Electrical Power - QSC PLD Series Application Manual

Electrical power

Electrical power has always been a potential constraint on portable production audio systems, whether they're run from a
building's AC mains or from portable power—i.e., a generator. Power amplifiers are by far the biggest consumers of electri-
cal power in an audio system, and through the PLD Series's significant advances in power factor, efficiency, and universal
voltage capability, the constraints presented by available power just aren't as limiting any more. With PLD, your system
can produce more audio power for a given amount of available AC, in more places and in more applications, than before
possible.
Power factor
Electrical usage has become a growing concern in recent years. AC-powered devices have a characteristic called power
factor that describes the relationship of the current waveform to the voltage waveform. If the current waveform through
a device coincides exactly and proportionally with the voltage waveform applied across it, its power factor is 1. Examples
of devices with perfect power factor are space heaters and incandescent light bulbs (without dimmers), which present a
purely resistive load to the AC mains. The total power used and/or dissipated by an AC device, in watts, is equal to rms
voltage times rms current times power factor. Therefore, power factor tells us how much of the current provided by the
mains actually gets used as power (this includes wasted power due to inefficiencies in the device).
In devices where the current waveform is still sinusoidal but shifted out of phase with the voltage, as often happens with
AC motors under various load conditions, or is not sinusoidal, as with most electronic power supplies, the power factor
is somewhat less than 1. That means the AC mains actually have to deliver more current than what actually gets used as
power. This may require building substantial excess current capacity into an AC system with no real return on that invest-
ment.
Unfortunately, electronic power supplies—even the heavy line-frequency (non switch-mode) ones that are colloquially
called "linear"—are actually highly nonlinear. As the use of electronic devices increases and resistive appliances such as
incandescent lighting decreases, this nonlinearity presents a growing challenge to the AC mains power generation and
distribution systems and may even require them to be oversized. This is because of "peak rectification," where the alter-
nating current (AC) is converted into direct current (DC) and fed into reservoir capacitors to smooth out the voltage. The
diodes—semiconductor "check valves" that allow current to flow in one direction but not the other—that do the rectifying
do not actually conduct current until the instantaneous voltage of the AC is slightly higher than the DC voltage stored in the
capacitor.
The result is that the current flows only at the peaks of the AC voltage waveform, leading to a current waveform that isn't a
sine wave like the voltage; instead it comprises sharp spikes alternating positive and negative every half cycle. Needless to
say, the power factor can be a figure much less than 1 (typically around 0.65).
PLD Series Amplifiers
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Application Guide—Portable PA
Rev. E, Apr. 2015