Generator exercising and load testing

R

Robert H. Wilkinson

New member
As quite a few Bratts have back up generators for hurricane season, I thought I would share this.

As some know I have lived off grid for the last 34 years but have never experienced this before. My main gen. is a diesel, but as it occasionally goes down and requires service - a backup is required. I bought a new 3500 watt Briggs in Nov. 2011 and used it for about 6 hours. I never used it again untill it was needed this Nov. It started fine but to my dismay it was not producing power.

After much research I found that many pull start generators rely on a small stored electrical charge to re-energize the field on restart. If left in storage too long it will no longer produce power because the field is not self energizing.

You can google the problem and there are several websites that suggest different remedies, but here is the one that worked for me. Take a short cord with a plug on one end and plug it into one of the 120v outlets. With the gen. running I took the 2 wires from the other end of the cord and very briefly touched them to a 12v car battery. This is referred to as "field flashing" - it re energizes the field and allows the gen. to start producing power. Thank you Internet!! :D

FYI - when you "exercise" your backup generator - in addition to making sure it starts, etc. you need to also apply a load and run it for a while with the load on. This "load test" confirms that it is ready for service and allows the gen. to replenish the stored charge for the field next time it is started. With small generators that utilize brushes, this also helps keep them "run in" as infrequent use can lead to brushes sticking and loosing contact.

Regards, Rob
 
Rob, we installed an 18kw generator when we had the house built in 2008. It has a 200a. Automatic Bus Transfer device that waits 15 seconds to switchover once the power is REALLY off. It runs on propane and we have 500gal of that liquid gold in the ground. Have to reset all the digital clocks once it starts but when power comes back, it switches back and shuts down w/o a blip. I change oil, filters and plugs each summer.

It exercises itself for 15 minutes each week. Longest it ever ran was for 5 days when IRENE visited us 18 months ago. It used about 100g of LPG.

So far, it's been great, I guess out here, we lose power often enough not to have the problem you describe. Thanks for bringing it up, good to know!

Charlie
 
I have never heard of this before. Now you have me thinking. I have not used the gen set for over 2 yrs (luckily). Looks like something to check on.

Thanks!
 
I run our home 6K on propane generator on the first of each month for about 15 minutes. I had to field flash our old generator once several years ago before replacing it with this new one.
 
We have a similar set up to what Charlie has except it runs on natural gas. The Honda Eu series is not subject to the lost of magnetic field, as the contractor type of generators are.

I do have a back up for the back up back up--and that is one of those Briggs Stratton Contractor types, and I should check it out to be sure that the field is still OK.
 
Anna Leigh":35qww08k said:
I run our home 6K on propane generator on the first of each month for about 15 minutes. I had to field flash our old generator once several years ago before replacing it with this new one.
Click to expand...

Just curious - when I was researching this problem I found several different methods to field flash a dead generator. One of them was to plug a drill into the running gen. and spin it by hand in reverse. This didn't work on mine , although it might if you coupled it to a cordless drill to get it spinning faster. Which method did you use to field flash your generator, and did it work OK?

Bob, my first gen. was a Honda 3500watt(1984) it served me well for 18 years and was still running good when I traded it in on my first diesel. Although it probably never sat for more than a month or so, I never had this problem with it. It was however an electric start unit so it might make a difference- having a battery hooked up to it..?

Regards, Rob
 
Like THATAWAY I have a smaller 15KVA GENERAC water cooled that runs on natural gas. It does an automatic test from 0800 to 0820 every Thursday. It did come with a jacket/oil heater for winter starts.

I got the water cooled unit because it ran slower (1800rpm verses 3600rpm) and is quieter. It runs everything in the house, a/c, dryer, etc., all but the stove.

The longest run was for 8 days. Just as Charlie mentioned, with power loss it takes 15 sec to start, but the automatic return to utility power is seamless. There is a variable cool down period after the load is removed.

Interestingly when I decided to add this to my home the propane companies would not install the tanks on my lot. It seems that since I already had natural gas, they declined.

My gas meter was located in the rear of the house on my patio with the gas line running around the perimeter from the street. To move the meter to the side of the house where the generator was to be located was $300. To upgrade the regulator from 1/2 psi to 2psi was $300. This was needed for I have both a gas hot water heater and furnace. The point of this is when I did both the move and upgrade I was charged only the single fee of $300. It pays to ask the right questions.

I am well pleased with the peace of mind this has given us; especially as to how I respond to excess heat. I guess getting old is not for the faint, but I still feel it at times.

Art
 
On some of the older(1950-60's) naval aircraft I worked on as an electrician (S2f, C45, H34, AD1, T1A, T34, P2V, C118 etc.) I had to 'flash' the generator fields now and again to remagnetize the residual field magnets. This was usually on the 28VDC/300A and 100A generators. The batteries in the a/c usually did this thru control regulators, but in the test unit in our shop we had other means.

The residual magnetic field from the core only helped the field to build as the generator began to spin with some energy fed back to enhance the fields strength thru a (carbon pile) regulator. Once the field developed the 28VDC for the load the generator could be switched 'on-line'.

Boy this got me back to the "old days". Now I remember why I changed into the (flight) simulator computer field - to work in a more challenging technical field for me and it was air conditioned. I must admit I did enjoy the year+ as aircrew chasing underwater targets (subs) in a S2F (Grunman Hunter Killer).

Art
 
Art, thanks for the info. Although diesel is the prefered choice for a prime power unit I think your natural gas gen. is a good choice as a back-up unit in a residential area. You don't need a fuel tank - they need to be inspected every 10 years and must be installed to code for insurance requirements. No fuel to buy and hope it doesn't go bad before it gets used, etc.

Some municipalities won't even let you install a propane tank if there is natural gas within a certain radius. Propane does have more than twice the btu of natural gas but price wise it often equals out.

There are also regulations regarding how close different fuel types can be stored to each other. I have a propane tank, diesel tank and a wood pile. They must all be a certain distance from each other.

Some things I read re. field flashing say there are capacitors which store power to energize the field on restart and that over a period of time they will loose that stored power. As a auto mechanic I can understand that because I know a cars alternator will not produce power untill the field is energized with power via the ignition switch / voltage regulator.
Other things I read talk about a loss of magnetism - and needing to restore it. That is a little more confusing to me. :?

Regards, Rob
 
Bob;
The early generators had permanent magnets mounted on the frame of the generators casing. They were wound with wire to enhance the magnetic's field strength. Just like when you take a coil of wire , run some electrical power through it and watch a compass needle move around from its affect.

These permanent magnets could lose their strength with time or if they were hit hard with an object. The magnetic strength residing in the field magnet is referred to as residual magnetizim. It is like the match being held ready to start a fire.

Once the the pulley to the generator starts to rotate, this magnetic field passes thru a series of coils mounted on the rotor( the part that spins). This action causes electrical energy to flow thru the rotor coils and exit via a series of brushes that act as switches to pass that energy to an outside load. Part of that energy is fed back to the field windings around the magnets to strengthen the field and raise the power output available. This is where the generator's regulator comes into play; it limits the energy to the field as needed.

Alternators do not normally have these permanent magnets and have to have the field generated from the battery source initially, also to hold the field till the RPMs are high enough to sustain the field.

Conceivably, you don't need a battery to get power from a generator if the residual field is OK. It will build as I described before and generate an output. That is why you could remove a battery from the older cars that had generators while they were running and still have everything work - just don't let the RPM get too low for it would then quit.

Hope I didn't get too deep on this....

Art
 
We are dependent on propane for heat and for our back up generator. I pray for natural gas.
And I would replace diesel for natural gas in a heartbeat.
 
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