Generating your own power doesn't mean you
have to give
up the convenience of AC living.
by TJ Byers
The progress made in developing alternative sources of
energy over the last decade has demonstrated that
independent power systems (those using other than fossil
fuels) are not only possible but are also very practical.
In fact, a wide variety of generating equipment is now
available to allow individuals to take advantage of just
about any renewable source of energy.
For a number of reasons, however, most of these systems
produce only direct current (DC), and often do so only at
low voltages. Nonetheless, it's generally agreed that the
greatest potential use for alternative energy in the future
will be to serve alternating current (AC) loads ... since
those exist in the vast majority of modern homes.
Conventional refrigerators, most televisions, and
all induction motors (which comprise 97% of the
U.S. supply of electric motors) simply won't work on direct
current. Furthermore, although there are DC-compatible
substitutes for some such appliances, they're typically
Of course, one way out of this dilemma would be to convert
direct current to alternating current ... and one of the
most effective ways to accomplish this is through the use
of an electronic inverter. In this ar ticle, I'm going to
tell you about the different sorts of inverters that are
available, and their applications. Then, in MOTHER
NO. 81, we'll get down to the details of putting one of
these devices to work.
To understand how inversion is accomplished, you should
first comprehend how AC power differs from DC. Compare the
voltage components of both types of power, as illustrated
in Fig. 1, and you'll see that in alternating current the
voltage is constantly reversed. The polarity (positive or
negative) switches rhythmically in the form of a sine wave.
On the other hand, the polarity of direct current is
constant ... that is, the positive lead is forever positive
(even though the voltage may vary).
The basic idea of an inverter, therefore, is to
periodically change the polarity of the DC source. Of
course, there is more than one way to approach the problem,
but semiconductors (such as transistors)—which
accomplish the metamorphosis by switching currents through
a transformer—are the most common solution.
INSIDE THE INVERTER
Fig. 2 is a simplified diagram of a typical inverter
circuit. The workhorses are transis tors ... solid-state
switches that can be changed from "off" to "on" simply by
applying a small voltage to their control elements. To take
advantage of this property, the emitter leads from
a pair of transistors are connected to opposite ends of a
center—tapped transformer. The collector
lead from each transistor is then wired to the positive leg
of a DC power source—a storage battery, for
example—and the center tap from the transformer is
returned to the negative lead of the power source.
If you're a little hazy about this, read on ... I think the
picture will become clear once the sequence of events is
described. First, a signal is applied to the left
transistor that "tells" it to allow current to pass. The
flow of electrons proceeds from the battery, through the
transformer, into the transistor, and back around to the
battery ... counter-clockwise as indicated by the arrow.
The transformer primary winding delivers the energy to the
output winding, and current moves in a clockwise direction
through the load as shown.
After about eight-thousandths of a second, the first
transistor is turned off and the other (on the right) is
switched on. As shown in Fig. 3, the current again flows
from the battery, through the transformer, into a
transistor (but this time it's the one on the right), and
back to the battery. However, the current is now traveling
in the opposite direction (clockwise) in the
transformer windings. This means that the electron movement
in the load portion of the circuit is counterclockwise.
As you can see, then, this switching mimics an AC current
pattern, and when it performs a backward and forward flow
60 times per second, you get the rough equivalent of
60-hertz electricity. (What's more, the transformer can be
designed to raise voltage to the desired level.)
Although transistors are employed in many small inverters,
they're not really very efficient when used as switching
devices. Consequently, for inverters larger than one
kilowatt (KW), a component called a silicon-controlled
rectifier (SCR) is substituted in the same basic circuit.
Units using SCR's are available with power capabilities of
10 KW or more.
Unfortunately, even an SCR absorbs a portion of the power
that it's controlling, and losses in the transformer core
and from wire resistance add to the toll. All in all, you
can expect an inverter to supply about 9007b of
the input power to the load ... but only
under ideal conditions.
An inverter usually performs best when it's fully loaded,
or close to it (see Fig. 4). For example, if you have a
500-watt unit, it will work most efficiently when you're
drawing 400 to 500 watts through it. As the load drops, so
does the mechanism's efficiency . . . since an inverter
requires a certain amount of what's referred to as "tare"
energy to keep it running. At low demand, operating power
can become a large part of the overall output. In fact,
this standby (or idle) current will be between 5 and
20% of the maximum power the device is capable of
supplying, depending on the particular inverter.
The obvious solution to tare loss (which can drain your
batteries of valuable energy) is to shut down the inverter
when no power is being demanded. Several models on the
market incorporate circuits that sense power demand and
switch the inverter off when there's no load.
You may have noticed that I used the word "mimics" when
describing an invert er's attempt at producing AC power.
While it is true that the simplified circuit shown in Fig.
2 would produce alternating current, the form of
the wave generated by the two transistors would be far from
sinusoidal ... which is the pattern of pure AC
utility power, as illustrated in Fig. 1. In fact, the
waveform from our simple inverter would be more properly
called square, because of its characteristic shape (which
is shown—interposed with a sine wave, for
comparison—in Fig. 5A).
Now some AC equipment can handle squarewave AC power just
fine, but other devices have real problems with it.
Induction motors, for example, are designed to work on the
purest type of AC current—the sine wave—and
when the transistors in an inverter deliver a square wave
(which represents at a minimum, a distortion of 40% from
the sine wave) to one of these devices, the motor must work
harder to overcome the counterproductive currents. The
result is a marked increase in operating temperature and a
loss of efficiency.
The ultimate solution, of course, would be to generate a
sine wave. Unfortunately, this is easier said than done,
since inverters capable of producing such a form are
inherently bulky, expensive, and inefficient. A simpler
remedy is to modify a square wave to remove as
much of the distortion as possible. For example, if we
delay the activation of the second transistor, we'll get a
waveform that looks much like the one shown in Fig. 5B. As
you can see, this modified square wave is an improvement.
The distortion has been reduced from 40% to less than 20%.
Some inverters extend this concept even further by stepping
in multiple currents to generate the "staircase" effect you
see in Fig. 5C. The intricate switching pattern, however,
requires a controller that borders on being a
microcomputer, which increases the complexity and cost of
the devices. In fact, as a general rule, you can assume
that the closer an inverter's waveform is to sinusoidal,
the more you should expect to pay for the device.
Inverter design and selection is further complicated by the
fact that not all AC loads react equally to alternating
voltage. A utility company can compensate for this inherent
problem within its sizable grid ...
but in a small, independent electrical system, the behavior
of the load(s) is reflected back into the inverter.
An induction motor, for instance, requires substantially
more power to get going than it does to operate at its
design speed. (It actually takes six times as much
juice to start up one of these motors as it does to run
it!) As a matter of fact, every electrical device
in your household has some surge demand, though none is as
demanding as the induction motor.
A surge current can last for anywhere from a fraction of a
second to almost a full minute ... and, during
this time, the inverter is placed under heavy stress.
Fortunately, inverters can be engineered to withstand very
short periods of extreme overload ... but this
sort of abuse is best handled by SCR-equipped devices.
Of course, you could just buy an inverter large
enough to handle the calculated surge loads of the devices
you want to run. The disadvantages to this approach,
however, are twofold. First, and most obvious, a bigger
inverter costs more. Second, if you'll recall our
discussion of standby (or tare) power, you'll realize that
the upsized inverter will be forced to operate in its
inefficient, low-demand range once the load has passed
surge and dropped to its conservative operating level.
Another aspect to consider when dealing with AC electricity
is the power factor of the load(s). The power
factor results from a time distortion between the voltage
and current components of the AC waveform, and is created
during an operation with a reactive load (such as
the running of an induction motor).
To make it more clear, let's consider inductance (a
reactive value created by the windings in a motor). By its
very nature, inductance resists changes in the
current flowing through it (it would rather the current
remained steady). But as we have discovered, both the
voltage and current in an AC system are changing
So, as the voltage (which is unaffected by inductance)
increases across the motor winding, the current must work
harder to stay in step with it. However, despite such
efforts, the current can't keep up and lags behind the
voltage, as shown in Fig. 6.
When you consider that the simultaneous combination of
voltage and amperage makes up power, you can begin to see
what problems this time distortion might present. If the
voltage and current are out of phase with each other, then
the power—which is calculated by multiplying the
instantaneous values of voltage and amperage—will be
less than you would get by multiplying the peak voltage by
the peak amperage in the ideal situation.
The discrepancy between the true power (the amount
of energy the load is actually using) and the
apparent power (the power the load seems to be
using when the voltage and current are multiplied,
disregarding timing) is the power factor. For example, an
induction motor with a power factor of 0.7 is really
consuming only 70% of the power available to it. To
distinguish between the two values, apparent power is
expressed in volt-amps (VA), while true power is
measured in watts. Consequently, the volt-amp rating of an
appliance is always higher than its actual wattage.
Unfortunately, this phase shift also reflects back into the
inverter, where it can do real damage. The situation is
easily corrected, though, by placing a capacitor across the
inductive load. You see, a capacitor has a reactive
component which is just opposite that of an inductor: It
resists changes in voltage while leaving current
unaffected. By paralleling the proper amount of capacitance
to the inductive load, the effects of the inductance can be
cancelled, and the sine wave will once again be in phase
Just as the nature of the load affects the inverter, so can
the inverter affect some loads. There are three important
cases in which this can come into play ... and
we'll discuss those situations briefly here.
PEAK VOLTAGE: The amount of power delivered by an
AC waveform is equal to the area under the curve
... even if that curve happens to be square. Now
if you refer back to Fig. 5B, you'll see that the sine wave
must reach a higher voltage if it's to develop the same
amount of power as the square wave.
Of course, the load (the power demand, that is) doesn't
really mind the voltage difference between sine and square
waves. It's interested only in receiving all the power it
has coming. However, some electronic devices are designed
to take advantage of the recurring peaks of a sine wave.
For instance, many color televisions utilize this voltage
to generate higher voltages inside the set ...
thereby eliminating bulky power supplies. Obviously, the
square wave can't deliver the voltage that such electronics
require, and the set won't work properly.
VOLTAGE REGULATION: In most inverter applications,
it's of primary importance to stablilize the output
voltage. Without an internal regulator, a square-wave
inverter will give a correspondingly lower voltage output
as the voltage of the input drops (which is a regular
occurrence in battery storage systems). And since most
appliances are designed to operate within a specific
voltage range, an extreme drop could result in
When you shop for an inverter, then, you should be certain
that it has voltage regulation and that its range of
tolerance of input is in keeping with the generator or
batteries that will be supplying it with power.
FREQUENCY. Devices that are sensitive to voltage
peaks and regulation are also likely to be sensitive to
frequency (clocks and timers are two examples). Ideally,
the output frequency of the inverter should always be 60
cycles per second (in the U.S.). But in actual application,
variations in input voltage, temperature, output power,
etc. will influence the frequency. And, al though
large inverters do an admirable job of limiting
frequency drift, many small, inexpensive transistorized
units have no means of stabilizing the output at all.
Yet another member of the electronic inverter family is the
synchronous inverter. Unlike the static units we've
discussed so far—which take power from a source,
invert it, and deliver it to a load—the synchronous
type is designed to be hooked up to a local utility.
As I'm sure you're aware, most alternative energy sources,
particularly the wind and the sun, are by nature
intermittent. To get through the calm or dark times,
therefore, people often use storage batteries.
Unfortunately, batteries are expensive and messy, and they
The synchronous inverter eliminates the problems of
chemical storage by allowing owners of alternative energy
systems to use utilities for backup power. Though different
utilities have different ways of making the necessary
connection, the essential elements of the arrangement are
that you can buy power from the grid when you run short,
and sell it to the utility when you've got an excess. An
interesting note is that the utility is required-by
law-both to make this connection and to pay you a
fair (but, of course, wholesale) price for the power you
deliver to it.
SELF-COMMUTATION: This sort of synchronous
inverter is quite similar to the static inverter we talked
about earlier ... with one important exception:
Utility power controls the unit's output waveform and
Therefore, since the utility's waveform is purely
sinusoidal, neither square- nor modifiedwaveform inverters
can be used for interfacing. They can be adapted, however,
with the proper filtering to remove unwanted harmonic
distortion. The basis for the synchronous inverter is
usually a stairstep waveform unit, though, since output
from such a device requires far less filtering and is thus
more efficient. The frequency of a self-commutated inverter
is locked onto the line frequency by phasing the operation
of the electronic switches with the AC input.
LINE COMMUTATION: Another way to handle current
switching in a synchro nous inverter is to rely entirely on
the sine wave from the utility for commutation. With this
approach, the inverter won't function if the power lines go
dead. Consequently, an independent power system connected
to the utility by a linecommutated inverter won't be of use
in the event of a public power failure.
POWER FACTOR: One major shortcoming of many
synchronous inverters is their low power factors. Because
of present design practices and intrinsic characteristics,
it's not uncommon to find units with ratings of 0. 5 or
Now power factor correction for synchronous inverters
can be accomplished with capacitors, as it is for
the static inverters we discussed earlier. However, an
inverter's power factor-and specific demand for
capacitance—is influenced by changing loads, grid
inductance, phase angle, etc., and as a result, it changes
constantly. To date, inverter manufacturers have relied on
the sheer size of the utility network to absorb these
defects. But as more and more independent producers are
placed on line, additional efforts will likely be
I'd like to stress, at this point, that the power factor of
an inverter itself doesn't affect its efficiency ... it
merely changes the unit's rating. For example, a 4-KW
inverter with a power factor of 0.5 is rated at 8 KVA. This
doesn't mean that only half the power is being delivered to
the load, though, since if the power factor were corrected
to unity (using capacitance), the unit's capability would
still be 4 KW.
WIRE NUTS AND BOLTS. Unfortunately, tying into a
utility isn't as easy as it may first appear. Whether you
use a self- or line-commutated inverter, there are
important safety considerations to be taken into account.
The self-commutated inverter, for instance, would keep
right on humming if the utility lines went dead. This could
have several serious consequences. First, if the disruption
were caused by a component failure—such as a
transformer—the inverter would feed power into the
defect, which could wreak havoc. At the same time, a
service worker could conceivably receive a lethal shock
while trying to repair equipment that was supposedly
In theory, at least, a line-commutated inverter should stop
working if the utility power is removed. This should
protect the grid from independent producers ... unless more
than one such plant is on the line. You see, it's not
impossible for two, or more, inverters to synchronize
between themselves and continue producing power.
A synchronous inverter, therefore, must include a sensor to
disconnect it from the grid when even one cycle (1/60 of a
second) is missed. Actually, though, it's not unusual for a
utility to skip a beat now and then, because of the many
switching stations such firms employ to manage power flow.
So the inverter's "fail safe" detector must be smart enough
to judge the difference between a slight case of the
jitters and an honest-to-goodness failure.
Living in an AC world may not be the best (or the worst) of
all possibilities, but it is one that we all have to deal
with. Through the proper use of an inverter, DC and AC
power can be made compatible—and
cost-effective—though ... and, for many of us, it may
be the only practical method of having our electricity and
using it too.
EDITOR'S NOTE: In the next issue of MOTHER,
we'll tell you how to go about selecting, sizing,
and using inverters. Don't miss it!
THE CURRENT PATH NOT TAKEN
Before the advent of solid-state inverters, DC power
was converted to A C by a couple of different techniques.
One, the vibrating inverter, is now largely gone ...
and for the most part, its passing is no great loss.
Most of the vibrators were pretty unreliable, and produced
alternating current of limited quality and
The DC to AC converter — often called a
motor/generator — is still with us,
how-ever and has some useful applications. Since
this device consists of a DC motor that drives an AC
alternator, its waveform is pure sine. Therefore, the
converter can be used for driving appliances that
require a sine waveform ... and it's
particularly attractive when you consider that it's
substantially less expensive than an equivalent electronic
On the other side of the coin, motor/generators are
considerably less efficient than are electronic inverters
(they usually operate at no more than 80% efficiency),
consume a large amount of idle current, and require
periodic maintenance. Currently, there's only one
manufacturer of motor/ generators. You can contact it by
writing Honeywell Motor Products Division, Dept. THEN, P.O.
Box 106, Rockford, Illinois 61105.