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Over voltage
An abnormally high voltage sustained for an extended period.

Load current is drawn from a voltage source. In ac systems, the voltage is a sine wave and for a purely resistive load, the current drawn is also a sine wave aligned perfectly (in phase) with the voltage sine wave. Most loads, however, are not purely resistive and the current drawn is delayed and lags behind the voltage sine wave (out of phase). The lag is measured in degrees. Power factor is equal to the cosine of this phase difference.

Power Factor
The relationship between actual power (W) and apparent power (VA). Calculated by dividing Watts by Volt-Amperes (W/VA) and usually in the range of 0.75 to 0.85.

PWM – Pulse-Width Modulation
Process of varying the width of a train of pulses to adjust the rms voltage and frequency and modify the waveshape, typically to sinusoidal.

Rectifier / Charger
Part of a UPS that converts the incoming ac utility power to dc power for the inverter and to charge the batteries.

RFI – Radio-Frequency Interference
Electrical noise resulting from some parts of the equipment or wiring acting as a radio antenna. This noise may be large enough to disrupt communications or cause computing error.

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