GMC generator review

Submitted: Thursday, Nov 02, 2006 at 11:14
ThreadID: 39063 Views:33452 Replies:11 FollowUps:15
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After some two years or so of the GMC generator, these thoughts may prove useful.

These are marketed under several brands, including the ubiquitous GMC sold by Bunnings. The so-called $98 one is rated at 500 watts, the dearer one at 650, with 850 peak.

I've tested both the AC and DC output with a digital multimeter. WIth no load, the AC fluctuates around the desired 230V mark, quite erratically, but within acceptable tolerances for my needs. Some switchmode power supplies don't mind that, otherw do.

We use the AC to run a 650 watt toaster oven, which has several heat settings. On the lower, warm-up setting the generator hums along, but speeds up considerably to max out at its rated 650 watts.

I also run small wattage power tools: 500 watt router, 500 watt drill, 150 watt orbital sander. It handles them with ease without any appreciable speeding up of the motor.

I've experimented with an old 10" Philips TV, and the generator produced wavy vertical lines. Not recommended.

The DC seems to fluctuate around 12.8 volts. That is insufficient to charge a battery. Placing the voltmeter in series with a 12v car battery, the input has never been greater than 2 amps. The supplied cable is extremely short, and pretty useless unless you a) place the generator right next to your in situ batter and kill yourself on carbon monoxide, or b) remove the battery and place it next to the generator in the open.

Until recently it always started on first or second pull. Now it requires removing the spark plug and cleaning it to start.

The unit tends to condense oil from the exhaust, which drips down one leg and can leave a stain on whatever it's sitting on.

Lately, a tiny leak has developed from under the petrol filter (which connects to the carburettor). That could be dangerous.

It weighs 22 kg, same as the Honda eu201. Quite easy to handle with one arm. But the Honda outputs 1600 watts, compared to GMC's 650. I have heard both and I would say they are just as quiet. Yes, we like the quietness of the GMC.

Petrol consumption is very light, but needs to be mixed 2-stroke.
To output 50hz, the motor must run at 3000 RPM. But with no load, it's much less, and quieter still.

The manual suggests storing empty -- but the only way to do that is to turn it upside down and let petrol spill everywhere. There should be a tap on the tank.

SUMMARY: For running small voltage insensitive power tools and appliances, the GMC 850, with 2-year warranty, is very economical to buy and run.

--Inspired



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Reply By: Mike Harding - Thursday, Nov 02, 2006 at 11:30

Thursday, Nov 02, 2006 at 11:30
I'm afraid most multimeters will not correctly measure the open circuit DC output voltage as this is a full wave rectified 100Hz, return to zero, waveform. You either need a true RMS meter or an oscillscope.

Mike Harding
AnswerID: 202500

Follow Up By: V8 Troopie - Thursday, Nov 02, 2006 at 21:42

Thursday, Nov 02, 2006 at 21:42
I would have thought a large capacitor should do the trick, cheaper than your suggestions too.
Klaus
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FollowupID: 462202

Follow Up By: Mike Harding - Thursday, Nov 02, 2006 at 21:59

Thursday, Nov 02, 2006 at 21:59
It'll charge to the peak voltage, not the RMS.
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FollowupID: 462206

Follow Up By: V8 Troopie - Friday, Nov 03, 2006 at 02:37

Friday, Nov 03, 2006 at 02:37
Exactly, the peak voltage a battery will be charged at if left hooked on indefinitely.

BTW, you can't have "RMS" voltages from a full wave rectified DC output, average yes, RMS no.
RMS is the DC equivalent voltage when the output voltage waveform HAS a negative (AC) component which a rectified voltage will not have. A 100Hz pulsed DC voltage is still DC, no AC is present. So, no need for a RMS meter.
Klaus
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FollowupID: 462285

Follow Up By: Mike Harding - Friday, Nov 03, 2006 at 09:09

Friday, Nov 03, 2006 at 09:09
RMS (Root Mean Square) is a mathematical (essentially statistical) concept not an electrical one. It is possible to derive an RMS value for any quantity which changes or varies. ie One could calculate the RMS value of a set of different sized baked bean cans on a supermarket shelf and arrive at an RMS baked bean can – let’s see you measure _that_ with an oscilloscope :)

Do not confuse the common use of RMS where it refers to the household AC mains, there is no requirement for any value in the set to have a negative value in order for an RMS value to be calculated however RMS _will_ handle negative values (which an averaging function won’t - not very well in the case of electricity anyway).

Any signal which changes is an AC signal there is no need for it to have a negative component – indeed, how do we measure “negative”? Such a measurement requires we set a zero baseline and we may choose to do that with any value – so if I have a 12V system and decide 6V is my zero then a value of +2V will be 8V and –2V will be 4V and my system can supply maximum voltages of +6V and –6V (if you see what I mean :) I suspect you are assuming negative voltages are referenced to earth (ie the planet) but that is usually not the case (although the mains supply in Oz is) and especially not with a free standing generator – what is your reference point for positive and negative with a gen? If you measure between earth and the gen’s active output you will not obtain any reading.

In regard to the battery charging; what you say is partly correct. The battery will charge to the peak voltage *eventually*. The issue occurs because the charging source (the 100Hz) is not there all the time, ie. it’s not DC, so there are gaps of time between the peaks. Consider a much worse case: suppose we have a charging source of 16V but it is a pulsed waveform of 5% duty cycle ie. we get 16V for 5 seconds and 0V for 95 seconds repeated ad infinitum. If you connect a capacitor across the charging voltage and measure the voltage you will read 16V (after a few pulses) but any calculations you make in regard to the time required to charge the battery will be out by a factor of 20 (or maybe 19? :) In reality with a full wave rectified 100Hz source it probably won’t be too far out but you would need to analyse it to be sure – these things can be _very_ deceptive.

The following links provide useful information in regard to RMS, the Wiki one is pretty mathematical but the other looks at the electrical application rather more.

en.wikipedia.org/wiki/Root_mean_square

www.ece.unb.ca/tervo/ee2791/vrms.htm

iirc when I looked at the DC output of my GMC gen on my scope it had a peak, unloaded, value of about 22V dropping to around 18V under load – use caution when charging batteries from these outlets, don’t leave them on charge too long - I don’t know that Honda et al are a lot better, DC wise, are they? I did an analysis of both the AC and DC outputs of my gen and posted it to this site a couple of years ago, if you can find it!

Mike Harding

mike_harding@fastmail.fm
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FollowupID: 462327

Follow Up By: inspired - Friday, Nov 03, 2006 at 17:45

Friday, Nov 03, 2006 at 17:45
Ummmmm, not being an electronics engineer....Are you saying that my digital voltmeter cannot correctly read a DC signal from my modified sine-wave generator?

--Inspired
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FollowupID: 462447

Follow Up By: Mike Harding - Saturday, Nov 04, 2006 at 09:42

Saturday, Nov 04, 2006 at 09:42
What's a modified sine wave generator? Do you mean an inverter or are you talking about the GMC gen DC output??

It depends: but it is unlikely a meter you paid less than, about, $250 for will have a true RMS facility therefore it will only read DC which does not have an AC component (battery type voltage) or a sine wave accurately - how inaccurate it is depends upon a number of factors and I cannot estimate that without knowing much more about your system.

Measurement instruments are subject to all sorts of issues which may, indeed do, affect their accuracy - never assume that simply because a measurement instrument produces a reading it is correct, especially with digital multimeters!

Mike Harding
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FollowupID: 462568

Reply By: Redback - Thursday, Nov 02, 2006 at 11:40

Thursday, Nov 02, 2006 at 11:40
Is this for 4WD/Camping applications or home handyman/sub contractor.

AnswerID: 202502

Reply By: traveller2 - Thursday, Nov 02, 2006 at 11:41

Thursday, Nov 02, 2006 at 11:41
Agree totally with your observations, mine too developed a fuel leak and it was the actual tap itself loose in the fitting on the bottom of the tank, 30 secs with a spanner to tighten it and end of problem.
Have also noted the drippy exhaust which can be slowed by mixing just enough oil with the fuel to meet the 50:1 mix, don't just chuck in 'enough' like we alll do ;-))
Very quiet and the TV problem can be solved by connecting an old PC UPS in the power before the tv. I had a stuffed/battery cactus 250w one sitting around which was about the size of 1 1/2 housebricks so used that.
I usually use it for power tools, lighting and it is better to use a 240v charger for batteries rather than the 12v lead which is way too short.
AnswerID: 202503

Reply By: Member - 'Lucy' - Thursday, Nov 02, 2006 at 13:09

Thursday, Nov 02, 2006 at 13:09
Inspired

I/ve had 4 of these suckers.

(1) Never fired a shot as the petrol tank seam peed fuel when filled.

(2) Fuel tap leaked

(3) Perfect (12V went up in a cloud of white smoke one day) Ran beatifully and quiet

ALL replaced on the 2 yr warranty thingy without a bat of an eyelid (Bunnings)

(4) Okay - bit noisier, runs power tools up to 850watts easily

No. 4 has killed to battery chargers thouh.

So last weekend at the Melb 4Wd show I bought a Honda 2.0i and all is well for every thing.

Takes virtuallly the same room and weighs the same.

Grandson now has his own GMC 850 Gennie for dirt bike riding trips with 'the boys'.
AnswerID: 202528

Follow Up By: Jimbo - Thursday, Nov 02, 2006 at 22:12

Thursday, Nov 02, 2006 at 22:12
That Grandson of your's must be mighty fit to be still riding dirt bikes at 50.
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FollowupID: 462211

Follow Up By: Member - 'Lucy' - Thursday, Nov 02, 2006 at 23:56

Thursday, Nov 02, 2006 at 23:56
Que'

He is 5 you wacker and rides a Honda 50 & 70 like there is no tomorrow.

Now where have you been old son. Very quiet of late I must say.
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FollowupID: 462272

Follow Up By: Jimbo - Friday, Nov 03, 2006 at 00:02

Friday, Nov 03, 2006 at 00:02
"He is 5"

That must be your Great Grandson LOL.

I'll call you tomorrow if I have a better day.
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FollowupID: 462273

Reply By: Wizard1 - Thursday, Nov 02, 2006 at 13:39

Thursday, Nov 02, 2006 at 13:39
Had a 850 Watt 2 stroke. It stopped for no apparent reason after a total of around 6 hours use, off and on, while we were at Coburg Peninsular, NT. Got back to Darwin got my money back from Bunnings and put it on as a deposit for a Honda EU20. What a great bit of equipment not a single fault after 3 years of use, including running the house fridge and freezer during SE QLD blackouts. Having a 4 stroke takes away the stuffing around with extra oil and mixing. Ever tried to buy pre-mixed in the outback?

I can't believe people that don't learn from the first one stuffing up, let alone 4 of them! I got the message the first time, you pay for what you get.
AnswerID: 202542

Follow Up By: Mike Harding - Thursday, Nov 02, 2006 at 14:06

Thursday, Nov 02, 2006 at 14:06
My little GMC 550W is still running well. Must be more than two years old now, it does everything I require - mainly battery charging via a 240V 3 stage charger - and for $98 I, most definitely, got much more than I paid for.

Horses for course, people.

Mike Harding
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FollowupID: 462057

Reply By: Robin - Thursday, Nov 02, 2006 at 14:04

Thursday, Nov 02, 2006 at 14:04
Love to know if anyone has got one of those cheap microwaves to run off these gennies.

I imagine you could get it to work however you would have to set the microwave down to 1/2 power or so.

$98 gennie and $69 microwave , could solve the problems of cooking on the
total firebans days we are expecting in Vic this summer.

Robin Miller
AnswerID: 202545

Follow Up By: Mike Harding - Thursday, Nov 02, 2006 at 14:10

Thursday, Nov 02, 2006 at 14:10
A 600Wish microwave probably has a current draw around 4A or 5A - off the top of my head - so I doubt a 550W/750W gen would have a prayer?
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FollowupID: 462061

Follow Up By: Mike Harding - Thursday, Nov 02, 2006 at 14:13

Thursday, Nov 02, 2006 at 14:13
Ah! Just re-read your post.
Microwaves don't "reduce" power as such - they cycle the magnetron on a relative time duty cycle which is proportional to the requested power. So 50% power gets you (say) 10 seconds at full power followed by 10 seconds at zero power. Your gen would still need to provide the 4A or 5A?

Mike Harding
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FollowupID: 462063

Follow Up By: Robin - Thursday, Nov 02, 2006 at 14:39

Thursday, Nov 02, 2006 at 14:39
Thanks, yes they cycle now Mike , should have remember that as mine cycle.

Used to own a Sharp 500w some 20 years ago , and it actually powered down
although limited, but there was a lot more inside it than todays models.

But these days you don't seem to be able to buy one(cheapie) less than 700w.

Never looked into them but I suppose one would have to upset the internal
magentron driving voltage some how.

I got a kilowatt 110v transformer somewhere so might try to run one of it
and see if it fires up.

Robin Miller
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FollowupID: 462072

Reply By: Robert - Thursday, Nov 02, 2006 at 14:40

Thursday, Nov 02, 2006 at 14:40
Whilst haven't used our 750 GMC camping much, have used it around home and works fine. Did try when first got it on old tv and had same problem where the picture flickered but when used on more recent tv was fine!
Haven't had any problems with leaks or starting problems and have charged a dead flat battery with it. Use it often to run small angle grinder and have used it to run portable dvd and portable fan with no problems. Did check freq and voltage output and didn't think it seemed to fluctuate much - guess it depends on how well the motor is tuned.

Wouldn't use it for laptop but don't take one of those away anyway.

Think they are well made, simply just the older style gen that was built with a cast iron engine hence heavy compared to newer ones. These are generators that in the past people would have paid quite an amount of money for.

I think it's about being realistic for you are going to use it for, for me I can't justify spending a lot more money on something that won't do anything more than I need of the GMC unit.

A more expensive gen is a target for thieves!
AnswerID: 202550

Follow Up By: Mike Harding - Thursday, Nov 02, 2006 at 14:51

Thursday, Nov 02, 2006 at 14:51
>A more expensive gen is a target for thieves!

Good point!!! Explains why I found that envelope tucked under the handle of my gen recently with a $10 note and a message inside saying "You need this more than I do" :)
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FollowupID: 462076

Reply By: TerraFirma - Thursday, Nov 02, 2006 at 14:53

Thursday, Nov 02, 2006 at 14:53
Your summary is very good.! Spot on. However it should only inspire others to buy a better quality generator in my opinion. You revealed the hidden costs and cons of this generator and when you add them all up, IMHO, they make this generator a bit of a pain in the arse. If you have high tolerance for these issues then thats fine by me, but I for one, will sleep peacefully with my Kipors, arrr, that is while they last..! LOL
AnswerID: 202554

Follow Up By: Robert - Thursday, Nov 02, 2006 at 15:48

Thursday, Nov 02, 2006 at 15:48
But not everyone has had problems - mine works fine.
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FollowupID: 462101

Reply By: Member - Jeff M (WA) - Thursday, Nov 02, 2006 at 16:12

Thursday, Nov 02, 2006 at 16:12
Well a little off topic, but might be handy for those thinking of buying the much more expensive Hondas. I borrowed my step fathers Kipor 2kva 4 stroke inverter genny last weekend for the Boyup Brook rodeo. What a pearler of a machine! Bloody quiet, we had it running 3 car fridges, charging my 2 x 100amp hr batteries (off my on board 240v 3 stage) a camp light, my car stereo and a caravan 240v fridge and caravans lights all weekend. Used a about 5L of ULP started first start every time and just for bleep s and giggles we started up the caravans aircon to see what would happen and other than the Kipor starting to bark to an acutaly noticble noise level above the music (we could barely here it previous to that and it was only about 4 meteres away!) it run that on top of everything else!! So thumbs up to the Kipor, I think he paid just under $1000 off ebay for it brand new and for that $$$ you can't complain I reckon! Not sure of the weight of this unit but I could carry it with one hand and it sat on the back seat of the surf on the way there and way back.
AnswerID: 202575

Reply By: Groove - Thursday, Nov 02, 2006 at 18:28

Thursday, Nov 02, 2006 at 18:28
I have a relative who works for a Bunnings store, he claims they send at least 1 pallet of gear back to GMC every week, sometimes more. When there are blackouts (the store is on the outskirts of Sydney and prone to extended power outages), they sell dozens of the GMC generators and a decent number come back a few days later when the power returns, not dead just not needed. Thats GMC's no questions asked return policy at work.

AnswerID: 202620

Reply By: Member - Omaroo (NSW) - Thursday, Nov 02, 2006 at 18:41

Thursday, Nov 02, 2006 at 18:41
We've run our 750W unit now for nearly two years too. We run it on the farm to power the van lights and other general duties. It would have several hundred hours up on it and it has never missed a beat - ever.

I replaced the spark plug for an NGK early in the piece. Runs far better - more consistently and with seemingly less throttle hunting.

Oil does condense. Silly problem to have but there it is. It has to go somewhere and I'd rather collect it than breath it.

The way I read the bit in the manual concerned with leaving it empty was in relation to the carby. I let it run dry before I put away and it's never failed to start first pull thereafter.

All in all - a great unit and by far the best money spent on any power tool that I have. I'd only replace it with a diesel one now.
AnswerID: 202624

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