Trace Engineering's SW4024 Inverter
Every once in awhile a product comes along that revolutionizes a
whole industry. The SW4024 power inverter is such a product.
Powerful, compatible, efficient, stand alone or grid intertie, this
machine has it all. With 4,000 Watts of
wave output power, a built-in 120 Amp battery charger, automatic
generator starting, and load diversion circuitry plus much more,
this device will tie it all together and do more to mainstream the
use of renewable energy than anything we've seen so far.
For newcomers to Home Power, an inverter is a device which changes
DC power stored in a battery to standard household 120/240 vac
electricity. Most renewable energy charging sources (PVs, microhydro
and wind) generate DC which is stored in batteries. Nearly all
lighting, appliances, motors, etc., are designed to eat ac power, so
it takes an inverter to make the switch from battery-stored DC to
standard house power (120 vac, 60 Hz).
The Name Game
Inverters come in two basic flavors, square wave and sine wave. That
sounds simple enough, but it ain't. Most inverters sold today
produce a modified square waveform. The marketing spin doctors tend
to call it modified-sine or quasi-sine wave. Whatever, it all looks
the same on an oscilloscope. Most ac devices will digest this
waveform OK, but there are some notable exceptions. Devices such as
laser printers, which use triacs and/or silicon controlled
rectifiers, fry and die when fed mod-sine power. Motors and power
supplies usually run warmer and less efficiently on mod-sine power.
Some things, like fans, amplifiers, and cheap fluorescent lights,
give off an audible buzz on mod-sine wave power. On the plus side,
mod-sine inverters make the conversion from DC to ac very
efficiently. They are relatively inexpensive, and many of the
electrical devices we use every day work fine on them.
To have the term sine wave applied to inverters at all is unique.
Sine wave is usually used to describe a waveform produced by a
rotary generator. In an inverter, direct current (DC) is switched
back and forth to produce alternating current (ac). Then it is
transformed, filtered, stepped, whatever, to get it to an acceptable
output waveform. This is not a trivial process! Much wizardry goes
on in a sine wave inverter. The more processing, the cleaner and
quieter the output, but the lower the efficiency of the conversion.
No free lunch, eh? The goal becomes to produce a waveform that is
acceptable to all loads without sacrificing too much power into the
The SW4024 Waveform
Trace calls their waveform a "stepped approximation of a sine wave."
I call it the "Mayan temple" waveform. Check out the photo. It's the
damnedest thing I ever saw. And it changes! Trace varies the number
of steps per cycle between 34 and 52 depending on battery voltage
and/or load. Low input voltage or a large load increases the number
of steps. More steps mean a closer approximation to a sinusoidal
We can discuss the theories and relevance of sine waveforms until
the cows come home, but the goal is the same: a waveform that is
acceptable to all loads without sacrificing too much power into the
conversion process. Put simply, if all loads will digest the
inverter's waveform as well as they do a sinusoidal wave, who cares
what it looks like? They gotta eat it, not me.
Packaging and Manuals
Trace has always packaged their machines as if each one had to go
"there and back again." The SW4024 is a BIG machine. It measures
21"W x 15"H x 9"D and weighs in at a whopping 105 pounds (48 kg).
Packaging such a behemoth to go around the world is no small thing.
Trace started out using the molded foam technique that works well
with their smaller inverters. After a few too many dings and crushed
battery terminal covers, they switched to two flexible foam cradles
which support the machine from all sides and keep it suspended in
its heavy cardboard shipping box. The whole shebang is put into
another close-fitting heavy cardboard box with cutouts for hand
grips and is stapled closed. Short of a jousting contest with
forklifts or practice for the caber toss, we think the 4024 will
In the past, Trace owner's manuals have been top notch. We were
happy to see this tradition continue with the SW4024, with only a
few flaws. The section on battery recommendations seems to have been
taken directly from the 2500 Series manual without any updates as to
sources of supply. The newer SW4024s are compatible with alkaline
batteries. Also, the section dealing with automatic generator
starting and stopping needed far more explanation than was given.
The manual for the inverter we tested was Version Two. By the time
you read this, Version Three should be shipping. It speaks to some
of the SW4024 improvements made in the past few months.
The manual runs forty-eight 8-1/2" x 11" pages plus two fold-out
diagrams and a five-page addendum of warranty and registration
information. None of it is fluff. This is a very serious machine.
There are so many features and user adjustable parameters it takes a
computer-like, menu-driven, LCD control panel just to display them
all. The manual does an excellent job of explaining the operation.
It starts right off with important safety instructions. Read these
first. This is a machine to be reckoned with.
The manual then goes into an overview and an explanation of the
modes of operation, including inverter, battery charger, UPS, and
generator or utility interactive mode. The manual proceeds to
explain the control panel and its functions. Following that is the
discussion of batteries. Finally, on to the actual installation
While the owner's manual is written in plain English with a minimum
of technical jargon, its job is to try and explain a very complex
and powerful piece of equipment. Read it before you unpack the
inverter. If it makes you nervous or gives you a headache, the
manual has probably done its job. Think about hiring an RE
electrician to help you out.
SW4024 Specifications & Features
The SW4024 is packed with features. So many, in fact, that we
haven't heard of anyone using all of them in one application. The
following is a brief description and synopsis of the main ones:
It's a 4,000 Watt sine wave inverter. Continuous. And then some.
It has a published idle draw of 16 Watts (we measured 14 W) and a
search mode where the inverter powers itself down to one Watt
while sending out pulses until a load is detected.
It's a three-stage battery charger. Adjustable charge rate from 0
to 120 Amps DC. Adjustable bulk charge voltage, bulk done amps,
float voltage, battery temperature compensation, and max charge
It's an uninterruptible power supply (UPS). If the grid (or
generator) goes down, it's on line in 32 milliseconds or less.
It's a utility interactive synchronous inverter. If you have
excess power from your RE sources or stored battery power, it will
synchronize to the grid waveform and sell back power. The battery
voltage sell point and maximum sell current are adjustable.
It's a peak load shaving inverter. It has an onboard clock which
can be set to power your loads during peak energy consumption
times and recharge the batteries off peak when grid electricity is
It provides generator support. Should your generator need
assistance starting a large load, it will switch from charging to
inverting mode, synchronize with the generator waveform, and give
up to 10 kW extra starting boost. The generator support voltage
and maximum current support are adjustable.
It provides a run and a cranking relay to automatically start and
stop my your generator based on battery voltage.
It has separate and interactive inputs for utility and generator
It has three independent voltage controlled relays which can be
used for load diversion, alarms, whatever. Both the relay trip
voltage and the change state voltage (hysteresis) are adjustable.
The control panel sets the inverter's operating parameters using an
LCD display and eight pushbuttons. One of the buttons is dedicated
to turning the machine on/off or into charger mode. This button is
colored red for easy identification. A green button switches an
automatically starting generator on or off. Two buttons scroll
backward or forward through 15 general menu headings. Two others
scroll backward or forward through the items under each of the menu
headings. The remaining two buttons set the programmable parameters
of the menu items. There are 47 user-adjustable settings, 9 metering
functions, and 8 diagnostic readings to indicate errors which would
cause the machine to shut down. All accessible via the control
In addition to the LCD display, there are eight LED indicators
showing whether the machine is inverting, charging (bulk or float),
selling power, has a malfunction, or has grid or generator power
present on its inputs.
The control panel also has an adjustment for the LCD contrast for
better visibility in different light conditions and an Oops! button
which resets all parameters to factory defaults in case you've
really got your knickers in a knot.
The SW4024 control panel has an ac ammeter and both 120 vac and 24
VDC voltmeters. The ammeters display inverter amps (- while
inverting, + while recharging), input amps either from grid or
generator, and load amps passing from the inverter to the loads. The
rms ac voltmeters indicate the inverter ac output voltage, and grid
or generator input voltages. The DC voltmeters show actual battery
voltage, and temperature compensated battery voltage if the included
temperature sensor is used.
The Test Bench
We tested the SW4024 using four Fluke 87 DMMs and a Hewlett-Packard
AN/USM 339 dual trace oscilloscope. The Flukes have a DC accuracy
rating of 0.1%. The scope was used to view the waveform and the
current phase. Home Power had never tested an inverter this big.
Even after rummaging through two households, we could only come up
with 3850 Watts of resistive loads!
In order to measure the inverter's input current, we used a Fluke 87
measuring the voltage drop across a 500 Ampere, 50 mV shunt. The
Fluke measuring DC input voltage was connected at the inverter's
input terminals. Output current was measured across a 100 Ampere,
100 mV shunt.
We used Bob-O's main battery (twelve Trojan L-16s1050 Ampere-hours
at 24 VDC) power the inverter. Even with this much stored energy, we
had to wait for a sunny day with wind to come up with enough power
to really twist the Trace's tail. This is a really big inverter and
feeding it requires lots of watts! We set up the test jig in the
basement, and testing lasted for over five hours. We had to stop
testing periodically to let the battery recover and recharge.
We measured the inverter's no-load power consumption at 13.31 Watts
(0.5 Amperes at 26.61 Volts DC). We then proceeded to add loads from
25 watts to 5236 watts. The Trace SW4024's efficiency was over 90%
from 95 watts to 2748 watts. We measured 87% efficiency at a full
4000 watts out of the inverter. For a sine wave inverter, this is
very high efficiency.
Both the rms and peak output voltages of the Trace were very stable.
We measured a low rms voltage of 118.5 and a high of 121.9 vac rms.
The peak voltage of the Trace's waveform varied from 161 to 172 vac
peak. This degree of stability is better than the utility grid
delivers to most homes.
The power efficiency chart speaks for itself. Our overload tests
showed us this machine really has lots of snort. After loading up
the 4024 with our 3850 Watts of resistive loads long enough to bring
on one of the internal cooling fans, we started a 1/2 HP split-phase
grinder. With the grinder and the other loads running, we started a
1/2 HP air compressor. Finally, with all of the going, we started a
7-1/4" Skilsaw. The Trace still had more to give, but our 250 Ampere
input circuit breaker popped off at 274 Amps input!
We didn't have the capability at the test site to adequately test
the charger features, but data observed from other SW4024
installations indicates it will do as specified at around 75%
efficiency (117 vac in to 24 VDC out). This is a big improvement
over the Trace 2500 Series chargers which run at about 55%-60%
Here are some comments from other SW4024 users.
"The table saw starts much faster, even with the electric water
"Wall cubes don't buzz (some cheap compact fluorescents still do)."
"The three general purpose relays are great! I have simply been
plugging/unplugging the electric water heater (dump load which also
serves a useful function). Now I can use one of the relays to
control a power relay."
"I like being able to use power tools while someone else is
microwaving without dumping the computer."
Warts, or, Ya Can't Please Everybody...
The lowest search threshold is 16 Watts. We'd like to see it a
little lower. Over the years, many of us have trained ourselves to
eliminate all phantom loads and replace our energy sucking
incandescent lighting with low wattage compact fluorescents. I've
heard of folks having to insert a phantom in a circuit just so their
favorite 7 or 11 Watt CF reading light will stay lit.
The pushbuttons on the control panel sometimes don't make contact
and you need to punch them a few times to change the display. The
menu items under the menu headings are not necessarily grouped
intuitively. We'd like to see two copies of a control panel flow
chart, one with the manual and one loose to tack up on the wall next
to the machine until you can find your way around.
While the SW4024 is pretty quiet on its power output, the inverter
itself emits a very audible hum, especially at high output power.
For most applications and installations this should be not problem
at all, but if you have to live or work in close proximity to the
inverter, then the noise will be irritating.
The stickem on the battery temperature sensor needs some help. It's
come unstuck from the sides of several battery banks I've seen.
If this sounds pretty picky, it is! We had to hustle to come up with
these! Given the incredible versatility of the SW4024, these are
pretty minor flaws.
This and That
The SW4024 is also available in a 40 VDC input version. It also
comes in a 3kW, 234 vac, 50 Hz export version, the SW3024E. Trace
plans to release a 2500 watt, 12 VDC model by this fall. Options for
either the 4024 or the 4048 include a conduit ready enclosure for
the DC input cables, a series interface cable and a full function
remote display. Everything else is built in. There is a stacking
interface that allows two 4024s to operate together. The warranty is
The Trace 4024 is gaining acceptance by utilities. It has been
approved by Pacific Gas & Electric for utility intertie in Northern
California. The Trace SW4024 is ETL certified to UL spec. 1741
(residential service), so there will be no problems with
installations on grid.
The Trace SW4024 is an incredible device. It is the missing link
between small-scale renewable energy and America's utility grid. It
makes a wonderful, stand-alone inverter for those off of the grid.
The retail cost of the SW4024 is $2980. This is very inexpensive (75
cents per watt) for a sine wave inverter. Our testing was rigorous,
and the 4024 met Trace's specifications. It works well and is
affordable. Great work, Trace!
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