Table 4-2. - Power Factor of Various Types of Electrical Equipment
|Incandescent lights||100%||In phase|
|Heating devices (all types)||100%||In phase|
|Induction motors (loaded)||80%||Lagging|
|Induction motors (light load)||20%||Lagging|
|Synchronous motors (underexcited)||Varies||Lagging|
|Synchronous motors (overexcited)||Varies||Leading|
and distribution circuits. The current and voltage are now said to be "out of phase." The current drawn by idle running induction motors, transformers, or underexcited synchronous motors lags even more than the current shown in the figure.
Occasionally, the current leads the voltage. An unloaded transmission line, an overexcited synchronous motor or a static condenser takes leading current from the line. When the current leads or lags the voltage, the power in the circuit is no longer equal to volts times amperes but is calculated from the expression:
Watts = volts x amperes x power factor
Power factor = watts / volts x amperes
POWER FACTOR CORRECTION CAPACITORS. - For general use, the most practical and economical power factor correction device is the capacitor. Capacitors are used at power stations where an elaborate and expensive synchronous condenser installation is not justified. The following paragraphs deal exclusively with power capacitors.
The "power factor" can thus be defined as the ratio of the actual power to the product of volts times amperes. The latter product is generally called voltamperes, or apparent power. The value of the power factor depends on the amount the current leads or lags behind its voltage. When the lead or lag is large, the power factor is small; and when the lead or lag is zero, as when the current and voltage are in phase, the power factor is unity. Unity is the largest value that the power factor can have. The power factor is usually between 0.70 and 1.00 lagging. An average value often used in making calculations is 0.80 lagging. Table 4-2 gives the power factors of various types of electrical equipment.
The cause of low power factor is an excessive amount of inductive effect in the electric consuming device, be it motor, transformer, lifting magnet, and so forth. Induction motors, when lightly loaded, exhibit a pronounced inductive effect. Idle transformers likewise have a strong tendency to lower the power factor. A low system power factor can be increased by adding corrective equipment to the system. There are many devices used for power factor correction, including synchronous motors and power factor correction capacitors.
SYNCHRONOUS MOTORS. - Any synchronous motor may be used for power factor correction by overexcitation.
Capacitance is the direct opposite of inductance, just as heat is the opposite of cold, and day is opposite of night. Capacitance is a property of a condenser, and a condenser is a combination of metal plates, or foil strips, separated from each other by an insulator, such as air, paper, or rubber. The capacitance, or the capacity of the condenser to hold an electric charge, is proportional to the size of the plates and increases as the distance between the plates decreases.
RATINGS. - Capacitors are rated in continuous kvar (kilovoltampere reactive), voltage, and frequency. They are designed to give not less than rated and not more than 135 percent rated kvar when operated at rated voltage and frequency. Capacitor units are available normally in voltage ratings of 2,400 volts to 34,500 volts and kvar ratings from 15 kvar to 300 kvar. Various manufacturers' medium-voltage units up to 200 kvar are interchangeable. Capacitors are generally rated at a frequency of 60 Hertz (Hz);Continue Reading