How an Inverter Works Clifford W. Mossberg The process of converting low voltage direct current (DC) electricity into high voltage alternating current (ac) is not particularly "hi-tech" but it has several levels of complexity. The most complex levels use high tech solutions and are frequently employed because they make efficient and reliable inverters. In this article I will not try to explain the esoteric details of such "techy" stuff - I'm not qualified - but I will try to give the novice user a basic idea of what is going on inside that heavy box hooked up to the batteries. Inverting really means Switching Polarity At the simplest level alternating current (ac) can be achieved by a simple switching device which just swaps connections to the battery leads. Insert Figure 1 In the very basic circuit and graph in illustration 1, a simple DC resistance is shown, for instance a light bulb. The "voltmeter" I have drawn is the type which registers zero in the center of the meter, so current flow in either direction through the resistance (bulb) will show up as movement of the needle to either the left or right of zero, depending on the polarity of the battery hook-up. When the resistor is hooked to the battery, current will start to flow. The voltmeter will measure a voltage drop equal to the voltage which the battery can produce. If the battery was 12 volts and the resistance was a automotive tail light bulb the reading of the volt meter would be 12 volts. The graph shows that this would start the instant the resistance was hooked to the battery and would stop just as suddenly when the resistance was disconnected. The length of time the voltage shows on our voltmeter could be anything in this example, from milliseconds to hours. Insert Figure 2 In illustration 2 the leads from the battery to the resistor have been switched so the resistor is hooked up opposite what it was in illustration 1. You will notice that the voltmeter is now deflected to the same reading (12 volts in our example) in the opposite direction. In illustration 2B I have extended the graph in 1B, drawing the new meter voltage reading as if it was turned on at the moment when the circuit in 1A was switched off. This graph (2B) is neatly drawn with both time intervals exactly the same length because in an actual inverter the interval that the electricity would be turned on would actually be the same, but in our crude example there is nothing which would necessarily make both periods of current flow equal. One of the necessary devices in an inverter is a timing mechanism that will make these pulses of electricity of about equal duration. The same circuit can produce opposite effects electrically just by reversing the flow of electrons, or current. Here we have the basic principle of an inverter. Suppose for timing we hooked an electric motor to a simple rotating switch? For half the rotation of the switch current could flow as in illustration 1, for the other half of the rotation current flows as in illustration 2. This simple timing device would assure that the electrical energy in each pulse would be about equal. Here we have the simplest of inverters. Each pair of negative and positive electrical pulses shown in illustration 2B would be one cycle. The number of cycles that occur in each second would be the frequency of the cycles. In this example, the frequency of this simple inverter would be controlled by the speed of the electrical motor, not a very elegant nor accurate method of controlling frequency, but--surprisingly--good enough in some cases. In actual fact I have only used this method to illustrate what is necessary and it is not a practical solution, however in some circuits a vibrating switch is used to reverse the current flow. While this is crude and does not produce an accurate frequency, it is good enough for some applications. For instance, old tube type car radios used a vibrator to invert 12 volt DC. INSERT FIGURE 3 The graph in illustration 3 shows two complete cycles of inverter operation. This type of current flow is known as a square wave for obvious reasons. It will repeat itself as long as the battery voltage is being switched. This repetition of full cycles is the frequency of the inverter and is given in "cycles per second" or "Hertz" in more modern terminology. Normal ac line frequency in North America is 60 Hertz (Hz.). (in some other countries it is 50 Hertz and in aircraft operation 400 Hertz is common). This 60 Hertz frequency is very important and it must be accurately controlled. A typical gasoline powered small generator may control frequency to plus or minus 1 Hertz; that is, the frequency of such a generator might range from 59 Hertz to 61 Hertz. Anything much beyond this and electrical gizmos begin burning up. Commercial electricity is controlled much more precisely. For all intents and purposed there is no variation in the frequency of electricity from your power line for the simple reason that even minor deviations will cause the source of the offending power to be disconnected. Accurate control of frequency in an inverter is very important and as yet our crude inverter has no control. If we go back to our example we can determine that the speed of the motor running our rotary switch is what controls the number of times per second that our electricity changes direction. Speed up the motor and thus the rotation of the switch and the electricity will change direction faster, it will increase its frequency. It would be possible to accurately control the speed of the motor but the solution to do that best would probably incorporate solid state electronics and at that point switching and frequency could be more efficiently controlled by an all solid-state circuit which would eliminate the motor and rotary switch entirely. A circuit similar to "The Time Machine" circuit shown on page 79 of Home Power #21 would do well either to control the speed of our motor in the hypothetical example, or if we throw away the switch and motor the same circuit--with minor changes--would act as the switching device. We have built a simple inverter now. It converts DC electricity into ac electricity of the same voltage. That's all very well if you have batteries that supply 120 volts DC but that's the exception not the rule. More commonly we will have 12 or 24 volt battery banks and so we will need to increase the inverter output voltage from some low value to the 120 volts which our ac appliances will require. However our inverter has made this easy for us. Once we have ac electricity we can use a simple transformer to increase the voltage, Enter the Transformer INSERT FIGURE 4 A transformer in its simplest form is just two coils of wire placed close to each other. Alternating current flowing in one coil will generate electricity in the second coil, so without any direct connection between the coils we have electricity flowing in on one side and out on the other. The interesting thing about this is that the electricity can be changed in voltage as it passes through the transformer. Input and output voltage will be directly proportional to the number of turns in each coil. For instance if there are 100 turns of wire in the input coil and 1000 turns in the output coil the ratio of input voltage to output voltage is 1:10. If you supply 12 volts of ac electricity to the input coil of the transformer in this example you will get 120 volts out! This is not nearly as simple as I'm painting it to be but the principle holds. So all we need to do to finish up our inverter is attach the proper transformer to the ac output and we have it, a supply of 120 volt ac square wave electricity. There is one little problem though, our appliances don't like to eat square wave electricity. "Rolling Stock" or Electromechanical Alternators Most ac electricity is generated by rotating a magnet or an electromagnet between coils of wire. The electricity is generated as the "magnetic lines of force" cut through the wires in the coils. The process of a rotating magnetic field within coils of wire is used by engine/generators and by commercial utilities. Illustration 5 shows this: Insert Figure 5 The lines of magnetic force can be thought of as directional. As the magnet in the generator rotates, the lines of magnetic force are sometimes far away from the coils of wire and moving toward them and at other times they are very close to the coils and cutting directly across the wires. In the former case there is no voltage at all being produced while in the later case it is producing maximum voltage. Sinusoidal, or Sine-Wave, Power INSERT FIGURE 6 Illustration 6 is a graph of the voltage produced through one half of a revolution of the magnet in Illustration 5C. Points A, B, and C correspond with the labeled positions of the magnet. Compare this with the square wave graph for our crude inverter. The graph in 6 is called a sine wave curve and it is a "picture" of the electricity which your local power company produces. Electrical appliances with motors in them use coils of wire in the motors to make them operate and these coils of wire (called inductive loads) feel very comfortable with sine wave ac electricity. This is logical if you think of it. A rotating magnet inside a coil in a generator should be well matched to a rotating magnet inside a coil in a motor. Their electrical properties are the same. Square Waves and Sine Waves INSERT FIGURE 7 Illustration 7 is a graph comparing sine and square waves of the same size and period. The interesting thing about a graph like this is that (mathematically) the area underneath the curves in each graph is a proportional representation of power. Even a quick look can see that there is more power underneath each half cycle of the square wave (more area in the rectangle) than there is underneath each half cycle (hump) of the sine wave. This is the problem with a square wave inverter. Motors (inductive loads) don't know how to use that extra power in a square wave. They will go on using the same amount of power they would use if a sine wave was supplied, and with evil single mindedness, they change the remaining extra power into heat! This extra heat builds up until it burns the insulation off the wire in the motor's windings and a trip to the graveyard for that appliance. All ac appliances like the gradual and elegant changes made by the sine wave. Many appliances have problems digesting the abrupt changes offered by the square wave. Problem appliances may do anything from protesting with a load audio buzz, to frying and dying. Modified-Sine Wave Enter the modified-sine wave or quasi-sine wave. Keeping in mind that the area under each curve is power, if a square wave could be produced that had the same area (power) as a sine wave it would take care of the major problems. We could do this by reducing the height (voltage) of the square wave but reduced voltage means reduced motor performance. The trick is to keep the voltage up, but reduce the length of time it is on so that the two curves look about the same to a motor. Using silicon magic, this is now fairly easy. INSERT FIGURES 8A&8B Our alphabetic friends, the ICs and the FETs Instead of a simple on/off circuit the modified-sine wave inverter requires a timing cycle which has a variable "off "segment to it. Illustration 8A shows a quasi-sine wave while Illustration 8B compares this with a true sine wave. From this you can see that the trick is to turn off the electrical pulse of the square wave for a longer time. This is quite easy to accomplish efficiently with modern integrated circuits (ICs) and super efficient field-effect transistors (FETs). This is the method most often used in a modern inverter found in our renewable energy systems. The Last Word? Not Likely... I hope this makes modified-sine wave inverters a little clearer to those folks who use them but don't want to know too much about them. The truth is that I have left out far more than I've included. The modern quasi-sine wave inverters such as the Traces, or Heliotropes, and PowerStars just to name a few, are wonders of design and efficiency. They use the basic ideas offered here, but employ them electronically to custom-tailor the inverter's power production to the particular job at hand. This electronic magic gives us reliable inverters with phenomenal efficiency. Inverters have done a lot to cushion the foibles of Ms. Nature in lonely places. I, for one, pray to the god Kilowatt every time I turn on my electric ice cream freezer and think of the complexity of the process that brings me this luxury from the sun, wind, and water. Access Author:Clifford W. Mossberg, POB 75273, Fairbanks, AK 99707