Power inverters

by Richard Perez

The modern power inverter has revolutionized the usage of battery stored electrical power. an inverter changes the low voltage dc energy of the batteries into 120/240 volt, 60 cycle, ac housepower. The idea here is to use the battery stored energy in regular household appliances.

The low voltage dc supplied by the batteries will not run standard consumer appliances, which accept only 60 cycle, 120/240 volt, ac power. Until the advent of modern inverters, battery people had to content themselves with 12 vdc appliances. These are specialized and very expensive. In many cases there are no 12 vdc appliances made for a particular job.

The inverter has changed this; now battery users can run just about any standard commercial appliance. in practical terms, the inverter allows us to run electric drills, power saws, computers, printers, vacuum cleaners, lighting, food processors, and most electrical appliances that can be plugged into the wall.

If the battery/inverter system is big enough, then large appliances such as freezers, refrigerators, deep well pumps, and washing machines can be accommodated.

All these standard 120/240 volt ac appliances can be powered from the batteries by using the appropriate inverter. The inverter draws its energy from the batteries, it does not require any other power source.

inverter operation is quiet and its power is available 24 hours a day, whenever it is needed. The addition of an inverter to a motorized system greatly improves the system efficiency. Power costs can be cut to 25 cents on the dollar by using an inverter instead of constant generator-only operation.

The generator can be run for only several hours per week, but the inverter's 120/240 vac power is constantly available. It is simply not efficient to run a large generator for a few lights and maybe a stereo.

The generator is used to recharge the batteries, and to power large intermittent loads. This approach results in the generator being run more heavily loaded, where it is much more efficient.

different types of inverters

inverters are manufactured in 3 basic types. These types are named for the kind of power they produce. the question is, "how close does the inverter come to reproducing the waveform of standard commercial power?" there are trade-offs involved in inverter design. The more closely the inverter replicates commercial sinusoidal power, the less efficient the inverter becomes.

this is a sad, but true, fact of physics. as the primary power source, efficiency is a very important factor in inverter operation. When we consider running large appliances such as freezers and washing machines on battery stored power, even small percentages of wasted energy are not acceptable. Battery stored energy is simply too expensive to waste.

square wave

of all types of inverters, the square wave inverter produces power that least resembles commercial power. This inverter is the cheapest type to buy. it will not run many appliances which require cleaner forms of power. stereos, televisions, computers, and other precision electronics will not accept square wave power.

the power produced by square wave inverters varies considerably with the voltage changes of the batteries as they are discharged. These inverters are designed to be inexpensive, and as such their efficiency is low, less than 70% when fully loaded.

if the square wave inverter is only partially loaded, its efficiency drops to less than 30%. These inverter cost about $0.50 per watt and are available in sizes up to 1,000 watts.

the square wave inverter is not suitable for homestead usage. It is neither efficient or versatile enough.

modified square wave

The modified sine wave inverter represents a compromise between efficiency and utility. the modified sine wave inverter is the best type to use in home power service. This type of inverter is capable of powering almost all commercial electrical appliances, even very delicate electronics such as computers.

The power this inverter produces is not identical to commercial power, but it is close enough to fool almost all appliances. the efficiency of the modified sine wave inverter is the highest of all types of inverters, in some cases consistently over 90%.

For example, we use a 1,500 watt trace inverter. this inverter is over 90% efficient at power output levels between 100 and 600 watts. its no load power consumption is less than 1 watt.

we leave it on all the time, ready for instant service. We have yet to use an appliance that will not accept its modified sine wave power. The inverter is fully protected against overloading. It even contains a circuit that prevents overdischarging of the batteries.

the output power of the trace inverter is very clean, far cleaner and more dependable than commercially produced electricity. These inverters are also available with built-in battery chargers. The battery charger senses when you have turned on the ac powerplant and recharges the batteries.

It also automatically transfers the household to generator produced power, and returns the household to inverter power when the motorized powerplant stops.

The cost of modified sine wave inverters is about $1.00 to $1.50 per watt. This type of inverter is available with output wattages between 300 and 25,000 watts. in most cases, the inverter is capable of surge wattages about 3 times its rated output wattage.

Many of the larger modified sine wave inverters have outputs of both 120 and 240 volts ac. This surge capability is very important when powering large motor driven appliances such as refrigerators, washing machines, and deep well pumps.

sine wave inverters

The sine wave inverter exactly duplicates the sinusoidal waveform of commercially produced power. It accomplishes this at the expensive of efficiency.

The sine wave inverter is necessary only for very delicate electronics. These inverters are usually sold to hospitals, airports, and government installations. They are the only ones who can afford to buy them and run them.

efficiency for sine wave inverters is less than 60% at optimum loading. at light and heavy loads the efficiency drops to less than 30%. these inverters are expensive, around $2.50 per watt. The sine wave inverter is not suitable for homestead power, it is too expensive and inefficient.

inverter sizing

Modern power inverters are available in many sizes. the process of determining the right size for a particular homestead can be confusing. the process is really simple-- just make a survey of all the appliances you wish to run from inverter supplied power.

List each appliance, its rated wattage, and the number of hours per day that the appliance will be operational. It is best to allow each person in the household the usage of a light-- one person,one light. We seem to average about five hours of lighting per day.

if this estimation process is to be effective all appliances must be included, be realistic. Be sure to allow some margin for future expansion.

average consumption

put a star beside all appliances that are required to operate at the same time. Include in this starred list all appliances with automatic controls, for example refrigerators and freezers.

Add the total wattage of all the appliances on the starred list. this wattage figure is the smallest amount of power that will do the job. the inverter must be sized larger than this figure if the system is to work as planned. If the inverter is undersized, it may shut itself off due to overloading and leave you in the dark.

The wattage of each appliance multiplied by the number of hours per day it is operational gives an estimate of energy consumption in watt-hours per day. This figure is used to determine the capacity of the battery pack necessary to do the job.

surge consumption

Appliances which use electric motors require more power to start themselves than they require to run. This high starting power consumption is called starting surge. Many motorized appliances require over 3 times as much power to start than to run.

these starting surges must be considered in sizing the inverter's wattage. If these surges are not allowed for then the refrigerator starting up may overload the already loaded inverter and shut it off.

Most power inverters worth having are capable of delivering 3 to 5 times their rated wattage for surges.

If there are several large motors in the system that may start themselves, then the situation becomes more complex. For example, consider a system where both the deep well pump and a refrigerator are being used. both the pump and the refrigerator may turn themselves on at the same time.

The resulting surge demand may be high enough to shut down the inverter. It is best to assume that all appliances on automatic control are starting at the same time. add their surge wattages and be sure this figure is less than the surge capability of the inverter being considered.

inverter wiring

Ihe inverter's output should be wired into the house's main distribution panel. A quick reference to books on house wiring will aid you in getting the power into the house with low loss and safety.

Remember that all the power being used in the house is traveling through these connections-- use big wire (6 to 2 gauge) and low loss connections.

Ac wiring

one of the major attractions of inverter produced power is that it is at normal 120/240 voltages. This is very important when placing older homes on alternative energy.

The wiring within the walls is designed for 120 volt operation. It has too much power loss to be used with low voltage dc energy directly from the battery. The wiring, switches, outlets, and all their interconnections have too much resistance to efficiently transfer the batteries' energy directly.

dc wiring- battery to inverter connection

The wiring that supplies the energy from the battery to the inverter is of critical importance and deserves special attention. These wires must be capable of transferring over 200 amperes of current efficiently. This means that the wiring must have very low resistance-- use 0 to 000 gauge copper wire.

Keep the length of these heavy gauge wires to an absolute minimum. Most inverters are located within five feet of their batteries.

the actual connections on the battery terminals are subject to corrosion. It is common practice to use battery cables from automobiles. These cables have ring connectors mechanically crimped to their ends. the sulphuric acid in the batteries eventually corrodes the mechanical connection between the actual wire and its ring connector.

If a more permanent connection is desired, make your own connectors by soldering copper tubing over the ends of the heavy wires. Flatten this assembly and drill the appropriate hole in it. This soldered connector is vastly superior to any other type.

visit richard perez' website at www.homepower.com

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