Covered Topics

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Saturday, September 12, 2015

Portable Generator Repair

Some friends of mine own this Homelite LR4400 portable generator, seen in the somewhat blurry cell phone pictures herein. It was stored in a garage for several years and they asked me to check it out and do whatever repairs were needed. Using some fresh gasoline and some spray ether (starting fluid), I managed to get the engine to start and run. The main problem, besides a somewhat dirty carburetor, is there is no power output.

With considerable effort, I found a PDF of the service manual with electrical schematic online. The manual contains a complete test procedure as well as coil winding resistance specs, disassembly instructions, etc. With that, I proceeded to take the end cover off the machine and check the electrical components. In the photo below one can see the rotor, stator coils and some of the wiring.

A Quick Word About Generators In General

All generators have what's called a field coil (a stator) and a moving coil (rotor). The rotor spins inside the stator. The stator often consists of 2 or more coils - one goes to the electrical outlets located on the end cap of the machine; the other serves as an "excitation winding" which feeds power to the rotor via a pair of carbon brushes which ride on copper slip rings. As power is fed to the rotor, it becomes magnetized. As the magnetized rotor is spun by the engine, it induces electrical current into the main stator winding, which powers the above-mentioned electrical outlets. In order to control the output of the generator under varying load conditions, either an electronic regulator or other circuitry must modulate the flow of power from the excitation winding to the rotor. There is usually a rectifier consisting of 2 or 4 diodes to convert the AC supplied by the excitation winding to DC to feed the rotor windings. Many generators also have a capacitor connected to the output of this rectifier to filter, or smooth, the rectified DC and thus provide the rotor with a "cleaner" signal. You may be thinking "Well, that's all great, Karl. Why are you telling me all this?" Quite simply - MANY electrical issues with portable generators are traceable to rectifier diodes, capacitors, or electronic regulator boards that have failed. Find and fix that problem and voila - your generator is back up and working again.

The photo on the left shows the parts mentioned above. Notice I am holding the filter capacitor in my hand - this component had an internal short circuit, which effectively prevented the rotor from getting any of the excitation current. With no current getting to the rotor coils, there is insufficient magnetic field to produce any current in the stator winding, and thus no power to deliver at the outlet.

I tested the rotor coils, stator coils, rectifier and filter capacitor with a Klein brand Model MM200 multimeter purchased from a big box home improvement store for $49. All components except for the capacitor tested within acceptable range.

A new capacitor is on order - we'll see what happens when I get it installed.

Some Things To Consider

If you are keeping a portable generator for any kind of grid-down emergency, extended power outages, etc., it might be a good idea to keep a repair manual and the spare proprietary electrical parts on hand for it. Since most small engines built after the 1970s have a solid state ignition, one should keep spares of those parts, too. Even if YOU can't or don't intend to do the repairs yourself, if you have a shop manual and parts, SOMEONE ELSE can for you.

Generators should be periodically run and tested under load. An emergency or an important job is NOT the time or place to discover that the capacitor in your generator has deteriorated in storage and failed. Also, many generators rely upon weak residual magnetism in the rotor's laminated iron core to trigger the excitation winding - this residual magnetism fades over time with non use. If this process goes too far, a repair technician must "flash" the system using a battery to renew the magnetic field. This is NOT something you want to be doing in an emergency - and different generators have different procedures for doing it. Do it wrong, and you may ruin your generator.

After use and BEFORE storing a generator, the fuel should be run out of the engine/tank/carburetor. If the gas tank doesn't have a drain cock, run the thing until it runs dry. Gasoline deteriorates and rapidly becomes such that small engines won't run on it. Also, gasoline as it deteriorates forms a gummy substance that will clog the carburetor's fuel and air passages. Once this happens, the only sure way to resolve this is to disassemble and clean the carburetor. So, as I'm running my gas tank dry, I go one step farther: Once the engine starts to sputter due to lack of fuel, I squirt some starting ether (spray carburetor cleaner works, too) into the air intake, which will enrich the fuel/air mixture and cause the engine to speed back up. Doing this a few times for a couple minutes will ensure that the carburetor float bowl and all its internal fuel passages are thoroughly cleaned and purged of gas.

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