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Portable AC Generators

 

Generators are available as built in or portable units. Built in models are usually only found in larger motorhomes, the emphasis here is the smaller portable units.


Generator Basics

The aim is to produce a stable and constant voltage and frequency, for Australia this is 240V at 50 Hz.
Portable generators are available with 2 stroke and 4 stroke engines.
Models are available with electrical outputs of 240V AC from 150 VA to over 5000 VA and often with a 12V battery charging output ranging from 60 to 150 Watts.

Many are understandably confused with the term VA and how it is related to Watts.
AC generators are almost always rated in VA (Volt-Amps) rather than Watts.
Put as simply as possible VA = Watt when the load is resistive (such as a heating element). 
However most AC loads are reactive (such as electric motors, transformers) where current and voltage are out of phase to varying degrees. 
The degree to which they are out of phase is known as the power factor (pf). 
By definition, power factor is the watts consumed by the load divided by the VA necessary to deliver that power.
The relationship between between watts, volt-amps and power factor is . . .

VA = W / pf  W = VA x pf pf = W / VA

Since the generator manufacturer doesn't know what the load will be they rate their product in VA or occasionally Watts @ pf.   For example the manufacturer may express the rating for a particular generator as 1 KVA or 800 W @ .8 pf .  (Of course 1000 VA is the same as 1 KVA) 


Conventional Two Pole Generators.

Ignoring the new inverter models, this requires an engine speed as per the following table.
 

Freq
( Hz )
Gen RPM
2 Pole
Gen RPM
4 pole
50 3000 1500
60 3600 1800

Four pole generators are almost the exclusive domain of larger stationary and marine units due to the physically larger generator and also engine which must produce the required power at lower RPM. 

These generators all require mechanical engine speed governors to maintain a constant frequency and some form of electrical regulation to try to maintain a constant voltage under varying load conditions. 

When selecting a generator you need to be aware that the name/model does not necessarily indicate the real output power for us in the 50 Hz part of the world.
Because most generators are made for the world market, the model name is usually related to the maximum short term output at 60 Hz.
The exception to this is inverter generators and locally manufactured or assembled conventional two pole generators for the local 50Hz market.
To use a fictitious example, the "Supagen 1000", refers to the maximum 60Hz output.
In this case a typical output would be . . . 

Frequency
(Hz)
Maximum
Output
(VA)
Continuous
Output
(VA)
50 800 640
60 1000 800

 
As you can see the "Supagen 1000" is really only capable of 640VA continuously at 50Hz, not 1000VA as many may expect.

Why the difference between the 50 and 60 Hz rating ? Because the power of these small engines in this RPM range is almost directly proportional to the RPM.  In other words engine power at 3000 RPM is 20% less than at 3600 RPM and this is translated to a corresponding 20% reduction of the electrical output capability.

Often the de-rating from 60 to 50Hz and maximum to continuous may be a little less than my example but it's fairly close, if you use 20% in both cases, you will be erring on the safe side.   


Inverter Generators

The new inverter generators are quite different in design.  The engine is coupled to a multi phase alternator which produces an output that is electronically converted to a stable single phase 240V at 50 Hz.

Because the engine speed does not govern the final frequency or voltage, only the output power, the engine can idle when power demand is low and can spin at 5000 RPM or more at maximum output. 

This results in both engine and alternator being considerably smaller and lighter for a given KVA rating.  Another benefit is a much cleaner and more stable output and lower noise levels when operating at the lower output.  
 
The photo on the left shows the new Yamaha 1KVA inverter generator weighing only 13 Kg.

The day is coming soon when all small generators, particularly portable ones will be inverter types.   Not only for the reasons just given but manufacturing costs would have to be lower too.    Pity that isn't reflected in the present retail price, hopefully it will when they become the norm.


What size generator do you need ? 

Of course this depends on what you want to operate, where you want to use it and how much can you comfortably lift. 

You must be realistic here, to comfortably lift it in and out of the storage area, you really don't want to exceed 20-25Kg and if you don't want to invoke the wrath of neighbours and rangers look for noise levels of 60dB/7M or lower.  
This will almost certainly rule out powering your air conditioner but may include the micro wave oven.
(An exception is the new Honda EU20i - with an output of 1.6/2.0 KVA, 59bD/7M, 21Kg -  which will run a small air conditioner**.)

I should also mention that the 12V output of these generators is almost always unregulated and suitable only to charge a 12V battery, it shouldn't be connected directly to a load.
The output, often peaking 18V and more under no load conditions will most likely be detrimental to devices directly connected.
Even 12V batteries will need to be monitored to ensure they are not overcharged, although a generator is not likely to be run long enough for this to be a problem in practice.

Remember when calculating a generator size to allow for loads that require a high initial starting current such as electric motors, particularly in compressor applications.
The micro wave oven is another appliance that is often misunderstood.  
When calculating the size generator you would need to run the micro wave, you need to appreciate that the advertised power rating is the RF cooking power, the total power consumed by the microwave oven is approximately double that.     For example, a 600W microwave will consume around 1200W.

In our application where we want to operate them with an absolute minimum of impact on the environment and any neighbours, we should be looking at a quiet four stroke model.  

In my own case the generator, a Honda EM650, with an output of 2A @ 240V and 8.5A @ 12V is an excellent backup for the times when the solar panel/battery/inverter system can't satisfy needs.   It has the ability to simultaneously run the fridge, TV, video, laptop computer, lights and charge the batteries. 
I find this a very useful size for caravanning.
This model weighs 22Kg and noise level of 54dB/7M.  
Reason for choosing this particular brand and model - small, very very quiet, light(ish), very reliable (always first pull starting), spare parts readily available, good "old fashioned" technology (repairable and rebuildable).  
Frequency stability is good, voltage stability is acceptable and wave form distortion, well that's another story although it's no worse than others in this capacitor regulated class.
Although rated at 8.5A for battery charging purposes, like most others, it will only do this into a dead flat battery.  Once the battery starts to charge, this falls off rapidly so that when battery is only half charged, charging rate is only a couple of amps.
To improve this, I have built a battery charger tailored to the characteristics of this particular generator.   It will now charge a battery initially at 20A falling to 15A when fully charged. (Obviously, putting 15A into a charged battery for any length of time is not desirable so this must be constantly monitored manually or I have provided for the ability to fed in to my PL20 regulator.)

I am sure there are other brands that fall into this category but when I have inquired about availability of parts, I have come away less than convinced.

For the same weight and noise level and three times the output, the Honda EU20i inverter model previously mentioned is the only new model that fulfils my same criteria that is available at the moment (except for the "old fashioned" technology), that I know about.  It also has the advantage of most inverter models, that of excellent voltage and frequency stability and low waveform distortion.   If I could justify an outlay of $2200, this would be my first choice. (Aust dollars Feb 05)

Update April 07

If you shop around and at caravanning shows, the EU20i can be had for A$1750.

Also, in and email from Pat & Ralph Michod,
"We also have a Honda 20i which is 4 years old and running sweet as a pie. It runs the A/C in the van with no effort, it is very miserly with fuel, runs caravan frigde & Waeco for 11 hours on a tank of 5 lt. Hope this info is of some help." and in a later email from them "The A/C is an Air Command Heron Q 2.5kw heat & 2.5kw cool max current draw of 5.5A and is tested by Air Command on that Gen."


If I had a medium size motorhome or much larger caravan, another Honda, the EU30iS on a slide out mounting would be my choice for $4100, (also inverter type, electric start, 3 KVA, 58dB/7M, 59Kg) or the new Honda EM50is for $4550 (inverter model, 5 KVA, 62-68dB/7M, 95Kg)

Also, another good choice in this larger class are three new Yamaha inverter models, the YG2800i (2.8 KVA, 67dB/7M, 30Kg - yes 30Kg), the EF3000iSE (electric start, 3KVA, 57dB/7M, 67Kg) and the EF3000iSEB (electric start, 10 sec 500W boost feature, 3KVA-3.5KVA, 57dB/7M, 68.3Kg) 


**
See Honda Aust brochure HE 5781 09/02

 
       
Update 2004

Recently a range of portable generators has flooded the Australian market originating from China.    They have various brand names reflecting the identity of the importer.   
I was originally sceptical of their worth but at the price asked, I have come to believe they may be good value.  

For example, what looks like a clone of the Yamaha ET900 that used to sell for around $900 is available for $149.  I even saw it briefly on sale for $98 by a major retailer.
( However, it is a two stroke so I wouldn't recommend it for reasons given earlier.)

Also available are fully enclosed "suitcase" four stroke models rated at 850VA @ 50Hz for around $600, almost half the price of a Japanese model.

As recently as last week, I looked at a 1.2 KVA inverter model for $800, half the price of the Japanese equivalent. 

How do they compare with the major Japanese brands, probably like a Hyundi compares to a BMW, although should be quite serviceable.

I have spoken to some owners of these generators and have not heard many negative comments.
While I haven't had the opportunity to personally explore the quality of construction of any of these generators, I have been favourably surprised with the quality of several Chinese built electrical devices that I have pulled apart recently (and not ones built for Western or Japanese companies).   But I have also seen some absolute garbage including some very poorly designed and dangerous DC/AC inverters.   The problem is that you can't tell without looking inside.

If you do purchase one, I think it would be wise to give it a lot of running time within the warranty period and if they come through that OK they should last with occasional use for some time. 

A major downside as I see it would be the availability of spare parts for the proliferation and turnover of "brands" & models.    You may well have a worthless piece of junk once the warranty period is over.
Another downside I fear will be that, unfortunately, many thoughtless caravanners will take advantage of the very cheap price of the two stroke generators and we will have to put up with the smelly things at our campsites.

They should have a beneficial effect of reducing the exorbitant retail price of Japanese models too.    
(Several years ago when I was pricing a Honda EU10i, the most often quoted price was (from memory) around $1600 but one retailer was happy to sell me one for $1200 claiming he was still making a reasonable margin.) 

Would I purchase one for a primary source of power - definitely not, would I purchase one for occasional non critical use - maybe.

But remember, buying one of these rebadged Chinese imports is a lottery, you may pick a winner - you may not.

A readers contribution on the subject of cheap generators.  Jan 05.

The following is an email I received which you may find interesting, I have included it un-edited.

Ray
I was looking at your website to find some information regarding generators, thank you for clarifying some issues, and noticed that you made some comments about the cheap Chinese imports maybe being worthwhile.
I will relate my experience with one of these and you may wish to reconsider this.

We bought a GMC from Bunnings as these carried a two year warranty against the Scorpion (one year) being sold by ( I think) Repco and various ther brands being flogged by others some with only six months warranty.  The cost was $189. I had been warned by other grey nomads to check that all the bolts were tight so I duly did this as several were a bit loose.  I then filled the tank with the required 50:1 mix and noticed that it leaked fuel like a sieve. 
Tightening the screw at the bottom of the carby solved that.  I then started it up and put a AC voltmeter on the output, it read 180 volts.  I found the throttle control and adjusted that to 250V.  Connecting it to our caravan batter charger/regulator (switchmode type) promptly dropped it back to 200V and then it fluctuated between that and 300V as the charger altered the load.  After talking with more grey nomads on the road I also plugged a 60W globe on the output and that seemed to stabilise it a bit, fiddling with the throttle I could get it to stay in the range of 200V to 270V.  That was fine while the charger was in the "BULK" and then "ABSORPTION" modes but as soon as it went to the "FLOAT" mode the load reduced and the voltage shot to 350V.  I have solved this by cuting it off in the absorption mode when the battery monitoring voltage gets to 14.2VDC, I realise that the battery is only about 85% charged at this point but after burning out a couple of powerboards by not cutting it off quickly enough I decided that I would live with this.  After reading your piece I now understand that the Hz rate probably also fluctuates wildly as a result although I have no means measure this.

We are now in the market for an inverter (ACV stable) generator.

I hope that this may help someone in a similar situation and prevent some possible damage.

Happy travelling.

John  (on the road somewhere)


It is worth noting that most small generators, including premium Japanese brands, utilising (cheap) capacitor regulation suffer from some degree of voltage instability and less than perfect waveform which is improved by adding a resistive load such as John's 60W light globe.
(However, expect voltage variations of 210V to 255V from no load to maximum load with this type of "regulation", often worse but anything like John's is totally unacceptable rendering the generator practically useless.)

On the occasions that I run our little Honda EM650 generator, I load it up with the 3-way fridge's 240V 175W element for this very reason (saves a bit of LPG too).

Generators with "proper" voltage regulation, often termed AVR (automatic voltage regulation) or some similar terminology are considerably better in this department. 
However I would only purchase a known and well supported brand with AVR.
(Almost all generator faults with which I have been associated over many years have been a failure of the electronic regulation circuit, and be aware, the cost of all the AVR circuit boards I have seen has been astronomically high, often rendering small generators to the scrap heap.   Most are not repairable, being either potted in epoxy and/or having their components defaced, forcing you to buy a new one.) 

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 Ray's Caravan Campervan & Motorhomes