Part 1 describes a standby power supply and an uninterruptible power supply and how to select the appropriate one. Part 2 covers Power Factor and is given for your reference. An understanding of the subject in not an absolute requirement for the average user but a basic understanding of the concept and benefits may be helpful.
What is Power Factor?
Power factor is associated with any alternating current (AC) power source such as standard wall outlet.
Power factor is the ratio between the Watts (W) and the Volt Amps (VA) drawn by an electrical load (computer, electric range, etc.), where the Watts is the "real" load power and the Volt Amps is the 'apparent" load power. It is the measure of how effectively the alternating current is being converted into useful work output, and more particularly, is a good indicator of the effect of the load current on the efficiency of the AC source.
All alternating current will cause losses in the supply and distribution system. A load with a power factor of 1.0, (referred to as unity power factor) will result in the most efficient loading of the supply. A load with a power factor of 0.5 will result in much higher losses in the supply system.
Figure 2: Oscilloscope Plot of 1.0 (unity) Power Factor
A poor power factor can be the result of either a significant phase difference between the AC Voltage and AC Current measured at the load, or can be due to a high harmonic content or a distorted/discontinuous load current waveform.
Poor load current phase angle is generally the result of an inductive load such as a motor, power transformer, lighting ballasts, welder or induction furnace.
Figure 3: Oscilloscope Plot of a 0.7 Power Factor Caused by a Motor Load
A distorted Load Current waveform can be the result of a non-linear load such as a rectifier, variable speed drive, discharge lighting, or a switching power supply typically found in most computers and sophisticated electronic equipment.
Figure 4: Oscilloscope Plot of a 0.7 Power Factor Caused by a Non-Linear Load
The National Electric Code limits the continuous current drawn through the equipment line cord to 80% of the rating of the receptacle. For the standard 15 A receptacle (NEMA 5-15R) the limit is 0.8 x 15 = 12 Amps
This limits the Apparent Power to S = V RMS x A RMS = 120 x 12 = 1440VA for a 120 volt utility line
Power Factor = Real Power (Watts) / Apparent Power (VA)
Real Power (Watts) = Apparent Power x P.F.
Apparent Power (VA) = Volts (RMS) x Amps (RMS)
But only the Real Power (Watts) is useable power and with the typical computer load power factor of 0.65 to 0.70 the available useful power
Real Power (Watts) = 1440VA x P.F. = 1440 x .7 = 1008Watts
If the P.F. is only 0.65
Real Power (Watts) = 1440 x .65 = 936 Watts
When using an "Off Line" or "Line Interactive UPS", the load power factor is not changed, and the efficiency may be 97%
Therefore, the useful power is:
a) 1008 x .97 = 978 Watts (P.F. = 0.7)
b) 936 x .97 = 908 Watts (P.F. = 0.65)
