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John Drake Services, Inc.
1427 E. 68th Street
Long Beach, CA 90805

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D.C. to A.C. Inverters

Basically an inverter takes direct current from a battery, or batteries, and turns it into 120 volt a.c. or higher.

There are a number of schemes to change battery power into a.c. for use in common loads.

There are/where pretty much four basic types of "inverters".

We will take a trip down memory lane and then come back to the present.

When I got started in this in the 1970's you had two "affordable" choices for d.c. to a.c.
power conversion.

There were a small number of "square wave" solid state inverters available.
These would operate motors and simple radios sorta of okay, if you wanted to use a television
sometimes they
would work but the picture was less than great and sometimes is was smaller than the screen.
Electric motors usually worked okay but they ran hotter than if they were running on 120 v.a.c.
from the power company.
All 120 volt a.c. devices were designed to see the curved sine waveform produced by
the mechanical generators of your utility company.
The further the wave form varies from a true sine wave, the poorer the device operates.
I remember using fluorescent lights on a square wave inverter - it was entertaining.

square wave inverter output square wave form

If you had to have a true (pure) sine wave, and did not have a rich uncle, there were
electro-mechanical units called rotary generators.
These had a shaft with a d.c. motor wound on one end and an a.c. alternator wound
on the other.
When d.c. voltage was applied to the motor, the shaft spun and created "120 volts a.c.
from the other end.
They were not too large, many were used in emergency vehicles at the time,
but they were expensive and heavy.
Besides the above there was another downside.
The output voltage was dependent on the input voltage maintaining the proper r.p.m.'s.
As the battery voltage dropped, the a.c. voltage output dropped accordingly.
They were also very inefficient as much of the input power was consumed in turning the shaft.

Types of currently available d.c. to a.c. inverters.

In the late 70's the modified sine wave (also known as the quasi-sine wave) inverter
became available.
It was similar to the square wave except that some had small steps on the side of the
square wave form and some spikes on the top flat part of the wave form.
These steps and spikes were an attempt to make it similar to the true sine wave
supplied by the electric company.
These were more efficient than the earlier square wave inverters and a.c. loads
worked better on them.
Laser printers usually don't work well on these inverters.

 modified or quasi sine wave inverter   modified sine wave form

The next to come along were "true" or "pure" sine wave inverters.
These took the modified wave form and added many steps to the sides
and the top.

A pure sine wave form dwells (rests or waits) in the middle of the wave form
(no potential) before continuing in the other direction.
Loads such as timers and some battery chargers have to see this dwell
time to work properly.
In many cases these inverters produced a cleaner and more stable output
than the utility company itself.
A motor load will usually operate cooler and more smoothly on a true sine
wave inverter than on a modified sine wave inverter.

Over a decade ago I purchased a 3000 watt modified sine wave inverter.
For giggles I set it next to an Exeltech 12 volt / 1100 watt inverter.
I tried all of my electric hand tools (8 amps or less draw) on each inverter.
The Exeltech ran each one smoother, cooler and quieter.
The sinusodial wave form is what all 120 volt a.c. loads want to see.

Inverter Ratings.

Inverters will have two wattage ratings.
The first, or lowest, is the continuous rating.
The second is the surge rating.
The surge rating can be 50 to 100% greater than the continous rating for starting
inductance (motor) loads which require a higher wattage than the running wattage.
Usually a modified sine wave inverter will have a higher surge rating than the same
size pure sine wave inverter.
You will also want to confirm how many seconds the inverter will output the surge
When you size the fuse or circuit breaker for the inverter you will need to consider
the surge rating so it does not blow when the inverter surges.

A word of caution.

When you look at an inverters surge rating please Read the Fine Print.
Many inverters have impressive surge ratings for starting electric motors for use
in compressors and pumps.
Some of these ratings show the surge capacity in milliseconds,
yep - thousandths of a second.
Some electric motors will pull 100 to 200% over the name plate rating in amps
for two to three seconds to get running.
That is two to three full seconds, not milliseconds (1/1000th).
You can see that some of the claims are meaningless sales hipe.
Remember, knowledge is power.  

Inverter Efficiency.

Most inverters over 250 watts have a fan mounted on one end.
It is there to exhaust heat generated by the electronics.
This heat is wasted energy from the batteries.
A percentage of the power put into the inverter never reaches the output,
it is converted into heat.
Usually the better quality the inverter, the higher efficiency it will have.
Efficiency is very important when the charging source for your batteries
is limited such as solar or wind.
If you were to compare two inverters, and one cost a little more but has
a higher efficiency, the more expensive one may be the best value.
Historically modified sine wave inverters were somewhat more efficient
that true sine wave inverters.
Another way to simplify efficiency would be that an inverter may be
outputting 1000 watts in 120 v.a.c. but drawing 1,200 watts from the
The inverter would have an approximate 84% efficiency.

Inverter search mode.

Most decent sized inverters made today have a search mode.
When there is no a.c. power going out of the inverter is lowers its power
draw to anywhere from five to twenty-five watts or more waiting to be
called on.
In this search mode the inverter "searches" for a 120 volt a.c. load,
when the a.c. load comes
on the inverter powers up to provide the wattage required and then
powers down when the a.c. load is shut off.
You want to take into consideration the idle or search mode power
draw when purchasing an inverter.
Phantom loads such as those from a wall wart, clock or other
electronic device that requires 120 volt a.c. to maintain a memory or
setting will keep the inverter from going into a low power idle mode.    

Inverter Sizing.

Many years ago Trace had efficiency charts on their website for
each inverter.
I remember a 1000 watt inverter that at 1000 watt output it had
an 88% efficiency.
That same inverter at 350 watt output had a 40% efficiency rating.
The closer the load wattage to the name plate rating on the inverter,
the less wasted energy from the batteries.
Bolted to my desk is an Exeltech 48 volt - 125 watt inverter to run
my monitor.
Bolted to the other desk is a Samlex 24 volt - 600 watt inverter to
operate other office equipment.
Outside are Exeltech 600 and 1,100 watt inverters with 12 and
48 volt battery banks.
You may be wise to be realistic.
Running three to four hundred watt loads from a whiz bang
3,000 watt inverter might not be such a good idea.

A strategy that usually works out when you have a wide range of
120 volt a.c. loads is to use more than one inverter.
A small inverter to operate the most often low power draw loads.
And a larger inverter to only operate heavy loads such as well pumps.
In the long run this can extend the life of your battery bank.

Choosing the type of inverter you need.

Before you purchase an inverter it would be a good idea to find
out what kind of wave form your loads require to operate properly.

Modified sine wave inverters will operate most loads.

A quality pure sine wave inverter will operate all loads within
their running and surge wattage rating.

Please note, if you are living off-grid and your inverter poops out,
the cheapest "bargain" can turn out to be the worst choice.

Many years ago we sold true sine wave inverters that were made in Texas.
They did not have any bells and whistles but the output was as clean as
could be found and their reliability was the best.

Then the "cheap" imports hit the market.
They put out lots of wattage and had loads of bells and whistles.
We have not handled inverters since then.