Ajax generator

[Motormowers]

Not sure if this is posted in the right spot but I was just given an Ajax generator made in 1983. Its 3000 watts 120/240. It runs but doesnt produce power. I cleaned the slip rings with fine emery then crocus cloth and nothing. I tried exciting the field with a 12 volt battery but all I get are some small sparks at the brushes when I do so. There is a small black thing that im not sure what it is or if its burned out. This is where im stumped. This thing looks like its had very little use and still has the original fuel filter and spark plug. I hope the pic comes through, if not email me and I can send it. Thanks in advance.

[BTPost]

The "Small Block Thing" is the Field Rectifier Diode Block, that is most likely
shorted out, or open circuited. You should test it with a Ohm Meter, and
determine if the diodes are still operational or not. These can be replaced,
and, or, substituted. If you remove the Block, and then use a battery to
energize the Field Winding, you should get a Flash of power on the Stator
wires.

[Motormowers]

Bruce thanks I will check it. Do you think its a rectifier though as it had slip rings and not a cummutator so I assume it produces AC current only? It looks like one wire off the brushes goes to an outlet and the other wire to that block which then comes out of it as two wires going to the field. Power then comes from the field in the black wires to the outlets. I also forgot to mention I tested the rotor with an ohm meter and it tested good at the slip rings and also to ground. Thanks.

[K D Redd]

There are two diode in that pack that rectify the AC to DC to enetgize the feild. The first one I ran into was toast. The way it was mounted suggested to me with my electronics background a full wave brigde. That is what I replcaed it with. The generator made power for about 30 sec. until the diode burn out again. I then measure how the winding were wired that powered the diode and saw I only needed two not FOUR. Wired in two and sent her home with the owner.

Kent

[Motormowers]

Thanks Kent, that makes sense. Any particular diodes I should get for that voltage?

[K D Redd]

I used a 6 amp 400 volt Radio Shack bridge and only used one-half of it. I did not connect the negative terminal to anything. In fact I cut it off close to the case. Why I used this bridge the first time was because the pair of diode was in a square pack with three terminals. This looked like a bridge with a screw mount in the middle. I assumed the screw mount in the middle was also the negative connection of the bridge. NOT This Radio Shack bridge could be mounted the same way. I do not think I need the 400 volt rating this was just what was on the shelf at RS.

Kent

[Fred M.]

Kent-

Sometimes 400 Volts may not be enough.

I don't know what this system's voltage is, but suppose you have 120 Volts each side of neutral being rectified. For 120 VAC, the peak is 170 Volts. When one diode is conducting, its output reaches plus 170 Volts. The non-conducting diode sees a minus 170 Volts peak added to the 170 Volts, or 340 Volts.

Allowing for ten percent high voltage, the diodes could be seeing 374 Volts. In this scenario, 400 is probably OK, but it is a good idea to do the calculations to be sure.

Fred

[Motormowers]

UPDATE: I tested the diode pack today and it ohmed out perfect. I tested the rotor again and it was perfect along with the stator. The only thing I could figure was the slight amount of corrosion where all the sections bolted together was not making a good ground. I tore it down and cleaned all the pieces and reassembled and still nothing after trying repeated times to excite the field with 12v at the brushes. After playing with it a few hours I came to the conclusion that there is an open curcuit somewhere and not worth repair. It was a freebie so the generator end came off and got tossed and the engine will get listed on Ebay as its almost new for the most part even though its a 1983. Thanks again for the help guys.