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Variable Frequency Drive
The terms Variable Frequency Drive (VFD), Variable speed drive (VSD), Inverter, and Adjustable Frequency Drive (AFD) are all used interchagebly to refer to AC motor drives. Motors drives have become an essential part of plant automation. They have a number of advantages over conventional methods. VFD's are used to vary motor speeds. They do this by varying the supply voltage and frequecy thus they are also known as Variable Voltage Variable Frequency (VVVF) drives. Modern VFD's provide numerous features like motor protection (over current, over heating and such), braking motors at precise position through DC injection braking, digital and analog inputs/outputs to build a control loop without excessive hardware, energy conservation, sleep mode and many others.
Advantages of using a variable speed control
The VFD controlled AC motor is used in all kinds of automated plants. Apart from optimizing the features of the AC motor, the variable speed control gives the following advantages:
Energy Saving
Energy is saved as the motor speed is continuously matched to the momentory demand. A good example of this can be seen in pump and ventilation equipment where the power consumption is reduced by the cube of speed.
Process improvement
Speed control according to the production process offers several advantages: Production can be increased, consumption of materials and rejection rate can be reduced.
Improved quality
The number of starts/stops is reduced. Unnecessarily harsh operation of the machinery can therefore be avoided.
Less maitenance
The VFD requires no maintenance.
Improved working enviornment
The speed of conveyor belts can be adapted to the working speed. In bottling plants the bottle noise is reduced by adjusting the speed to suit the production rate. In ventilation applications the fan speed can be matched to suit the demand so that noise and drafts are avoided Most of the AC motors used in industry are induction motors that are controlling various kinds of processes. In most of the application motors are not performing to their full capability, they are used under their nominal ratings although some are performing at their maximum limits. For motors whose full potential is not fully realized takes excess current under low load levels. A VFD can be used to decrease the excess current and therefore conserve expensive energy. Many motors require huge starting torques, mostly compressor motors come under this category. People complain that their generators trip when the compressors come online or other loads are affected severely when compressors start. A VFD is used here to start the compressor without causing any disturbance to other online loads.
Other benefits include delivering a more constant pressure, and energy saving. Further reading: www.joliettech.com/what_is_a_variable_frequency_drive.htm
en.wikipedia.org/wiki/Variable-frequency_drive
http://www.plantservices.com/articles/2006/278.html
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