Cyndi Nyberg
Former EASA Technical Support Specialist
Power factor can be best explained with a short illustration. Figure 1 below shows the three elements to consider. First, true power, measured in kW, is the power that does useful work. Reactive power, measured in kVAR, is the power that is stored and returned by all inductive machines, such as motors and transformers. The apparent power, measured in kVA, is the voltage multiplied by the current in the system. Even though the true power is doing the work, the power company has to distribute the apparent power.
Mathematically, the power factor is the cosine of the angle between the true and apparent power. Power factor is defined as the ratio of true power used in an electric circuit to the apparent power, or the power that is apparently being drawn from the source. In a sense, apparent power is “borrowed” from the power company. Since AC power is continuously reversing, the borrowed power is sent back to the system every time the alternating current reverses.
In electric motors, power factor varies with load (although not directly). A motor running without a load will have a very low power factor, whereas at full load, the power factor may be relatively high.
A facility that has a very low power factor in the total system may be charged a penalty by the power company for “borrowing” the power without fully using it. This is because more current is being transmitted than is actually doing any useful work.
Power factor correction
There are two common methods of correcting plant power factor. One is the use of synchronous motors. Typically, motor inductance causes the current to lag the voltage by a certain angle, the one between true power and apparent power. An ideal situation would not have the current lagging, and therefore the angle between them would be zero, resulting in a unity power factor. Using a synchronous motor in the system will provide power where the current is leading the voltage by a certain angle. This “opposite” power can offset the lagging current that is already present, increasing the total system power factor. A synchronous motor can be adjusted to leading power by increasing the excitation voltage to the rotating fields. Use of synchronous motors can be an expensive alternative if a new motor must be purchased (there aren’t any already in the system), but in certain applications may be the best choice.
Power factor correction capacitors (PFCC’s) are the second method to increase overall system power factor. Figure 2 below shows the typical placement of PFCC’s in the system. When the capacitors are located at position 2, no changes are required for the overloads. However, if the PFCC’s are located at position 3, the current through the overloads will be lower, so the relay must be rated lower to protect the motor.
It is possible to calculate the drop in amperes as a result of adding the capacitors. You need to know the original power factor and the corrected power factor. Then, using the following equation:
old PF
% current drop (A) = 100 x (1 - old PF )
new PF
For example, suppose that the system power factor was originally 78%, and you improve it to 90%. The percent drop in motor current is:
% drop= 100 x (1-.78/.90)
= 100 x (1-.87)
= 13% drop in current
For applications like elevators, multispeeds, jogging applications, or starting methods such as wye-delta, part winding, or auto-transformer, the PFCC’s must be placed in position 1. In this case, the power factor shouldn’t be corrected above 95% to prevent over-excitation.
Sizing Ccapacitors
Table 1 at right shows the appropriate capacitor rating (kVAR) based on the original power factor and the desired power factor after improvement. The selected multiplier is applied to either the entire kW load in the system, or to the individual kW load of a motor. The kW multiplied by the multiplier gives the appropriate kVAR rating of the capacitor.
Warning for power factor correction
When power factor correction capacitors are used on an induction motor, the kVAR value of the capacitor should not approach or rise above the power factor above unity. Over-correction can cause over-excitation, causing damage to the motor or the driven equipment in the form of high transient currents, voltages, or torque.
Note: Power factor correction capacitors pose a potential hazard if they are not properly discharged before they are touched. This is true for any power circuit capacitors, such as surge capacitors and starting capacitors.
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