The Ins And Outs Of Inverters

By converting battery power to AC voltage, these handy devices make it possible to operate numerous appliances and help to put the “home” in motorhoming.

By Bill Hendrix, F761S
November 2004

High-efficiency inverters are here to stay in the RV world. They enable us to watch television, brew coffee, enjoy computing, and perform many other activities aboard our motorhomes without giving them much thought. But how many of us understand what an inverter is, and what it does?

First, let’s discuss the differences between convertors and inverters. A convertor changes alternating current (AC) from the landline or generator to direct current (DC). These devices are useful for drawing on generator or shore power to charge the batteries and operate DC equipment, such as the water pump, furnace, and low-voltage lighting. An inverter is just the opposite; it changes direct current (DC) from the batteries to alternating current (AC), so we may use the energy stored in the batteries to operate 120-volt AC appliances.

Possibly the earliest attempt to glean AC from a DC source was a DC motor coupled to an AC generator, but with many shortcomings: size, weight, cost, and virtually no control of the output. As battery voltage decreased, the DC motor would slow, and AC voltage and AC frequency would suffer.

Inverters have been around for many years. They were available mostly in square wave form initially. Most of these units were bulky, heavy, produced a large amount of heat, and, therefore, were not very energy-efficient. The square wave form inverters of today are much better and still the most economical, but just not practical for RV applications.

Electronic technology has evolved to now include modified sine wave and sine wave form inverters with efficiencies above 90 percent. Efficiency is measured by dividing the energy output by the energy input; so, for example, if an inverter produces 90 watts with a 100-watt source, it would be 90 percent efficient. Today’s quality inverters should be above 85 percent efficient, with some as high as 95 percent.

The specification sheets on inverters will rate them at peak efficiency without stating the load factor of the rating. You could expect the efficiency to degrade somewhat when pushing the load to the maximum. This is one of many reasons we like to see inverters generously sized for their application.

The more popular high-quality inverters of today may incorporate many features, such as high-amperage, multiple-stage battery charging; battery temperature sensors; generous surge capacity; transfer relays; built-in output circuit breakers; load sharing; automatic generator start; and remote controls that could have as many as 18 menu headings with 80-plus selectable settings. I just waded through an owners manual that was 140 pages long. Most of the menu items were for one-time use during setup or for special situations, however.

There are three popular forms of inverter output current. In describing each of them, it’s worth noting that some countries have different voltage and cycle standards, as much of Europe is 220 volts at 50 hertz (cycles), but the numbers used here will be for the North American venue, which is 120 volts at 60 hertz (cycles).

The square wave (SQW) abruptly changes current from zero to 120 volts positive and then abruptly to 120 volts negative, repeating the cycle 60 times per second. Some types of AC equipment may not perform well with a square wave inverter. Simple resistance loads, such as incandescent lights, coffeemakers, toasters, hair dryers, and such would be no problem. You would not expect to find this style inverter today in anything other than the most basic recreation vehicle.

The modified sine wave (MSW) “” sometimes called quasi sine wave “” changes current from zero to about 150 volts positive in stair steps, to about 150 volts negative in stair steps, and back to zero, repeating the cycle 60 times per second, which is the simplest form of MSW. Some technicians refer to this as a modified square wave. The higher-capacity and more refined MSW inverters will have several steps in each wave segment, and most electronic and inductive equipment should perform very well with this type of inverter. However, some audio equipment may produce a slight background hum.

Sine wave (SW) inverters are much more refined and utilize microprocessors to mix the output of multiple transformers to attain a waveform that simulates a sine wave. The wave is divided into scores of segments per AC cycle, and this gives a waveform that is compatible with all AC equipment. Some technicians also profess that this is still a modified wave. However, it is so refined that it will appear as a sine wave on an oscilloscope like the sine wave from the power grid.

Inverter electronics
Two basic styles of inverter electronics exist “” low frequency and high frequency. The low-frequency inverter, which was the first style available, uses iron-core, copper-wound transformers for the primary voltage conversion. They generally are considered to be more durable and to offer better surge capabilities and marginally better efficiency. High-frequency inverters use a completely different technology (H-bridge, high-voltage) and are a bit of a hybrid that is starting to evolve to prominence in the RV industry. The advantage of high-frequency inverters is weight reduction that results from the absence of the heavy transformers, and a more competitive price. High-frequency inverters weigh approximately 40 to 50 percent less than the other types with comparable capacities, and this is an attractive advantage in the RV world.

Inverter ratings
Inverters are capacity-rated by their ability to sustain a specific level of AC current output with a specified DC voltage input at a rated temperature. Surge capacities are normally stated in the length of time the inverter can handle the higher-than-rated output. When the batteries start going down, the inverter output capacity also decreases. Another major factor is temperature. The inverter needs to be in a well-ventilated compartment, as the power rating may be stated at a temperature as low as 70 degrees to 80 degrees Fahrenheit and may fall to as little as half its capacity at 150 degrees Fahrenheit. Also, some inverters are rated at temperatures of 100 degrees Fahrenheit and above. Check the specifications sheet if ventilation is an issue. The other nuts-and-bolts factors that influence performance are appropriate cable sizes, cable lengths, and comparable connectors.

The inverter output normally is stated in watts for continuous duty and may also state the above-mentioned surge capacity for various time durations. Some data plates may show the inverter’s rating as “VA” (volts times amps), which is another way of saying watts.

Testing inverter output
When testing inverter output, it is imperative to use a true RMS (root mean square) meter rather than an averaging meter. The averaging voltmeter may give erroneous readings when used on inverter current. If the meter is a true RMS, it will so state on the meter.

What size inverter?
The sizing of the inverter for RV application is rather subjective, as folks operate in different manners. One couple may never use the television, the computer, the microwave, the toaster, and the coffeemaker all at the same time, while another might want to do so on a daily basis. Given the costs involved, it would be more prudent to add the loads that are a “must have” and use this figure to determine the minimum inverter size required. The other AC-powered items would have to be managed so as to not exceed the inverter capacity. This is probably a moot point for most motorhome owners, as the RV manufacturer has done the math with a broad-brush average, and buyers take a coach as it comes equipped. The accompanying chart gives the wattage loads of some of the more common appliances.

Power Requirements Of Some Common Appliances
 
Appliances Watts
Fluorescent light 10 – 25 watts
Stereo or VCR 50 – 100 watts
20” Color TV 90 – 125 watts
Computer 100 – 400 watts
Microwave, small 600 – 800 watts
Microwave, large 1,000 – 1,500 watts
Toaster 800 – 1,000 watts
Coffeemaker 800 – 1,000 watts
Hair dryer 800 – 1,000 watts
Hot plate 1,000 – 1,800 watts
Electric skillet 1,000 – 1,800 watts
 
All agency-approved appliances have either the watts or the amps stated on the data plate (amps times volts equals watts). Your appliances may vary greatly from the above generalities.

How many batteries?
Determining the size of the needed battery bank is a fairly involved process. Factor in all the expected loads by calculating the average number of hours used each day, then triple that number and divide it by 12 volts. This will give you the amp-hours of battery power needed for a day. For example:

Estimated total of load hours used/day   X   3  í·  12 volts  =  amp-hours of battery power needed per day

1,600 watt-hours  X 3 = 4,800 í· 12 volts = 400 amp-hours

This example would indicate the need for four 100 amp-hour-rated 12-volt deep-discharge batteries wired in parallel (or four 200 amp-hour-rated 6-volt golf cart batteries wired in series/parallel) for one day’s operation before recharging the batteries. If you charge the batteries twice a day, you could do with half the number of batteries. If you need to go two days before a recharge, have twice the number of batteries. This does not include the battery power needed for the furnace and water pump. The water pump doesn’t consume much current, as the run durations are very short. However, the furnace can be a big factor for dry camping in cold weather, and that should be considered.

Finding a suitable location for additional batteries may or may not present a problem, but if ventilation is not possible, use sealed batteries to avoid the hazards of hydrogen gas.

The reason we need to multiply the load by three is because we should not run the batteries all the way down. To preserve battery life, don’t discharge more than 50 percent. When recharging with the generator, we will normally only recharge to around 85 percent as it is not economical to charge back to 100 percent; that last 15 percent takes a long time.

So we actually are using only 35 percent of the batteries’ rated capacity, then toss in a little for the loss of efficiency, and we multiply by three. Some charts use a factor of two instead of three but also add 10 to 15 percent for loss of efficiency. This is assuming the batteries will be recharged to 100 percent. Take your pick. Resting voltage of a 50 percent discharged battery will be 12.1 to 12.2 volts. Fully charged, the voltage will be 12.6 to 12.9, depending on battery type.

Inverters are available from a number of distributors and dealers. (Check the Business Directory in the January and June 2004 issues of FMC magazine, or visit www.fmca.com/bd.) However, only a few manufacturers are active in the RV industry. They do offer an abundance of models suitable for RV applications, with a sweeping array of features and functions. Most have good surge ability, high-capacity battery chargers, and remote controls available. Some offer automatic generator start, battery temperature sensors, and load sharing, plus a large menu of selectable parameters for setup.

Popular Sizes of Inverters for RVs
 
Dimensions Unlimited 1,600 to 3,600 watt
Magnum Energy 1,800 to 4,000 watt
Tripp Life 1,000 and 2,000 watt
Vanner 2,600 to 3,600 watt
Xantrex markets three distinctive brands  
SW previously Trace 2,500 to 4,000 watt
Freedom previously Heart Interface1,500 to 3,000 watt
Prosine formerly Statpower Technologies2,000 and 3,000 watt

Ballpark list pricing (obtained from one aftermarket supplier, Jolyn Enterprises, C7065; www.jolynenterprises.com) ranges from $0.75 to $1.10 per watt, depending on features and capacity, plus $195 to $350 for the remote control.

The development of high-efficiency inverters has made a tremendous contribution to the versatility and livability of recreation vehicles. Being able to operate a variety of household-style appliances when camping without shore power or starting the generator enhances the appeal of the RV lifestyle.

Inverter Manufacturers

Dimensions Unlimited Inc.
St. Paul, Minnesota
(800) 553-6418
www.dimensionsunlimited.com

Magnum Energy Inc.
Everett, Washington
(425) 353-8833
www.magnumenergy.com

Tripp Lite
Chicago, Illinois
(773) 869-1234
www.tripplite.com

Vanner Inc.
Hilliard, Ohio
(800) 227-6937
www.vanner.com

Xantrex Technology Inc.
Burnaby, British Columbia, Canada
(800) 670-0707
(800) 446-6180
www.xantrex.com