Variable frequency drives (VFD’s) can provide significant energy savings and increases overall process/system efficiency by effectively matching the power applied to the level the process requires. By controlling motor speed, changes in load demands can be adjusted for quickly and automatically to maintain optimum process conditions. Also, the energy the driving motor needs to begin rotating, manifested as a high amperage commonly referred to as “in-rush” current, can be slowly increased to ramp up the motor while minimizing current draw. The VFD shares this soft-start functionality with the reduced voltage starter, often referred to as a soft-starter, but goes beyond this by allowing adjustable speed control. Let’s examine these two characteristics of the VFD – speed control and controlled starting and stopping – to understand how energy savings and other cost benefits are achieved.
VFD controls motor speed by comparing a reference signal to a pre-set value. The reference signal can be generated externally, for example via a process setpoint, or internally by the VFD using software to model motor parameters. The latter is accomplished by most VFD’s through the auto-tuning process during initial drive setup. It then adjusts the frequency and voltage to match the reference signal, which in turn results in adjustments to motor speed – frequency and speed are directly proportional. If the reference signal indicates the motor is not required to run at full (base ) speed, the drive slows it down accordingly. For variable torque loads such as centrifugal pumps and fans, power is produced in relation to the cube of the speed, so if speed is reduced by just 10%, power is reduced by (10%) cubed, or approximately 27%. Although constant torque load savings are not identical, they are significant as well.
Debate continues as to the savings generated by reducing motor speed in some applications versus the capital cost of upgrading to VFD’s. In essence, if the process is significantly over-sized, or is dominated by friction losses, then there is value in reducing the speed of process flow. Various on-line tools exist to assist in determining potential energy savings; for example, ABB and Weg have links to such tools on their Corporate web sites. Keep in mind also that more care must be exercised when evaluating processes requiring high torque or constant torque, since in general the savings through VFD use in such systems is not as great.
During motor starting, significant current must be supplied to the motor windings to overcome the inertia of the load and the mass of the motor armature. When motors are started “across-the-line”, the resultant current draw can be as much as 6 – 12 times the full-load amperage, and can place great stresses on driven equipment. In cases where motor loads comprise a significant portion of the total electrical demand, in-rush current can result in high electrical peaks and thus higher utility costs. A soft-start function, such as that provided by ramping up a motor slowly via VFD, can reduce in-rush to 3 – 6 times the full-load amperage, reducing peak demand. Also, costs for preventive maintenance of couplings, shafts and other driven components are reduced because they are not subjected to the larger stresses caused by full-voltage starting.
In addition to support available from most manufacturers, Joliet Technologies can assist you in determining whether a VFD is right for your application. Please call or email us at simon.fan@vtdrive.com with your application. And as always, visit our Comments section to provide any thoughts, suggestions, or questions you might have. And remember to visit us on-line at www.vtdrive. com.
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