UNDERSTANDING PORTABLE INVERTER GENERATORS



COMPARISON, REVIEW AND A SELECTION GUIDE





In conventional electric gensets, the output is a raw 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 portable gensets produce THD>15%.

In addition to this, cheap portable gensets may not 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. 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. The cleanest power of all sources is produced by sine wave inverters, which are best for sensitive electronics. That is why many genset manufacturers introduced the inverter technology into their product lines.

WHAT IS INVERTER TECHNOLOGY?


By definition, inverter generator (I-G) is a device that includes an internal-combustion engine, an alternator and an electronic DC-AC converter integrated into a single appliance. Inverters obviously require a DC input. That's why the voltage from an engine-driven alternator first has to be rectified. Unlike regular gensets, an alternator in an inverter-equipped model usually produces a 3-phase voltage. It is rectified and then transformed to a constant frequency AC. Usually it is a clean sinewave with THD below 3%, although cheaper models may produce a modified sinewave. The DC-AC conversion is normally done by a microprocessor-controlled solid-state SMPS power electronics circuit operating in a PWM mode (see this for 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 portable 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. In addition, the engine does not have to run at full speed constantly. It can automatically adjust depending on the electrical 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 inverter generator models available on the market are low-power portables. Because of their low wattage you can't use them as home generators. Their light weight and low noise make them a popular choice for camping applications.
On the negative side, inverter generators just like all engine-driven devices, produce harmful carbon monoxide exhaust and should be used only outdoors. They are also quite expensive as we will see below.



PRICE COMPARISON OF DIFFERENT MANUFACTURERS


I-Gs 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 inverter generators. 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 models 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.