Manufacturers might change their wiring at any time, consider this an example only.
Update! April 2002, Some manufacturers have changed their wiring! Newer heads have what they think is a handy terminal strip on the side of the dog house.
The first step in trouble shooting is to assume nothing. Verify that your head is indeed wired as above. If you find it is wired different, you might consider making a note of how it is wired now and then wire it as above for the purpose of testing. Note: if you have a center tapped head, it will be just like the drawing above.
There is a piece of test equipment called a 'megger', it's nice to have for testing electrical appliances. The one I had experience with in the NAVY had a hand crank, you'd hook it to the windings and then to the case or the ground terminal on a gen set and crank it up. This performed a high voltage test between the windings and ground. If the insulation was failing or breaking down, you'd see it. If you can get your hands on such a tester, do so , if not use the good ole multi meter like most of us, and proceed.
Use these test procedures at your own risk, this information is for educational purposes only! We'll assume your generator head is dry and clean and that you have grounded the head to earth ground to protect yourself. We'll also assume that it's hooked to the engine and that you can spin it up to 1800 RPMs. We also must assume that you have a good idea where 1800 RPMs is, or that you have a device like a mechanical tachometer to verify that the speed is correct. (It's a lot of assumptions!) If all this is true, proceed to step one. Remember! the following readings were taken off the 10KW head.
1. Remove the slip ring cover, lift both sets of brushes off the slip rings, a little messing around and you'll see they stay up on their own when lifted up far enough. Make a tool out of a paper clip to pull the spring back over the top of the brush when you wish to seat them again.
Place your ohm meter across the slip rings and take a reading on the R times 1 scale, you should see around 17 ohms. If true, go to step2. If you find a different resistance, the field is suspect, consider pulling the rotor, you'll find four windings in series, inspect the solder joints, and connections to the slip rings. Open the individual windings and compare them using the ohm meter. Check the field to the shaft, you should read infinite (OPEN). Once you have corrected problems here or have proven it good, go to step two.
2. Place the meter on the ohms times one scale, and place it across the Z1 and Z2 leads when they are removed from the diode bridge. g. The reading should be around 5 ohms, if true go to step 3. If not, Investigate the harmonic winding in the stator, and replace it if bad.
3. connect the volt meter across the Z1 and Z2 leads. Set the meter for AC volts, 100 volt scale. Start the engine and bring the set to 1800 RPMs. You should read approx 53 volts AC, if true go to step 4. If your reading was different, verify you are turning 1800 RPMs, if true, investigate stop the unit, take a 12 volt battery and FLASH the field with the brushes lifted on one ring, put the brushes back on the rings and repeat step three.
4. With the set stopped, re connect the Z1 and Z2 leads to the diode bridge. Start the engine and verify the set is running at 1800 RPMs connect your meter to the two screws that normally connect the two leads going to the slip rings, and note the DC voltage. It should be around 69 volts DC. If this reading is good go to step 5. If it is off, replace the diode bridge and retest.
5. If you got this far, you've proved you have a correct voltage from the Z winding, that it is being converted to DC by the diode bridge, and that you have a proper resistance in the field winding. If you have an amp meter that can accurately measure 2 amps AC , place the meter in series between the lead Z1 and the diode bridge terminal screw. Check all the brushes, slip rings and connections. Start the set and run it up to 1800 RPMs. The meter should read approx 2.1 amps if true, go to step 6. If you're reading was different, verify your meter, the RPMs, and the field excitation circuit, also look at the diode bridge for a flaw. read out the diodes with the ohm meter and prove it good.
6. You've proved that everything is working on the excitation side. The only thing left is the output side. Continue to step 7.
7. Verify that all test leads are removed and all the leads in the field excitation circuit are tight. Set the ohm meter on the R times one scale, place one test lead on the U1 terminal and the other on U2, you should see a very low resistance, .02 thru .06 ohms. If not verify that the strap is in between U3 and U4. Using the ohm meter, measure between u1 and ground, the reading should be infinite, if true go to step 8. If there is any reading other than infinite (open) to ground look at both windings further, replace if necessary.
7.5 NOTE: It doesn't matter which slip ring the plus and minus side of the rectifier are connected to, as long as you flash it in the proper direction, use a 12 volt battery, and 'flash' the field according to the plus and minus markings of the rectifier. The smart DIYer will make one of the brush rigs with a plus for quick reference later. If you take things apart and swap the leads, you only need flash the field accordingly! Some have found the output of the rectifier opposite of the drawing provided with their generator, or opposite of above, this is not a problem, as long as you note the polarity before you attempt to flash, and as long as you know that rolling these leads will kill the excitation voltage till you flash again!
8. Place your voltmeter across leads U1 and U2, set on a scale to read 230 volts AC. Start the engine and set at 1800 RPMs, if you're reading is not close to 230 volts AC, you most likely have a problem with your RPMs, your meter, or your U windings or associated leads. Get a MEGGER and test all the windings with it. If you got here and didn't clear your problem, you are one unlucky person.
Please report any errors or omissions you might find in this text.
05/2006 note: To date, the leading troubles in the ST design are noisy bearings, and rectifier failures. The old Chinese designed rectifier appears to be a first generation solid state rectifier that immediately followed the ancient and bulky selenium design, this first generation unit is air cooled, made of too many parts screwed together and far more prone to failure. Utterpower is consulting on this, and hopefully PS heads will have the better rectifiers stock, until then, there is a retrofit kit called the ST-Mod kit that eliminates the dog house all together and provides a far better rectifier.
As for bearings, leading causes of bearing failures is the assembler used cheap bearings, attempted to ship the bearings with little or no grease, or packed them in Yak fat instead of a proper grease. There are other problems, some assemblers will cheat the importer out of a shielded bearing on the fan side, this means the bearing and grease is open to pick up all the grit and dirt blown into it by the fan, and soon, you'll have a grinding compound in there working to take that bearing out in record time.
Regardless of what the importer ordered, you will only know it's right if you check. Don't have time to check? buy something else.
These comments are are about all Chinese ST heads,