76 electrical systems troubleshooting
ALTERNATING CURRENT AND WAVEFORMSA perfectly formed AC sine wave, characteristic of AC
power as delivered by a utility company.AC current forms sine waveswhen voltage
is charted against time. The waveform dis-
played on the oscilloscope is a true sine
wave. Although the nominal voltage is
120 volts at 60 cycles per second, the
scope shows the voltage as slightly less
than that, at just over 117 VAC. This is com-
pletely normal; voltage variations of 10%
are not uncommon.
Some of the equipment we use on boats
doesn’t always deliver a perfect sine wave,
however. Generators sometimes develop
mechanical or electrical problems that can
affect their ability to maintain precise engine
rpm, and frequency and voltage, as well as
the waveform, can become skewed. The
oscilloscope will display this faulty waveform,
as well as frequency and voltage values that
fall out of acceptable parameters (generally
10% for voltage and 5% maximum for
frequency). Many inverters don’t even attempt
to deliver a perfect AC sine wave, but rather will
deliver AC current in a stepped-square-wave,
or modified-square-wave, output. Charles
Industries, one major vendor of inverters,
offers at least one unit that bills itself as a
quasi-sine-wave inverter. This appears to be
more marketing hype than anything else. The
bottom photo shows the waveform as deliv-
ered by one of these inverters. These inverters
show these “stepped” waveforms because
the unit is taking DC current, which shows as a
flat straight line on an oscilloscope, and con-
verting it to what I’ll call a usable form of AC
current. The “steps” are created by the elec-
tronic switching that is occurring in the inverter,
which is really nothing more than the power
turning on and off as it gets “stepped up” to
AC voltage potentials. More sophisticated
inverters will have more electronic switches,
which in effect creates less profound “steps”A modified square wave, or “quasi-sine” wave, as
Charles Industries calls it, the company that distributes
the DC-to-AC inverter this reading was taken from. These
quasi-sine waves represent a certain level of distortion. If
you have an inverter, you have to figure out whether the
appliances you’re attempting to power with the inverter
can operate in spite of the inverter’s distortion.