20-04-2013, 04:30 PM
APPLICATION OF VARIABLE SPEED DRIVES (VSD)
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INTRODUCTION
Although there are many methods of varying the speeds of the driven equipment such as hydraulic coupling, gear box, variable pulley etc., and the most possible method is one of varying the motor speed itself by varying the frequency and voltage by a variable frequency drive.
Concept of Variable Frequency Drive:-
The speed of an induction motor is proportional to the frequency of the AC voltage applied to it, as well as the number of poles in the motor stator. This is expressed by the equation:
RPM = (f x 120) / p
Where f is the frequency in Hz, and p is the number of poles in any multiple of 2.
Therefore, if the frequency applied to the motor is changed, the motor speed changes in direct proportion to the frequency change. The control of frequency applied to the motor is the job given to the VSD.
The VSD's basic principle of operation is to convert the electrical system frequency and voltage to the frequency and voltage required to drive a motor at a speed other than its rated speed. The two most basic functions of a VSD are to provide power conversion from one frequency to another, and to enable control of the output frequency.
VSD POWER CONVERSION
As illustrated by Figure 5.1, there are two basic components, a rectifier and an inverter, to accomplish power conversion. The rectifier receives the 50-Hz AC voltage and converts it to direct current (DC) voltage. A DC bus inside the VSD functions as a "parking lot" for the DC voltage.
Factors for Successful Implementation of Variable Speed Drives
a) Load Type for Variable Frequency Drives
The main consideration is whether the variable frequency drive application require a variable torque or constant torque drive. If the equipment being driven is centrifugal, such as a fan or pump, then a variable torque drive will be more appropriate. Energy savings are usually the primary motivation for installing variable torque drives for centrifugal applications. For example, a fan needs less torque when running at 50% speed than it does when running at full speed.
Variable torque operation allows the motor to apply only the torque needed, which results in reduced energy consumption. Conveyors, positive displacement pumps, punch presses, extruders, and other similar type applications require constant level of torque at all speeds. In which case, constant torque variable frequency drives would be more appropriate for the job. A constant torque drive should have an overload current capacity of 150% or more for one minute. Variable torque variable frequency drives need only an overload current capacity of 120% for one minute since centrifugal applications rarely exceed the rated current.
If tight process control is needed, then you may need to utilize a sensor less vector, or flux vector variable frequency drive, which allow a high level of accuracy in controlling speed,
torque, and positioning.