analyzing harmonic distortion 79ending up a bit out of phase and waveform
distortion will occur as a result. (Remember
that resistive loads do not have these faults and
are generally thought of as linear; there is no
inductance or capacitance to worry about.)
Regardless of whether an AC circuit is sup-
plying an inductive load, a capacitive load, or
a resistive load, there are factors that will mod-
ify the perfect AC current sine waveform.
Excessive harmonic distortion may cause
equipment to overheat, motor failure, capacitor
failure (especially in motor circuits), and exces-
sively high, neutral conductor current. Com-
monly used devices that may either contribute
to or be affected by excessive distortion include:
- fluorescent lights
- some HVAC systems
- computers, printers, and fax machines
- dimmer switches for lighting
- audio equipment
- video equipment
- UPS devices
Here’s an example of how harmonic distor-
tion is created. In order to work, a fluorescent
light first requires a buildup of voltage. Once
the voltage is high enough, an arc occurs
across its pair of electrodes. At this point, cur-
rent flows easily, especially when compared to
the current flow before the arc occurred. In
effect, the light draws nearly no current ini-
tially and much more current once the tube is
activated and producing light.
Multiply this effect by a power grid’s cumu-
lative nonlinear loads, and the net effect is a dis-
torted sine wave delivered to the boat. Extreme
distortion can be seen on an oscilloscope visually
as a sine wave that has uneven peaks and valleys,
often with a “flattened” top to one of the wave
peaks (see top photo page 81).
Harmonic distortion can be cumulative
within a power grid, and will be distributed
throughout the grid, potentially causing
equipment problems. Determining whether
excessive distortion exists is one of the more
advanced troubleshooting exercises a boat-
owner or technician may find themselves per-
forming at some point.
Finding Harmonic Distortion
Finding the source of harmonic distortion is
not always easy. Harmonics can migrate
through the power distribution system from
one building or marina to another, and from
one boat to another in a marina, if they are
connected electrically via the utility power
grid. Furthermore, the problem can be inter-
mittent, as electrical loads that cause distor-
tions may cycle on and off. Monitoring must,
therefore, be done over an extended period of
time. In fact, some power analysis specialists
recommend site monitoring over a period of
one month to gather quantifiable data! This
makes a strong case for the data-logging
methodologies we’ll look at in Chapter 8.
If you have equipment that is exhibiting
harmonic distortion symptoms—the most
common ones are overheated or damaged AC
neutral conductors or cable terminations, and
circuit breakers that trip continually and mys-
teriously—you can use your oscilloscope to
confirm your suspicions.
Connecting an oscilloscope to an AC cir-
cuit is easy. Plug the meter’s red lead into the
meter’s AC volts socket and the black lead into
the “com” socket, as in the photo on page 75.
Then connect the other ends of the leads as if
you were measuring voltage: red lead to the
hot terminal, and black lead to the neutral ter-
minal. Next you’ll select the appropriate func-
tion for the meter you are using.
Calculating Distortion Using Peak
Capture
Although special power-analyzing meters are
currently available, they are cost prohibitive,
especially when you consider that even
advanced marine electricians will not be using
this equipment that frequently. But with an