22

Home Power #32 • December 1992 / January 1993

Inverters

lternating current or direct
current? How shall electricity be
sold to the public? Over a

century ago the battle raged, George
Westinghouse versus Thomas Edison.
Edison had electrified New York City with
DC power, to the wonderment of the
world. Then along came this upstart
Westinghouse, with his foreign friend
Nikola Tesla, and changed everything.

A

What’s an Inverter?

or Why can’t the world run on
DC and make life easier for
everyone?

Elliot Josephson

©1992 Elliot Josephson

Volts to run a 12 Volt motor, and then to use that motor to
drive a 120 Volt generator. It worked, but it was terribly
expensive and wasted a lot of power.

Transformer Magic!
A bit of magic was needed to overcome these problems.
Ac provided that magic because, unlike DC, it could
operate transformers. Transformers are devices that can
change the voltage up or down easily, inexpensively, and
efficiently.

To appreciate the importance of the transformer, put
yourself in the position of the electric power utility. It costs
you money to generate power at your power plant, and
you can sell that power only by delivering it to the user,
wherever he may be. Any power that gets lost along the
way comes out of your pocket. The user will only pay for
the power he actually receives.

Suppose that a reasonably sized power transmission line
loses 60 volts per mile. If you start with a 120 volt signal,
you’ll lose half of the voltage sending it one mile. But if
you transform the signal up to 600 volts and send it a
mile, the 60 volt loss is only 10% of the total. And if you
transform the signal up to 6000 volts and send it a mile,
you only lose 1% of the voltage. In fact, the utilities
transform electricity as high as 500,000 volts to send
power over long distances, and then transform it back
down to 120 volts to supply their customers.

Standards
Once the ac versus DC battle was settled, it took a long
time before the voltage, frequency, and socket spacing
was standardized, but finally a manufacturer could build a
toaster and know that it would plug in and work anywhere
in the United States. Appliances of all sorts were
designed and built to operate on ac power, and DC was
used primarily for automobiles and flashlights.

The First Inverters
Where it was necessary to convert DC to ac, a rotary
inverter was used. It consisted of a DC motor driving an
ac alternator at the proper rotational speed to create ac at
60 Hertz (cycles per second). Some of these inverters are
still being sold under the trade name Redi-Line™ and are
primarily used in utility vehicles.

As automobiles became more sophisticated, it became
desirable to install a radio, first for police and emergency
vehicles, and then for the general public. Automobiles
used 6 Volt batteries then, and the transistor hadn’t been
invented yet. Radios ran on vacuum tubes which needed
over 100 Volts to operate. Clearly, a device was required
to change the 6 Volts DC into ac so it could be
transformed to a higher voltage.

Everybody knew DC was simpler. Direct current flowed
from positive to negative continuously and did its job very
nicely. What foolishness it was to talk of alternating the
direction of current flow sixty times a second. No good
could come of that!

Editorials were written. Lawsuits were argued. Millions of
dollars were at stake. In the end, the battle was decided
by economics. No amount of philosophizing could
overcome the fact that alternating current was just plain
cheaper to make and distribute than direct current.

Why Not DC?
But why? Doesn’t the simplicity of DC electricity make it
easy to work with? Yes, as long as you’re reasonably
close to the source, and as long as you’re happy with the
voltage that’s available. But suppose that your source of
power is a great big 12 Volt battery, located a mile away.
You’ve seen how heavy the battery cables are in your car
just to carry the current a few feet to your starter and
engine. Imagine how heavy (and costly) the wire would
have to be to carry DC current a mile and still have
enough voltage left to light a headlight!

Or suppose 12 Volts DC were available, but you really
needed 120 Volts to light your bulb. In Edison’s time, the
only way to make this transformation was to use the 12

23

Home Power #32 • December 1992 / January 1993

Inverters

One of the earliest inverters, used in
the radio for the family car, was an
electromechanical vibrator. The
vibrator was a type of buzzer, with
contacts that opened and closed
many times a second, to switch DC
into ac. This ac could then be
transformed up in voltage. After
transformation, another set of
contacts switched the high voltage ac
back to DC to be used by the radio.
Although effective and relatively
cheap, it had very poor reliability and
had to be replaced fairly often.

Square Wave Inverters
When the transistor appeared on the scene, it replaced
the vacuum tube. Now you could operate a car radio
directly from the battery voltage, so radio vibrators were
not needed. At the same time, it was also possible to build
a transistorized inverter for general purpose use, getting
rid of the unreliable vibrator. This type of inverter
consisted of little more than a transformer and a pair of
transistors. The transisortized inverter is also called a
static inverter because it has no moving parts. It is still
being sold today as part of the Tripp-Lite inverter line.
Instead of the sine wave delivered by an electric utility,
this inverter produces a square wave. Because current is
a switched, or turned on and off, the generated current
had very abrupt changes and over time looks like a wave
with square corners. This is very unlike what comes from
the power company which has much more gradual
changes that look like sine waves. With this type of
inverter neither the frequency nor output voltage is
regulated, the inverter has little surge capability, and it is
not protected against overloads except by a fuse or circuit
breaker. However, it is simple and low cost and suitable
for many non-critical applications.

The shortcomings of the square-wave inverter become
most evident when running a motor. Ac induction motors
draw a substantial current surge on startup. They really
prefer sine-waves to square-waves and may overheat on
the latter. Ac induction motors also store energy during a
portion of each cycle, which will create problems if not
returned to the battery or otherwise controlled.

Enter the Modified Sine Wave Inverter
In response to these shortcomings, Heart Interface
pioneered the development of the “modified sine wave”
inverter, and today most of the inverters sold are of this
type. The “modified sine wave” is neither a sine wave nor

1

⁄

60

th second

-200

-100

0

100

200

Sinewave

Squarewave

"Modified

Sinewave"

Volts

a square wave, but a moderate-cost compromise which
runs most loads in an acceptable manner.

The inverters described up to this point are fairly large
and heavy. This is simply because they generate power
through a 60 Hertz transformer, which, by the laws of
physics, must weigh about 30 pounds for 1000 watts.

A smaller, lighter alternative is found in the inverters
pioneered by PowerStar and now offered by both
PowerStar and Statpower. Instead of converting 12 Volts
DC directly to 120 volts 60 Hertz ac, they use a multi-step
process. First 12 Volts DC is changed to 160 volts peak
high frequency ac (25 kiloHertz). This is converted to 160
Volts DC, and finally inverted to 120 volts rms, 60 Hertz
ac. At the heart of this process is the high frequency
transformer, which is less than one-tenth the size and
weight of a 60 Hertz transformer for the same power
level. Thus a 5 pound inverter can do the job of a 50
pound one.

Sine Wave
A few true sine wave inverters have appeared on the
market, notably from Dynamote and Exeltech. This kind of
inverter will run motors cooler and may offer less
interference with radio and TV. However, they are
inherently more complex and less efficient, so they may
not be justified in many installations.

Ask your local dealer
If you are confused as to which is the best inverter for
you, nothing can beat a truly knowledgeable dealer who
carries a broad line of inverters. He will know what’s
available, what works, and what it costs, and he can
translate your present and future projected needs into a
solid recommendation.

Meanwhile, look for technology to progress and for
inverters to become more efficient, better protected, more

24

Home Power #32 • December 1992 / January 1993

Inverters

reliable, smaller, lighter, less expensive, and easier to
use. This trend will undoubtedly continue for many years
to come.

Access
Author: Elliot Josephson, PowerStar Products Inc., 10011
North Foothill Boulevard, Cupertino, CA 95014 •
408-973-8502 • FAX [phone]

Things that Work!

UPG & POW 200

tested by Home Power

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And now you don’t need to worry about
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ratings are continuous and not time derated like many other inverters. Ask about our
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Try a PowerStar inverter for 30 days. If you are not completely satisfied, return it to
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1

/

3

the size and

1

/

10

the weight of the other guys. — RISK FREE!

For Home Power readers only: PowerStar Products, Inc. is offering a $50 rebate on all UPG
inverters, purchased in December of 1992. Just fill in the information below, and mail to PowerStar
with your warranty card and a dated copy of your receipt.*
Name: ______________________________________________________________________
Address: _________________________ City ______________ State _______ Zip _________
Phone: ________________________________ Date of purchase: _______________________
Dealer name: ___________________________ Inverter Serial# __________________________

*Must be received by 1/15/93. This offer is valid for UPGs sold between the dates of 12/1/92 and 12/31/92 only, and must be
accompanied by this original coupon in Home Power and a self addressed, stamped envelope.

For more information, call PowerStar at: [phone]

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