Reactive Power
While reactive power may seem to be loss of
power, it isn’t a complete loss. In fact, reactive
power is necessary to generate the magnetic
field that is essential for the operation of
motors, transformers, and similar devices with a
wound coil. When a refrigerator compressor
motor is turned on, there’s a brief time lag
between the moment the thermostat trips and
the moment the motor begins to turn. It is dur-
ing this ever-so-brief interval that reactive
power is working. This is part of the reason why
motors on start-up typically draw anywhere
from three to six times their running current.
Reactive power needs to be controlled so
that the devices function, but the public elec-
tric utility (or the boat’s generator or inverter)
doesn’t recognize the inherent disparity, which
in effect requires delivery of more power over
a period of time. The amount of reactive
power required needs to be minimized. All of
this equates to more than simple inefficiency.
Ultimately it affects the entire power-delivery
system—things like transformer sizing, wire
gauge appropriate to handle the increased
current demands, and the like.
When you apply this concept to equip-
ment and appliances, you move into the area of
power consumption. How do your appliances
and equipment rate? Low-power-factor items
can have a significant impact on your boat’s
electrical system. For example, a low-power-
factor fluorescent light ballast will draw more
current than a high-power-factor ballast for the
same wattage lamp. The result is that fewer AC
appliances can operate on a given circuit.
On a boat with many AC circuits powering
pumps and motors for refrigeration systems, air
conditioners, and such, it’s possible to have a
fairly low cumulative power factor (i.e., the
power factor for the total amount of watt-hours
consumed). In extreme cases, this may require
additional shore-power service to the boat.Improving Your Power Factor
Power factor is most commonly improved by
incorporating capacitor banks in a device. The
capacitors charge and discharge on demand to
supply the reactive power needed by the load.
By engineering the capacitor bank correctly,
an electrical designer can achieve, or at least
get very close to, the desired 1.0 unity factor
mentioned above. Once power factor correc-
tion circuitry is installed, the need to draw the
reactive power from the AC supply source is
eliminated or greatly minimized.
You can test the equipment and appliances
on your boat to determine their power factor
(see below). You may be able to upgrade to
appliances and equipment that have a better
corrected average power factor. All appliances
that are “Energy Star” qualified have high
power factors, which can significantly improve
your power efficiency. For example, in the
home construction field, a home that is certi-
fied as Energy Star qualified can be as much as
15% to 30% more electrically efficient, in
terms of watt-hours used per month.Determining Power Consumption
Performing an AC system load analysis is an
important part of ensuring that there is
enough available capacity to operate a boat’s
equipment based on typical use and con-
sumption habits. (Note that the same applies
to DC service, but we will be dealing with
AC service only.) In many cases, boaters are
attempting to add air conditioning or refrig-
eration systems to older boats. Such systems
typically use quite a bit of power and often
require installing additional, isolated shore-
power service to the boat, which can be
expensive. (Using an Energy Star appliance
might make the difference between having to68 electrical systems troubleshooting