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KiwiJohn
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Joined: 01/12/2005
Location: New Zealand
Posts: 691

Posted: 08:09pm 25 Feb 2011

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My friend in Christchurch had a problem after the earthquake left him without power and his generator was unable to start the freezer.

I did a bit of head scratching and came up with what is probably an old idea and the fact that I had never heard of it means nothing!

Anyway, I thought that if the generator could start a heavy motor, such as a table saw or bench grinder, even if the machine had to be given a 'rope start', that could be used to start the freezer.

The idea is to start the grinder, saw, whatever and when it is up to speed switch the freezer into the circuit. As the generator sags with the extra load the inertia of the induction motors will take over some of the load and so kick start the freezer. Once the freezer is running the saw etc can be turned off.

I suppose if the generator cannot start the grinder you could instead switch on a number of smaller units (which I presume must have induction motors) until you have enough inertia in the system to start the freezer.

I have not been able to get confirmation that this will work and by the time I managed to contact my friend again he had mains power on and his need had passed.

I present this here in the possibility that someone may care to experiment (I dont have the equipment required) and if sucessful this could be added to the useful knowledge store of The Back Shed.

John (80ks from Christchurch and alls well...)

Edited by KiwiJohn 2011-02-27

dwyer
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Joined: 19/09/2005
Location: Australia
Posts: 574

Posted: 10:25pm 25 Feb 2011

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Hi Kiwijohn
Yes I know what you mean as l did had similar problem with my freeze first is put plug in with 150 watt outdoor light while is running and add other power lead and plug to the freeze will work as l know something weirdo with generator will not run until some load on. This happen on many cheap chinse generator .and beware that these Poor quality generator can burn out moderm,Tv and electronic devices as has very dirty sine waves, voltage sometime are unable regulate probely So it ok for use for power tool,light tranformer batteries charger
all the best

dwyer

davef
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Joined: 14/05/2006
Location: New Zealand
Posts: 499

Posted: 07:26am 28 Feb 2011

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Unfortunately, my "cheap Chinese" generator is not here or I would fire it up and put an oscilloscope on the output under the test conditions you have alluded to.

Being a "rotating machine" I would have thought that the output would be a good sine wave. Voltage regulation could be an issue. I did not think there was any electronic regulation on my unit. I thought it just used some mechanical feedback system to keep the speed relatively constant, like a lawn mower. I could believe that the voltage regulation may be comparatively poor.

I'll check mine out next weekend and try to provide some details.

Dave (10kms from Christchurch and mostly well!)

Edited by davef 2011-03-01

VK4AYQ
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Joined: 02/12/2009
Location: Australia
Posts: 2539

Posted: 01:43pm 28 Feb 2011

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Hi Dave

I have a alternator that has a boost field built in to assist regulation, the first section of the field windings runs the voltage without load and thats set by a rheostat and the second set of windings is made of thick wire and carries the output current so as the generator loads up the current in the secondary field winding increases the excitation of the field proportional to the load on the generator. This is accomplished with two bridge rectifiers, it works and has never broken down so never pulled it to bits to draw a circuit, sorry.

All the best

Bob

Foolin Around

davef
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Joined: 14/05/2006
Location: New Zealand
Posts: 499

Posted: 09:06am 01 Mar 2011

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Bob,

No need to be sorry!

I am only interested because I'll be running several bits of gear off my generator, at times. Probably mostly BIG load stuff, but I would like to know the reason for this problem in case I have to run a fridge or freezer off mine.

Cheers,
Dave

VK4AYQ
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Joined: 02/12/2009
Location: Australia
Posts: 2539

Posted: 10:29am 01 Mar 2011

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Hi Dave

My little generator has trouble starting things at times, so what I do is manually open the revs up slightly as I switch on the load, this overcomes the governor droop in the in the loading increase, as there is a finite time required for the Governor to react to the load, and the engine to react to the Governor in that time the voltage droops with the extra load and the generator bogs down. if you have a frequency and voltage meter on the machine you can see this happening.

I use the rule of 55 cps as maximum rpm, on my unit that takes the volts to 250 instead of 230 it prevents the motor from bogging down.

While this isn't a good idea on a permanent basis for emergency it is OK, as it does less damage to the generator and the appliance than sitting in a semi stalled high power draw situation until some form of overload protection cuts it out or the magic smoke escapes. Do not hold the extra cycles once the fridge has started as it increases the internal pressure in the sealed unit and is detrimental to the unit.

If you don't have a frequency meter on the generator as most small units don't have, try to get a volt and frequency meter and make a power board up that has a few power points on it and connect a single lead to the generator at least a 2.5mm rating lead and no more than 4 meters long. The meters are quite cheap on ebay.
Induction motors can draw 5 to 8 times their run power to start, so once they are running all is OK.

I set the Governor on my unit to 53 cycles off load and it load droops to 50 cps under load, I found the factory setting of 51 cycles resulted in a loaded cps of 48, this is a bit low for the efficient running of other motors as it causes them to draw more power than their correct cycle rating.

By using the described method I can run my fridge on a little el-cheepo generator rated at 850 watts.

When running a fridge or freezer set the thermostat lower than usual so it doesn't cycle when you are not there and put a few bottles of water in to absorb the cold to give more thermal inertia. Make sure that you are the one that turns it off not the thermostat.

Using this method with a freezer, I can get it to work by a 1 hour run in 5 hours if the OAT isn't to hot.

Prior to getting mains power I lived on generator power for 20 years so learned a few tricks of the trade so to speak.

With the worldwide happenings of late it is a good idea for all of us to be prepared with some emergency equipment to avoid joining the hand out lines at disaster centers, and ongoing stressful arguments with insurance companies.

All the best

Bob

Foolin Around

dwyer
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Joined: 19/09/2005
Location: Australia
Posts: 574

Posted: 12:29pm 01 Mar 2011

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Hi Bob
l agree with you and the trouble with little generator's Governor to react to the load, and the engine to react to the Governor. In that time the voltage drops with the extra load. The generator has no mechanical or electronics to control the voltage or power load and the Governor is controlled by a simple engine fan and has linkage and spring to carburettor. When power load comes on the engine rpm decreases so fan linkage increases engine RPM. The load to compensation voltage , power and frequency changing are too late, same with when the load is taken off the engine�s RPM will go up and down in the matter of a few seconds before running back at normal a speed. It�s all controlled by fan the same as a lawn mower

regard Dwyer

KiwiJohn
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Joined: 01/12/2005
Location: New Zealand
Posts: 691

Posted: 08:07pm 02 Mar 2011

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Thanks for the comments everyone. However it would be really interesting is someone with the means can do a test of the flywheel concept involving a spinning motor such as the bench grinder or table saw I mentioned. If this is effective it is a much easier thing to describe to someone like my friend who maybe not so familiar with the inside of his generator.

Regarding frequency, I suggest that a high frequency is much better than a low frequency. Most appliances will suffer increased 'iron losses' at a lower frequency. I dont think an extra 10 or even 20 volts would be a problem either.

VK4AYQ
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Joined: 02/12/2009
Location: Australia
Posts: 2539

Posted: 11:23pm 02 Mar 2011

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Hi John

The problem with a electric flywheel as in an induction motor with or without flywheel is the motor works on a trailing phase angle to extract power from the generator, thats why most motors that would be synchronous at 1500 rpm are actually rated at 1440 rpm, that means that the generator would have to slow below that rpm to re extract power from the motor.

You are right about about the volts as most things we use are OK between 220 and 250 volts, frequency is much the same within smaller limits, however a fridge relies on a stable frequency because small domestic units use a fixed orifice in the pressure circuit, and if the frequency is increased very much the pressure internally goes higher and loads the sealed unit more.

For a few seconds at a start it wont hurt but in the longer term it could overheat the unit and cause burnout or at least overload dropout.

The problem with generators these days is that they don't have big enough flywheels as they are made to be light and portable. My old generator is a 5 hp lister running at 1500 rpm and has a flywheel weighing 250 KG, it will start most things with just a bit of a grunt, my portable 2.5 KVA generator only weighs 50 KG all up including the frame so even with it running at 3000 rpm the rotational inertia is nearly nothing.

One way you could try is to place the induction motor on line to load the generator up and get the governor active then using a double pole switch flick to the fridge more or less instantly, the generator will then transfer load setting to the fridge.

If your friend doesn't know how to work the governor manually it may be a waste of time playing with a too small generator, hopefully power is back on to him now, and tell him to trade in his small one on a bigger one for next time, that should stop further emergencies, as they only happen when you aren't prepared, according to Murphies law.

All the best

Bob

Foolin Around

KiwiJohn
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Joined: 01/12/2005
Location: New Zealand
Posts: 691

Posted: 12:05am 03 Mar 2011

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Thats a good explanation Bob regarding the slip angle. If anyone is going to use the double pole switch idea then a heater might make a useful load, one that can be switched LOW, MED, HIGH.

My friend got power back soon after he and I were talking so his crayfish were saved!

I once worked at an airport where we had a small hydro generator with not the best frequency regulation and that used to play havoc with out American made Collins radio transmitters especially the power transformers which were already on their limits at 50Hz.

mid north Matt

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Joined: 06/01/2011
Location: Australia
Posts: 58

Posted: 12:50am 03 Mar 2011

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what about a large bank of AC caps to provide the extra grunt to aid the fridge/freezer to start up?after all they only need the extr power for 2 or 3 seconds than they run on next to nothing.another way to make a generator is a 5hp motor/24v truck alternator/BIG bank of DC caps and a 1.5KVA apc smart ups box to use as a inverter which has a surge/crest factor of 5 which would start a very big load and give pure sine wave regulated power just like what comes out of your powerpoint

Pt Wakefield Matt

isaiah

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Joined: 25/12/2009
Location: United States
Posts: 303

Posted: 09:09am 03 Mar 2011

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Why wouldn't a regular motor start cap work for this problem.
I have a old Montgomery Wards generator 1960's era 3500 watt and it will run every thing in the house and our smurageble pump but it wont run the central air conditioner.
It will run the furnace side of the system but not the air.
If a motor starting cap would work one could wire one in permanently in the lead in cord from the generator or in the box where you plug the generator in.
I have a small Kohler generator that doesn't start every thing so when the weather breaks a bit more we may try this out with some caps.
The old M.G.9 hp Briggs generator was like new when I bought it several yrs back and it as yet has the paint on the rope start cup. But due to father time taking his toll on me It may get updated to electric start soon.
We a should have are equipment up in shape if possible It looks like this is the year the dodo will hit the fan and those that are prepared may have a chance at survival.
We all have seen or lived through a lot all ready this year.
And as actor Michale Gross says in one of the Tremors movies
" These people in government are not your friends"
Isaiah

URL=http://www.motherearthnews.com/Renewable-Energy/1973-11- 01/The-Plowboy-Interview.aspx>The Plowboy Interview[/URL>

davef
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Joined: 14/05/2006
Location: New Zealand
Posts: 499

Posted: 09:43am 04 Mar 2011

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Bob,

Thank you for the detailed account of your experiences. Certainly much more to this than I first thought.

I'll look into a frequency meter. Hopefully, all I'll ever use the 2.5KW genny for is to pump out the holding tank in the sewerage system, for the drip lines.

Dave

mid north Matt

Regular Member

Joined: 06/01/2011
Location: Australia
Posts: 58

Posted: 10:40am 04 Mar 2011

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Davef
you can buy a frequency meter from briggs and straton,it works on vibrations coming off the motor with no electricial connection at all or buy a new multimeter with a frequency function on it

Pt Wakefield Matt

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