier circuits are also used. Since the incoming power is converted to DC, many units will accept single phase as well as three phase input power (acting as a phase converter as well as a speed controller), but the unit must be under- sought by using single phase input as only part of the bridge rectifier borne connected.
    As new types of semiconductor switches have been presented, they were promptly applied to the inverter circuits at all voltage and current ratings for which suitable devices are available. Presented in the 80, the insulated gate bipolar transistor (IGBT) has become the device used in most circuits inverter VFD in the first decade of the 21st century.

    Characteristics of AC motor require the applied voltage to be proportionately adjusted whenever the frequency is changed to provide the rated torque. For example, if an engine is designed to operate at 460 volts at 60 hertz, the applied voltage must be reduced to 230 volts when the frequency is reduced to 30 Hz. Thus the ratio of volts per hertz must be set at a V / Hz constant value (460/60 = 7.67 in this case). For optimum performance, a further adjustment of tension may be necessary, but nominally a constant volt per hertz is the general rule. This report can be changed to change the torque of the engine.
    The usual method used to adjust the motor voltage is the pulse width modulation PWM. With the voltage control PWM inverter switches are used to divide the waveform output quasi-sine series of narrow voltage pulses and to modulate the pulse width.
    The operation at synchronous speed above is possible, but is limited to conditions that do not require more power than the estimate of motor nameplate. This is sometimes called "field weakening", and for AC motors, runs volts / hertz at least as assess and above synchronous speed. The example, an HP 100, 460V, 60Hz motor of 1775 rpm (4 pole) supplied with 460V, 75Hz (6134 V / Hz) would be limited to the torque of 60/75 = 80% power at the rate of 125% (2218.75 RPM) = 100%.

    Embedded microprocessor governs the overall operation of the VFD controller. The primary programming in microprocessor software is inaccessible to the user VFD. However, some degree of configuration programming and parameter adjustment is usually provided so that the user can customize the VFD controller to suit specific engine and equipment requirements ducts. (Campbell, 1987)

    At 460 volts, the recommended maximum cable distance between VFDs and motors can vary by a factor of 2.5:1. Are allowed longer cable distances for frequencies below the switching carrier of 2.5 kHz. Frequencies below the switching carrier can produce audible noise of the engines. The switching frequency carrier of 2.5 kHz and 5 kHz pose fewer problems running engine that caused by Carrier Switching frequency to 20Hz.  Shorter cables are recommended for higher frequency switching carrier 20 kHz. The minimum carrier frequency switching to synchronize the flow of multiple conveyors is 8 kHz.

    The operator interface provides means for an operator to start and stop the engine and adjusts the operating speed. Service operations include additional operator could switch between reverse and manual adjustment of speed and automatic control of an external process control signal cycle. The operator interface often includes alphanumeric display and / or indicating lights and meters to provide information about the operation of the order. A unit of keypad and display operator interface is often provided on the front of the VFD controller as shown in the photograph above.
    The keypad display can often be connected by cables and mounted a short distance from the VFD controller. Most are also provided input and output terminals (input / output) for connecting push buttons, switches and other operator connecting devices or control signals. A serial communications port is also often available to allow the VFD to be configured, adjusted, monitored and controlled using a computer.







References
Campbell, Sylvester J. (1987). Solid-State AC Motor Controls. New York: Marcel Dekker, Inc.
Carrow, Robert.(2000) Electrician’s technical reference: Variable Frequency Drives, Cengage Learning, 175 pages.

Finney, David,(1995) Variable Frequency Ac Motor Drive Systems, IET, 391 pages

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