UNDERSTANDING INVERTER GENERATORS
COMPARISON, REVIEW AND A SELECTION GUIDE
WHAT IS IT?
Inverter generator (I-G) is a device that includes an internal-combustion engine, an alternator and an electronic DC-AC converter (inverter) integrated into a single appliance. Its main advantage is it produces clean sinusoidal output and has reduced noise compared to regular generators.
I will explain you how it works and how it differs from conventional generators. In conventional gensets, the output is an "unprocessed" voltage induced in a coil by a rotating magnetic field. Unlike grid-supplied electricity, this voltage is not a pure sine wave and may contain significant level of higher frequency harmonics. While total harmonic distortion (THD) of less than 5-6% is generally considered acceptable, some gensets (especially portable ones) produce THD>15%. That's why their manufacturers rarely specify THD. In addition to this, cheap portable gensets may not even have an automatic voltage regulator (AVR), can't react to rapidly changing loads and will produce power surges or sags. They may even destroy an appliance sensitive to voltage fluctuations, such as a refrigerator's compressor. In some cases, if you connect your electric load via a UPS power supply, it may not even recognize such a genset as a normal power source and may switch to battery mode. sine wave inverters produce the cleanest power of all sources, and are best for sensitive electronics. That is why many genset manufacturers include the inverter technology into their product lines.
Inverters are electronic devices that convert DC to AC, so obviously, we need DC input to run them. Since an engine-driven alternator produces AC voltage, first it has to be rectified. Unlike small regular gensets, which are single-phase, an inverter-equipped model usually produces a 3-phase voltage. It is rectified and then transformed to a constant frequency AC- see below a conceptual schematic diagram of inverter-generator power train.
IG output usually is a clean sinewave with THD below 3%. On the market there are also so-called modified sinewave inverters, which may have higher THD. See this review of various types of inverters. The DC-AC conversion is normally done by a microprocessor-controlled solid-state SMPS circuit operating in a PWM mode (see SMPS tutorial). The reason a 3-phase system is used is a 3-phase rectification results in much lower ripple factor that a single-phase one. Accordingly, such a rectifier needs much smaller filter capacitor (if any) to lower AC component to a level acceptable for the inverter. Besides generating high quality power output, I-Gs are more fuel-efficient and quiet- see for example our review of low-noise gensets. Note that the frequency of the voltage induced by a rotating magnetic field is directly related to the frequency of the rotation. Therefore, in conventional gensets the engines have to run at a constant speed. For example, for the 60 hertz U.S. market it has to be 3600 RPM for 2 poles or 1800 RPM for 4 poles. As the result, even when the motor is idle, its fuel consumption can be as much as half of the consumption at rated load. In contrary, in I-G the engine can run at lower RPMs because the induced voltage is rectified anyway so you don't care about its frequency. In addition, their engine does not have to run at full speed constantly. It can automatically adjust depending on the load demand. All this makes it more fuel efficient and much quieter. Another advantage of inverter generators is with proper design they can be paralleled for higher power. Note that usually you can't just connect them together- you need to buy an optional parallel adapter that forces two devices to synchronize their operation. Most consumer-grade portable inverter generator models available on the market are low power, typically below 8kW. Because of their low wattage you can use it to power a few critical appliance, but you can hardly use them as home generators. Their light weight and low noise make them more appropriate for camping applications.
On the negative side, IGs just like all engine-driven devices, produce harmful carbon monoxide exhaust and should be used only outdoors.
REVIEW AND PRICE COMPARISON
Inverter-generators typically cost 2 to 5 times more than conventional gensets. You can find a cheap gasoline genset for less than $100/kW, but a device with inverter technology typically sells for at least twice as much. There are a number of manufacturers of portable IGs. Honda and Yamaha models have high ratings based on the user reviews, but they also are priced at the high end of the market price range. These two brands cost typically $500 to $800 per kilowatt. Domestic and imported brands are generally less expensive (as low as $200/kW), but based on the customer reviews at Amazon, some of them may be more noisy, may have lower reliability and lower overall ratings.