How does it work - inverter generators

Thread Starter

theoldwizard

Joined Jul 17, 2005
20
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.
 

SgtWookie

Joined Jul 17, 2007
22,210
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.
 

Thread Starter

theoldwizard

Joined Jul 17, 2005
20
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.
 

SgtWookie

Joined Jul 17, 2007
22,210
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.
 

Thread Starter

theoldwizard

Joined Jul 17, 2005
20
Well, almost 4 years later, technology has made the back half of an inverter generator "trivial". As long as the the engine and alternator, along with what ever kind of rectification is used, can generate somewhere between 180-200 VDC, this little chip (EGS 8010), a couple of FET drivers and some power MOSFET is all you need. Conveniently, you can buy the chip and the FET drivers on a daughter board on eBay, cheap !
 

MikeML

Joined Oct 2, 2009
5,444
They use the same technology as a VFD. The alternator makes DC internally. A SMPS circuit makes the 60Hz AC waveform. When lightly loaded, the engine runs at idle speed. As the load increases, the engine speed increases to match the load power.The engine does not run at synchronous speed.

Unlike a traditional machine where the shaft is spinning at synchronous speed, the "inverter generators" are not good at starting induction motors. A tremendous amount of energy is stored in the rotating mass of the engine and alternator rotor in the traditional machine, while the inverter has a whimpy capacitor.

I have a 5000 Watt conventional generator that will start a 1.5 h.p. 240V induction motor (inrush current ~40A@240V), but a 5000W Honda inverter won't even make the AC motor shaft twitch...
 

redcentinela

Joined Feb 26, 2015
49
Thanks mike for letting me know that. I don't plan to use this generator in induction motors, I plan to use it in my home for emergency for the fridge, a couple of lights and the tv and in order to have voltage an all the outlets, I need 220v. Just want to know how to convert it to 220v since this XG-SF5600 inverter generator.
 

Thread Starter

theoldwizard

Joined Jul 17, 2005
20
Specs say it is a single phase. The same model number for both voltages, but it comes either with two 120v outlets with two separate breakers or two 220v outlets also with two separate AC breakers ( 20A, 50A )

This is the one I bought and sold in USA:
http://m.ebay.ca/itm/261786184664?nav=SEARCH

Since it has independent breakers, may it be split phased? ( remember it is a single phase generator)
Possibly. You will have to open up the control panel and inspect the wiring.
 

withoutego

Joined Dec 22, 2015
26
Has anyone actually measured the generator output on one
of these? I was hoping for something like 60 volts @15A per
phase for a total of 3.6 KVA (3 phase generator). What I want
is a hybrid electric bicycle. I figure the diesel electric locomotives
have been successful for the railroads, why not a gas electric
hybrid bicycle. Can't seem to get the internal details on the $500
to $700 gas electric generators. Lighter than SLA and cheaper
that 2KW in Lithium Ion batteries.
 

shortbus

Joined Sep 30, 2009
7,767
Has anyone actually measured the generator output on one
of these? I was hoping for something like 60 volts @15A per
phase for a total of 3.6 KVA (3 phase generator). What I want
is a hybrid electric bicycle. I figure the diesel electric locomotives
have been successful for the railroads, why not a gas electric
hybrid bicycle. Can't seem to get the internal details on the $500
to $700 gas electric generators. Lighter than SLA and cheaper
that 2KW in Lithium Ion batteries.
Just a question on the motivation for doing this, since your not the first to ask about it lately. Every time you convert energy from one type to another there is a loss of that energy. Wouldn't it be better to just use a internal combustion engine directly to drive the bike? The bike will need 'x' watts to drive 'y' speed and distance. Why subtract the 'z' energy losses from the end result? Not trying to be difficult but to understand the reason for this idea.

A car is not the same as a bike in this. A car can have a much bigger battery and store more energy, making it more feasible.
 

MaxHeadRoom

Joined Jul 18, 2013
20,927
The main reason it is done on the likes of a locomotive, is these have a motor on each of their many driven wheels in order to control or monitor each wheel independently, therefore the diesel generator distributes power to each motor as required.
Max.
 
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