Advanced Marine Electrics and Electronics Troubleshooting A Manual for Boatowners and Marine Technicians

78 electrical systems troubleshooting

on the order of 165 to 170 VAC. But it does

vary slightly due to forces within the devices

that are powering up, which either slow things

down a bit or speed them up. When this hap-

pens, the waveforms become slightly dis-

torted, and we end up with slightly skewed

waveforms. The term used to describe this

effect is harmonic distortion.

Harmonic Distortion

Harmonic distortion is defined in the

National Electrical Code as “a load where the

wave shape of the steady-state current does not

follow the wave shape of the applied voltage.”

As we just discussed, the bottom line is that

nonlinear loads—such as audio equipment,

computers, fluorescent lighting with ballast,

and variable-speed motors—will put AC-sup-

plied voltage out of phase with amperage. The

net result is a waveform that becomes slightly

distorted when viewed with an oscilloscope.

Linear loads—such as a water heater or an

incandescent lightbulb—will not create this

phase imbalance between voltage and current.

But all of this is quite esoteric, and really drags

us into advanced electrical engineering con-

cepts that go beyond what I am trying to con-

vey here. What’s important for the boatowner

or technician to understand is that excessive

harmonic distortion can create problems with

some common onboard AC equipment.

The illustrations show a change in the rela-

tionship of voltage and current in an inductive

load scenario (top illustration), which could

be a transformer or a motor, and in a capaci-

tive scenario (bottom illustration), which

could be a noise filtration circuit on some-

thing like an audio amplifier. In the inductive

scenario, the voltage is leading the current. In

the capacitive scenario, the current leads the

voltage. These are typical scenarios for these

broad load categories, but don’t get too caught

up in all of this. Again, what the boatowner or

technician needs to know is that things are

inductive load

voltage leads current

by

90 ̊ in phase

90 ̊

V

I

AC power source

Voltage (V) is leading the current (shown as amperage, I)

because it takes time for the voltage to force the buildup

of current to its maximum across an inductive load.

voltage lags current

by

90 ̊ in phase

90 ̊

V

capacitor

AC power source

I

Another example is what happens in a capacitor as it

charges and discharges. In this case the voltage lags

behind the current because the current must flow to build

up the charge in the capacitor. The effect is known as

capacitance, which is the ratio of the electric charge trans-

ferred from one to the other of a pair of conductors to the

resulting potential difference between them.