What are Some of the Things that can Cause Poor Power Factor?Basically any non-linear loads can impact it. The following list contains some examples:
A couple of comments about motors. Just as motor loading can have an impact on motor efficiency it also has an impact on power factor. If a fully loaded 75 hp motor has a power factor of 88%, when it runs at half load the power factor drops to 79%. Sometimes motors are oversized, or operations change, or they are sized for the maximum load of a cyclical process.
What are the Benefits of Improving Power Factor?
Depending on your utility you may be able to save money on your energy costs. Many utilities have power factor incorporated in some form in their rates. They may bill your demand in KVA or have credits and debits depending on your power factor. The utility has to provide distribution facilities, transformers, etc based on your Total Apparent power. Its like the beer mug illustration. You order a mug of beer (can be non-alcoholic). You pay for a full mug. You really want more beer than foam; but if it is not efficiently dispatched you may be paying for a high percentage of foam. With your power bill you could be paying for a lot of “foam” if your power factor is not high.
What can You do to Improve Power Factor?
The discussion of the causes of poor power factor should lead naturally to some possible solutions or ways to improve your power factor.
The installation of capacitors can in many cases be the most straight forward and cost effective approach to improving power factor.
Ideally for the maximum system benefit the capacitors should be located as close as possible to the offending equipment.
For induction motors, capacitors can be installed at a number of locations such as A, B, or C in the illustration. At these locations the properly sized capacitors are energized when the motor is energized.
Location A is ideal for new installations so that the thermal overloads can be sized to match the reduced current draw or for existing motors where the overload protection can be changed. However, NEMA does have some cautions about switching the capacitors with the motors.
Location B between the starter and overload relay is good for existing motors where overloads are already sized for existing current.
Location C works best for motors with frequent or special start and stop requirements or variable speed.
Capacitors can also be located at the distribution point for a number of motors; but some automatic switching may be required to match capacitance to the changes in motor loads.
A capacitor bank can also be located at the main service. This application is usually the lowest cost installation. Automatic or a combination automatic and fixed capacitor bank may be required to match the facility load. This application allows you to take advantage of any utility cost reductions from improved power factor but does not improve the loading of your facility’s power distribution systems.
Synchronous motors are good in situations where constant speed operation is essential. Unlike an induction motor a synchronous motor requires DC power as well as AC; but many are self excited. A synchronous motor delivers leading kilovars, much like a capacitor.
The solutions discussed mainly address the displacement power factor issues. The presence of harmonics in a system require tuned harmonic filters, a combination of inductors and capacitors, to correct power factor. The filters are “tuned” to specific harmonic frequencies.
ASDs and other sensitive equipment may be susceptible to transient voltages due to capacitor switching.
These issues should be identified and addressed as power factor improvement measures are implemented.
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