All About Circuits Forum How does it work - inverter generators
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#1
03-12-2010, 02:25 PM
 theoldwizard New Member Join Date: Jul 2005 Posts: 8
How does it work - inverter generators

Honda, Yamaha and others now sell "inverter generators". These use a small 5-10 hp gasoline engine and output 110V (I'm certain they have European voltage generators also). The most unique feature is that the engine speed is varied as the load varies. This results in much lower fuel consumption compared the other portable generators that use a constant engine speed controlled by a mechanical governor.

My limited knowledge of electric generation is that the output voltage varies with the shaft speed.

So how do you take a varying voltage (more or less constant current) from the generator section and convert it into a constant voltage, varying current source for the load ?

A circuit would be nice, by I'm more interested in a theoretical discussion.
#2
03-12-2010, 06:32 PM
 beenthere Senior Member Join Date: Apr 2004 Location: Missouri, USA (GMT -6) Posts: 15,815 Blog Entries: 10

The engine may drive the alternator through a variable speed pulley - http://www.speedselector.com/
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#3
03-12-2010, 08:32 PM
 theoldwizard New Member Join Date: Jul 2005 Posts: 8

Quote:
 Originally Posted by beenthere The engine may drive the alternator through a variable speed pulley - http://www.speedselector.com/
Nope. Direct drive.
#4
03-13-2010, 12:43 AM
 SgtWookie Expert Member Join Date: Jul 2007 Location: In the vast midwest of the USA; CST Posts: 22,032

Well, if the output power were being produced directly by an alternator, the motor speed would be held constant to maintain the output frequency, and the field winding would be fed current to keep the output RMS voltage stable.

Using an inverter to generate the output pseudo-sinewave (or true sinewave if they were using, say, something resembling a class D amplifier with a low-pass filter) would mean that they would just have to generate enough current to keep the output voltage up for the load. This sort of thing is done with DC-DC buck/boost converters over a fairly wide range. Of course, as the input voltage drops, more input current is required to keep a given load current stable.

The use of a stable circuit as an inverter to synthesize the output frequency eliminates certain problems, like another member posted several months back - they used a large, low RPM diesel engine to drive their generator. Unfortunately, the RPM varied significantly due to the high compression and violent power stroke, and the variation of frequency in the output power was too unstable to run a number of their motor-driven appliances. His options were fairly limited; increase engine RPM and use a reduction drive, possibly using a flexible coupling between motor and generator with a heavy flywheel on the generator, and I don't remember what all else.

Anyway, advances are being made in power generation efficiency all the time. It's a good thing, too - we're running short of fossil fuels.
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#5
03-13-2010, 11:46 AM
 theoldwizard New Member Join Date: Jul 2005 Posts: 8

Just to keep this discussion going, I'll give you my SWAG !

The alternator section uses a rare-earth magnet rotor, spinning inside a 3 phase (or maybe 5 phase) stator. Diodes convert the output of this section to DC. Here is my first "assumption" ! The windings are such that output voltage from the diodes is greater than the peak-to-peak voltage of a 120V 60Hz sine wave (> 170v) !

Next is a filter section, probably with a hefty inductor. This needs to supply the inverter section even when a load is added.

Many of these portable generators have a secondary low voltage winding for charging wet cell lead acid batteries. This is perfect for running some of the electronics.

The inverter section is based on a crystal controlled, sine wave generator (like a XR2206). This feeds a Class D MOSFET amplifier which gets it supply from the high voltage DC.

A microprocessor monitors the output voltage and then runs a stepper motor which open and closes the throttle.
#6
03-13-2010, 04:01 PM
 SgtWookie Expert Member Join Date: Jul 2007 Location: In the vast midwest of the USA; CST Posts: 22,032

Yes, they could be using neodymium magnets, and it may have more than 3-phase output. Synchronous rectification would be much more efficient than a traditional rectifier bridge.

I sincerely doubt that they are using an old linear oscillator like an XR2206; much more likely that they are using something like a microcontroller that has a sinewave table in the programming, or an otherwise dedicated PWM IC. Throttle control would more likely be a servo instead of a stepper.
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