Features April 15, 2005 Issue

DC-to-AC Power Inverter Test

We gave our 30-amp shore cord a rest for a while as we tested six of the most popular 2000-watt DC power inverters in the marine market.

All of the inverters PS tested are good, solid products. Above, Robert Laska of L&L Electronics studies the waveform generated by the IQ 2600 (seen at rear).

Quiet and emission-free, DC power inverters can provide a high-quality supply of AC power so you can enjoy most modern-day conveniences while away from the dock.

Acceptance of DC power inverters by most leading boat manufactures is finally starting to make its mark on the marine after-market industry. With more and more inverters being offered as a factory-installed option on new vessels, the prices for pure sine wave inverters are starting to come down. That means that competition between DC power inverter manufacturers is heating up and many new inverter features are hitting the market.

Several years ago when we first reviewed DC inverters, the products we tested were basically power transformers stuffed into semi-weather-resistant cases with the remote panels consisting of a few indicator lights and an on/off switch. Today's power inverters are pushing the envelope of 90% power efficiency. Many inverters contain multistage battery chargers and a few top-of-the-line models are now showcasing active LCD remote monitors that have the capability of sharing intelligence with other on-board power devices via the new NMEA 2000® data network. What this means is that the engineers are designing for the future when on-board power management will be viewed as a complete system, instead of the generator, engine alternator, and house batteries being treated as isolated components.

Background
By definition, marine DC inverters are electronic devices that change low-voltage DC energy (usually from a battery bank) into AC line voltage. Inverters are basically divided into two different classes: True Sine Wave (TSW) inverters and Quasi or Modified Sine Wave (MSW) inverters. Each of these classes of inverters uses a different circuit design to produce AC voltage. Correspondingly, each inverter design has pros and cons regarding the compatibility of electric appliances to the type of waveform produced.

True sine wave inverters produce an AC output with a smooth, sinusoidal waveform that is identical to shore power, but command an average 20% price premium over modified sine wave inverters. TSW inverters will power any electric appliance within in its load range without restrictions.

Over at the other camp, modified sine wave inverters produce an AC output that has a stepped waveform. MSV inverters are considerably more affordable than TSW inverters, but should only be used to power basic electric appliances, those not containing sensitive electronics, due to possible appliance incompatibility with the distorted waveform.

A modified sine wave inverter uses a system of electronic switches and a transformer to invert stored battery energy into AC power. Although this switch and transformer design is much simpler (and less expensive) than that of the true sine wave inverter, the AC waveform of the modified sine wave inverter is different from the power produced by an electric utility company.

The MSW inverter's waveform varies in amplitude rapidly from zero volts to a maximum volt value and then rapidly back to minimum volt value with a pause each time it passes the zero-volt base line.

The workings of a modified sine wave inverter involve passing the DC input voltage into a pair or multiple pairs of MOSFET power transistors. MOSFET (Metal Oxide Semiconductor Field Effect Transistors) transistors are highly efficient electronic switches capable of handling high current. The "switches" are rapidly turned on and off, with their output flowing into opposite sides of a transformer. It is the combination of rapidly switching the power transistors on and off, and the power flow to opposite sides of the transformer that create an alternating magnetic flux in the transformer's core. In essence, the transformer sees an alternating DC voltage on its primary windings and a resultant AC voltage is generated on the transformer's secondary windings.

On paper, this circuit design will yield a higher conversion efficiency than that of the more complex true sine wave inverter. The drawback to this design of inverting is that the AC waveform that is output will contain harmonics (frequencies) above 60Hz (cycles). These harmonics or modifications to a true sinusoidal wave can significantly affect the operation of certain AC appliances.

Compromises to consider when choosing a MSW inverter over a TSW inverter would be for example; LCD TVs operating on a MSW inverter will exhibit visible noise lines across the screen, microwave ovens operated by a MSW inverter will most often not produce full output power, which will effect cook times. In another example, the all too popular "wall cube" chargers that recharge everything from your portable VHF to your digital camcorder, will most likely have overheating problems when used with a MSW inverter. Drop-in battery chargers for cordless tools that are powered by a MSW inverter usually do not shut off when the battery has been fully charged, resulting in overcharging and overheating the tool's battery.

A true sine wave inverter uses a transformer and a case full of specialized electronic circuitry to produce an AC waveform that's clean and symmetrical. The TSW inverter's output is completely comparable with that of the power provided by an electric utility company (and most times superior to it).

While TSW inverters are significantly more expensive than MSW units, they are the best choice when powering today's advanced entertainment and communication electronics.

The abbreviated explanation of TSW inverter design begins with the conversion of the low voltage (12-24V) DC battery input line into high voltage direct current via a DC-to-DC converter. The high voltage DC then enters a high frequency Pulse Width Modulator circuit that converts the high voltage DC into chopped/stepped AC. The AC then passes through an LC (capacitance inductance) low-pass filter circuit which shapes the AC wave into a symmetric sinusoidal waveform that is free of harmonics and is oscillating at the desired 60 Hz/cycles per second.

Selecting an Inverter
The first step in choosing the right inverter for your vessel will be to define your AC appliance needs. Start by deciding which AC loads you plan to operate at the same time, and then add together the wattage rating of each of these appliances, plus a 20% safety margin. The sum of the wattages plus the safety factor will equal the size of the inverter that you will need, expressed as an inverter's continuous power output rating. If you plan on running appliances that draw a large surge when they first start up (plasma TVs and refrigeration units are good examples), then you will also have to check the inverter's surge capacity to make sure that the inverter is capable of handling the starting surge loads that may start at the same time. As a rule, MSW inverters usually have a higher surge capacity than TSW inverters.

The second step in selecting an inverter will be choosing which type inverter, TSE or MSW, will best power the appliances that you intend to use. The last step in inverter selection usually involves price and case size vs. weight and performance. And for that part of the equation the best place to look at is our test comparison results.

The following are desirable options to consider:

Integral Battery Charger — Since the inverter is connected to its designated battery bank by some seriously large conductor wire, it only makes sense to incorporate the ability to quickly recharge the battery bank via the same large conductors not in use when the inverter feature is turned off. By integrating a battery charger into the same case as the inverter, the manufacturer can not only offer a cost savings over a stand-alone battery charger, but can also control the type and quality of battery charger that is to be included. The current trend is toward multistage battery chargers that recharge batteries in three distinct modes: bulk, absorption, and float. Each stage of the recharge process has precisely regulated voltage and current levels to ensure accurate recharging without the risk of overheating and battery damage—a must for recharging deep-cycle batteries.

Battery Temperature Sensor — Some upper-tier model inverters with an internal battery charger offer a temperature sensor as an option. As its name implies, it is a temperature sensor that is physically attached to a battery’s case and is wired back to the inverter. By monitoring the battery's case temperature, the internal battery charger can adjust its charge rates so as not to overcharge in hot conditions and conversely not undercharge in cold conditions. We reckon that buying whatever options are out there to help extend the life of the house battery is money well spent.

Automatic Power Management — Here's the scenario: It's the end of the day and you pull into port and your inverter house batteries are 50% discharged. You plug your 30-amp shore cord into the dock, and immediately your inverter's integral battery charger draws 25 amps of current. Next you plug in your AC vacuum cleaner to tidy up a bit, and pop goes the dockside breaker. With Automatic Power Management circuitry employed, the inverter will automatically reduce its power to the battery charger so that the load on the shore breaker will not exceed 30 amps. This in turn will eliminate nuisance tripping of the shoreside circuit breaker—definitely a nice convenience.

Remote Panel Display — Remote panels are shipped standard with most of the top-tier inverter units, and are an optional accessory with others. We would highly recommend installing one. Not only does a remote panel allow you to conveniently control the inverter from your nav station, but it gives you an instant indication of the status of your house battery bank. The more advanced remote panels can even display what mode the internal battery charger is operating in (bulk charge, absorption, float), along with the percentage of available battery capacity.

Test Parameters
Our test setup consisted of two 8D deep-cycle, wet-cell batteries, wired in parallel to give us a capacity of 490 amp hrs. of stored energy. The battery bank was in a fully charged state, at room temperature, prior to the start of each inverter evaluation.

Each inverter was initially loaded with two 1,000-watt incandescent light bulbs for 15 minutes, during which time the AC inverter output voltage was constantly measured and logged with a Fluke 177 true RMS voltmeter. A Tektronix dual-trace oscilloscope was connected to the inverter output line to verify proper waveform and the percent of total harmonic voltage distortion. We then connected a Fluke 336 Clamp-on amp meter onto to the positive lead of the inverter's DC input line and took a DC input current reading with both industrial light bulbs on (2,000-watt draw), and then again with only one bulb on (1,000-watt draw). The DC current reading gave us a base line for inverter efficiency.

At the 16th minute, we turned back on the second light bulb (2,000-watt total load) and readied our stopwatch. Waiting in the wings we had a group of 1,000-watt quartz heaters, matched to each inverter's surge rating (in 1,000-watt increments) to come instantly on line via a circuit breaker. Each inverter had to keep pace with the surge for the 3 to 5 second rating time to receive a passing grade.

The Results
There really were no surprises with five out of the six units submitted for our test. These five were UL-listed, and we can say with certainty that each unit we tested met or slightly exceeded their rated specifications. Of particular interest in the waveform oscilloscope test was the Charles Industries IQ2600 modified sine wave inverter. Charles Industries claims that they have engineered an advanced circuit into their unit, whereby they are able to break down the stepped waveform of a modified sine wave inverter into 19,000 individual micro steps per second. C.I. claims that this waveform enhancement will be compatible with 95% of all AC appliances. We were able to confirm the presence of the micro steps with the oscilloscope, and when we plugged our 15" RCA LCD TV into the inverter, there were no bars or lines of interference present.

Charles Industries
The IQ2600 is a very robust and well-built UL-listed inverter. This unit is a popular OEM choice on new vessels. Charles Industries also makes the IQ2600 in a true sine wave model, but we chose to test the modified sine wave model because of the factory's claim that the waveform on their MSW unit is far superior to that of any competitor's model. As we didn't experience any bars or lines of interference with our LCD TV, it appears that their technology supports the claim. The IQ2600 had the highest surge rating of our test group and had the highest maximum efficiency score.

We were a bit disappointed that the company's website did not have a listing of authorized marine retailers, but they did promptly answer our inquiry regarding where we could purchase an IQ2600. This product is manufactured in the U.S. by the Vanner Corporation and retails for $2,199.

Freedom Marine 20
Heart inverters were the single most popular line of inverters in the mid '90s, and for good reason—they worked. The Heart line was acquired several years ago by Xantrex, given a name change to Freedom and a cosmetic makeover. The new Freedom Marine 20 is packaged in a stylish case, which is more in line with the other units in Xantrex's wide product line.

The unit doesn't come standard with a remote LCD display panel (add $150 to $200, something we highly recommend), but does come with a battery temperature sensor. Much to our surprise, this unit is not UL safety listed and is manufactured in China. Taking into account this unit's history of exemplary reliability and its attractive price (around $1,199), The Freedom Marine 20 is our choice for best value in a modified sine wave inverter.

Newmar
The 12-1801C is a solid and well laid out unit that carries a UL safety listing. This unit completes Newmar's line of voltage converters and electrical panels, and delivers its rated output with little fanfare. Unfortunately the higher price of this Newmar ($2,099) places it squarely in the middle of the pack. Its optional remote panel consists of a bank of LED lights, far less informative than the active LCD panel on the Prosine 2.0. The 12-1801C is a crossover product from the land mobile industry. It's manufactured in the U.S. by Dimensions Unlimited, which is based in St. Paul, MN.

Prosine 2.0
Another product from Xantrex Technology Inc., the Prosine 2.0 is manufactured to exacting specifications in China and is UL listed. The lightweight and compact case size of this unit, combined with a street price of around $2,000 (including remote LCD panel) makes this inverter a strong choice. The active remote LCD panel that is provided with the unit is excellent and the unit comes standard with a battery temperature sensor. The operation manual and installation support documents that are included in the box are geared toward DIY installations. With 2,000 watts at its disposal, and an all-up weight under 25 lbs., this is one powerful package.

Trace SW2512MC
Trace Engineering, is yet another line that was acquired by Xantrex Technology (see a trend here?). Trace inverters have a loyal following in the OEM custom yacht market. This unit weighs in at 90 lbs., making it the undisputed heavyweight of our test group.

The engineering department at Trace designs inverters that are so durable and well engineered that they are almost considered scientific grade. Although the Trace is priced well above all the other units in our test group ($2,795), it is one of the few units on the market that can be installed in a "stacked" configuration. By this we mean if the application calls for an output in the 4,000-watt continuous range, then two SW2512MCs can be stacked in parallel to provide the desired capacity, or redundancy. This is definitely a professional-grade inverter. It's manufactured in the U.S. and UL listed.

Xantrex MS2000
When we uncrated the MS2000, it was apparent to us that this unit was not a crossover product from another industry, but truly a dedicated marine inverter. The MS2000 is manufactured in China and assembled in Canada. Xantrex's newest model features a case fabricated from of die-cast aluminum. (All of the other units included in our test have bent-metal cases.) The cable attachment points made to this inverter are via exterior stud mounts. Removing the unit's case lid to gain access to the cable attachment points (as you must do with Charles Industries' and Newmar's products) is unnecessary. Installed on the topside of the MS2000 case is a massive (20% of its total case area) aluminum heat sink, which ensures that this unit will run cool.

The MS2000 comes embedded with Xanbus power management firmware, Xantrex's NMEA 2000 compatible data communication network. The Xanbus software allows communication between devices connected to the network so that they are aware of each other and can function in unity. This "awareness" is being currently showcased with Xantrex's first Xanbus device, the automatic generator start module. When a Xanbus network is equipped with a genset start module, the MS2000 can signal the generator to turn on automatically when the system detects that more power is needed than the inverter can deliver. In keeping with the mission of building a highly advanced inverter, the MS2000's software is field-programmable via flash memory. This will ensure that the MS2000's feature list will be kept current, much like the field upgrades that are being offered by the top-tier GPS/chartplotter manufacturers.

With a retail price of around $1,600, (more for the optional remote panel) superb case construction, a long list of advanced features, and built-in future expandability, the MS 2000 by Xantrex gets our nod as the new leader of the pack.

Conclusions
We tested 342 lbs. of some nicely engineered products. Every unit included in the test should be considered a recommended choice, which should make your decision easier once you've determined the size of the inverter that you require. If a modified sine wave inverter fits your intended application, then we would give the nod to the Freedom Marine 20. The Freedom has a long established track record, and is attractively priced.

The Prosine 2.0 is an excellent choice for a powerful true sine wave inverter packaged in a weight-saving 23-lb. case. But our overall choice for best quality and value in a true sine wave inverter would be the Xantrex MS2000, hands down. This unit is priced right, has plenty of power, and is just downright pleasing to the eye, both externally and internally. (Don't worry about that little "Warranty void if removed" sticker that we removed to take the internal photos. We put it right back on with 3M decal adhesive.)

The Xantrex MS2000 with its Xanabus digital communications network is the very first DC inverter that is capable of exchanging power management information and commands over a NMEA 2000® network. The MS2000's software pack is upgradeable via flash memory, and this unit is so advanced that it can even automatically start your generator when it detects that the voltage in the house batteries has dipped too low. The MS2000 is definitely an intelligent unit and we give it top honors for its forward-thinking design.

 

Also With This Article
"Caveat Emptor"
"Spec Sheet: DC Inverters"

Contacts
• Charles Industries, Ltd., 847/806-6300, www.charlesindustries.com
• Newmar Corporation, 714/751-0488, www.newmarpower.com
• Xantrex Technology Inc., 800/446-6180, www.xantrex.com

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