Inverters
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Introduction
Inverters are devices that change the direct current (DC) electricity produced by photovoltaic (PV) and many wind and hydroelectric systems, or their battery energy storage components, into alternating current (AC) electricity, which powers most home appliances. Inverter technology has been advancing significantly in the past few decades, and manufacturers are continually improving their products. The following is a basic discussion of issues that you may wish to consider when selecting an inverter.
Most inverters manufactured in the U.S. accept an input of 12-, 24-, 32-, or 48-volt direct current (DC) and produce 120-volt, 60 cycle AC. Many U.S. manufacturers will produce 220-volt, 50 HZ models for application in countries outside of North America. Popular sizes provide between 50 and 5,000 watts of AC electricity. Small inverters can power individual appliances; a large inverter can handle the load for all the appliances in a house. The quality of the AC current that the inverter produces in waveform, frequency, and voltage, determines the types of appliances and equipment it can operate.
Waveform
A waveform is a graphic representation of the shape that indicates the wave's frequency and amplitude. The frequency is the rate at which something occurs over a specified period of time. Amplitude is the measured height above or below a reference point. In an AC waveform the reference point is zero volts.
Inverters produce one of three basic types of waveforms. The ideal shape is a pure "sine wave" alternating current, similar to that supplied by the electric utilities. A modified or "quasi sine-wave" approximates a sine-wave form. The third is a "square-wave," which looks the least like a sine-wave.
A sine wave looks like a sideways "S" with a horizontal line passing through its center. The horizontal line represents zero volts. For one half of the cycle, the voltage of a sine-wave is positive (above the zero volt line). For the second half of the cycle it is negative (below the zero volt line).
A modified or quasi sine-wave looks like the side view of a set of steps. The series of steps, like the sine-wave, also forms a sideways "S" with a horizontal line representing zero volts passing through the center.
A square wave looks like two adjacent squares. One of the squares is above the zero volt line, the other is below it. As with a sine wave, for half of the cycle the voltage is positive, and for the other half of the cycle it is negative.
Square-wave inverters can handle all heat devices, such as electric resistance heaters, and hair dryers. Other devices that can be run by square-wave inverters are incandescent lights, moderate quality stereo equipment, black-and-white televisions, and some motorized devices, such as blenders, mixers, vacuum cleaners, and sewing machines.
Modified or quasi sine-wave inverters will run almost any piece of equipment except those most sensitive to harmonics, such as laser printers and computers. Harmonics are the amount of distortion or variation that occurs in a pure or perfect waveform.
Pure sine wave inverters produce an electric waveform that emulates the sine wave produced by a utility's electricity. Pure sine-wave inverters are typically used to run equipment that is very sensitive to harmonics. The degree to which the waveform of the sine-wave inverter actually resembles a pure sine-wave varies. Contact the manufacturer to determine what tools and appliances their inverter can and cannot handle.
Frequency and Voltage
Equipment that is sensitive to the purity of the waveform is often sensitive to the inverter's ability to maintain a frequency of 60 cycles. Timing devices, such as clocks, are particularly affected by a change in frequency, and may also require voltage regulation.
Most inverter manufacturers supply their inverters as 120-volt, 60 cycle AC units. Some inverters can be connected in series to produce 240-volt, 60 cycle AC. If you wish to produce 240-volt, 60 cycle AC, consult the manufacturer before purchasing an inverter, to see if it can be connected in this manner and for recommended connection procedures. A second option is to use a step-up transformer. The 120 volt, 60 cycle output of the inverter is fed into the step-up transformer and steps up the voltage to 240 volt, 60 cycle AC.
Power Ratings
Inverters have two output ratings: continuous and surge (or intermittent). The former is the amount of power available on a continuous basis, and determines the size of load it can handle. On startup, electric motors require a large initial surge of electricity, which lasts a second or two. This surge may reach six times the rated continuous wattage for some motors. An inverter must be capable of handling this momentary surge, or it will not start the device and may shut itself off. The manufacturer should be able to supply information on the most powerful motor the inverter will start. Make sure that the inverter is able to run every device you intend to connect to it. Some inverters will damage certain appliances or tools that they are not designed to power. Be especially wary of capacitor devices, such as chargers for battery-operated drills. If connected to the inverter, the drill's battery charger and rechargeable battery may overheat, damaging the battery pack and the charger.
Types of Inverters
The following are different categories of inverters, based on the method by which they convert DC to AC, and the application in which they are used.
Rotary Inverter
In a rotary inverter, DC electricity input powers a DC motor that turns an AC generator. Rotary inverters are reliable and produce a pure sine-wave output. These inverters have automatic load demand: they begin operation once a load is activated, and shut down when the load is removed. Disadvantages of a rotary inverter are lack of frequency control, low surge capability (50% above maximum rating) and lower efficiency (50% to 80%). Rotary inverters are not as common as electronic inverters.
Electronic Inverter
In an electronic inverter, solid-state electronic components convert DC to AC. Electronic inverters have efficiencies ranging from 60% to more than 90% at full power.
There are two types of electronic inverters. Each employs a different method of producing AC electricity; both work equally well. One method uses large transformers to handle the large amount of current or amperage entering the inverter from the battery. Transformers are capable of converting low voltage, high current/amperage AC into high voltage, low current/amperage AC. This makes the inverter bulky and heavy, because each step up in output wattage from the inverter requires progressively larger transformers. In inverters with large transformers, before the power enters the transformer it goes through a "chopping" circuit. This produces low voltage, 60 Hertz (Hz, cycles per second) AC power. In the United States, 60Hz is the same frequency available from the outlets in your home. The power then enters the transformer, which changes the low voltage 60 Hz AC into the 120 volts, 60Hz AC that your appliances use.
The second type of electronic inverter employs a very small transformer, and weighs much less than the large transformer-based inverters. The power entering the smaller inverter goes through not one but two chopping circuits. The first changes the low voltage, high current DC into high frequency, low voltage AC (approximately 25,000 Hz). The high frequency switching allows the use of a small transformer. The transformer changes the low voltage, high frequency AC into 165 volts high frequency AC. The high voltage AC is then rectified or turned into high voltage DC. The DC voltage then goes through the second chopping circuit, which produces 120 volt, 60Hz AC.
Stand-Alone Vs. Synchronous Inverters
Stand-alone inverters, also called static inverters, are designed to be used with independent power systems and to draw power from battery storage. They operate totally independent of a utility grid. ("Grid" is a term used for an electric utility transmission and distribution system.)
Synchronous inverters, also called utility-interactive or grid-intertie inverters, are used in systems connected to a utility power line. Synchronous inverters must produce AC electricity in synchronization with the power line, and of a quality acceptable to the utility company. In these systems, the utility company serves as the primary energy storage medium. No battery is necessary, unless the system is configured to have emergency backup power. Then the system may also incorporate a battery, a stand-alone inverter, or an inverter that can operate as a utility interactive and stand-alone inverter.
One side of the synchronous inverter is connected to your DC power source, and the other side is connected through a meter to the circuit breaker box. This enables the utility to measure the amount of power you produce. Some areas only use a single meter: the meter turns backwards when you are producing more power than you are consuming. This is called "net" metering. When the alternative power source generates electricity, the inverter powers any appliances in use. If the generated power exceeds the load from the appliances, the inverter feeds excess power into the utility line. If the generated power cannot meet the load, power is drawn into the house from the utility line. Most utility-interactive inverters shut down when the alternative power source is generating little or no electricity. They may also shut off if the quality of the grid power is outside the range acceptable to the inverter.
Synchronous inverters are also designed to disconnect from the grid during a utility grid power failure, so that utility company workers can safely make repairs on the lines. Installation and operation of a synchronous inverter must comply with safety requirements, local electrical codes, and utility regulations. Most utilities will also require inverters approved by the Underwriters Laboratory, or some other nationally recognized testing organization, to make sure that the equipment meets their safety requirements. The homeowner interested in purchasing a grid-intertie inverter should first consult with experienced designers and installers of these systems, as well as with local utility officials.
Battery-Charging Inverters
There are inverters available that include a DC source battery charge controller and/or that can charge a battery using an AC source such as the grid or a standalone AC generator. The first type avoids having to purchase a separate battery charge controller for systems with a DC power source. The second is similar to an uninterruptable power supply (UPS) for computers and other equipment, except that it can power (from the battery) a range of household appliances. If/when the grid power is unavailable, the inverter takes power from the battery and/or from a DC power source (such as a photovoltaic array) to meet specific loads. When grid power is available, the inverter monitors the battery and keeps it fully charged. Some models will manage the operation of an AC generator to maintain battery charge.
Inverter Efficiencies
While there are standards for ensuring the safety of inverters and their installation in stand-alone and grid-intertie applications, there is not yet a standard procedure for measuring the performance or efficiency of inverters (though a standard procedure has been proposed).
Manufacturers' efficiency ratings are typically based on a partial power output point where maximum efficiency is achieved. Some inverters operate very efficiently at one-quarter or one-half of their rated capacity. With some inverters, efficiency may drop off on either side of this output. Consequently, it is prudent to match inverter capacity with the load as closely as possible. Quasi or modified sine-wave inverters typically have a higher operating efficiency than the newer pure sine-wave inverters. Manufacturers are continually improving designs. Check with the manufacturer or dealer for their efficiency rating and the wattage it is based on.
Some inverters draw power even when no appliances are operating. This idle or standby power drain can be as low as one-half a watt, or as high as 10% of the maximum continuous power rating. Some inverters have a "load-sensing" feature. This feature senses when a device is using the inverter, and turns off the inverter when all loads are shut down. It uses only a small amount of electricity in stand-by mode.
