Thursday, March 24, 2016

A Closer Look at Inverter Generators

By: Kirk A. Kleinschmidt, NTØZ

Inverter Generator Basics
Unlike conventional generators, which typically use a two-winding core that must turn at 3600 RPM to produce 120 V ac power at 60 Hz, inverter generators produce multiple-phase ac power at high frequencies, which is electronically “converted” to dc, then “inverted” back to rock-solid, low distortion, 120 V, 60 Hz ac. Because an inverter generator can draw more power per revolution from the generator core (thanks to several hundred overlapping sine waves per revolution), the core doesn’t need to operate at a fixed speed to maintain regulation. Inverter generators are lighter, quieter, and more fuel efficient because generator speed is automatically adjusted to match load conditions.

Inner Workings
A conventional generator rotates two large coils inside its core, and each full engine rotation produces one complete ac sine wave. By design, the engine must spin the generator at 3600 RPM to produce the desired ac output. If, because of varying loads, the generator spins faster or slower, the voltage and frequency of the output vary in step. In an inverter generator, the core uses multiple coils and multiple magnets. Each full rotation of the engine produces more than 300 three phase ac sine waves at frequencies up to 20 kHz, which produces more electrical energy per engine revolution. A microprocessor-controlled inverter module converts the high-frequency ac to dc (about 200 V in at least one unit) before “inverting” it back to clean, stable 120 V, 60-Hz ac power. Typical inverter generator regulation specs are 3% voltage, 1% frequency and 3% sine wave distortion. That’s essentially “power company” ac in a hand-carried unit!

Sound Design
A compact, tightly regulated ac power source just wouldn’t be quite so handy if it sounded like a jet engine. To minimize noise and vibration, manufacturers designed extra large internal exhaust mufflers, multi-chambered inner structures that isolate intake air from output exhaust, innovative internal cooling and air transport fans and enclosures that use unconventional laminates. The generators are compact and “friendly” looking, but the internal construction is carefully tailored to minimize sound and provide adequate cooling. If you look closely, you’ll notice that, unlike typical construction site generators that use open frame designs that let everything “hang out,” the smaller inverter gens are completely enclosed. Instead of using a single layer of plastic for the outer shell, at least one manufacturer sandwiches a layer of sound and vibration-dampening foam between two thin layers of plastic. This dramatically reduces sound levels and prevents the enclosure from “resonating” with the whirring and rotating internal parts and exhaust notes.

Size and Efficiency
If you’re wondering whether portable generators have been getting smaller these days, you’re right! Thanks to relentless design efforts and the fact that inverter generators are some 20% more efficient than their conventional counterparts, engines, generator cores and enclosures have been shrinking, in size and weight.

Generators can create RFI so you may need to put a filter on the line.  Check out the 2012 June issue for a review of 4 small generators.

Four Field Day Generators

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