AC Generator Theroy and repair

Hi. There is plenty of info on a 3 phase system out there. But, basically it has 3 independent phase wiring 120degrees apart physically and electrically, hence the 3 phase name and 6 terminals for those 3 coils. The rotor is fed thru 2 slip rings a DC voltage (regulated in most cases) to generate a Steady magnetic field. That sets the output voltage of the three phases, but it does not participate in the power generation. That comes from the physical power supplied to the shaft or axle.

Now to the repair. First, disconnect all wires. Fire up the engine, to know, that at least that works and an idle rpm will be maintained. You will be able to measure a few 10 Volts AC on every AC coil output just from the remaining magnetism of the rotor. Lubricate everything.

Once successful, beg, borrow or .... a variable power supply of some strength, or batteries and power resistors (light bulbs, toasters, whatever) you can lay the hand temporarily to set and limit the DC current to the rotor. Start low and work your way up. You have to know, what is the maximum allowable DC current is. Or work your way up carefully, until no more output is produced, then back off 10-20% if it is too hot in the rotor. Measure the AC output. They better be more or less the same. Putting some crude loads on them you can see if they produce even currents.

Past that stage you are ready to produce real power with it and test it under real load conditions. If you can secure resistive loading, you even can measure something meaningful. But at this stage, real loads it was intended to supply will do fine too.

I cannot help you specifically at this stage, as how the regulation of a constant AC output voltage is achieved in your case (if at all), but that remainig problem is up to your ingenuity.

My point is, that such generators are not fussy sissies. Unless and until you overload them, they happily produce (some) power for you. So, just go ahead, and experiment.

A last note. These old devices are fussy about lubrication. If you run them at 70 -80% max., they ought to last then forever.

Thank you for all the information ! Yes we have about 8 Volts out on each phase. This sounds like we are not powering the rotor. This was our guess but finding out why is going to be tricky. The control panel is just a maze of wires, relays and power resistors. But now we know that the rotor is powered from the 24 Volt DC side for sure I'll start tracing it back.I'll let you know how we make out.

Thanks !

The 24 volt battery should be plenty to reestablish the residual magnetism if that is your problem.

Send me a private email and I'll try to walk you through some basics on the phone. If you are not 100% sure about things you can get an arc flash and burn your face off while working on this thing whilst running. It happens all the time so be careful.

Be prepared to tell me how many leads it has coming from inside the junction box. 12 leads, 6 leads or 4 leads or whatever. Pictures would be nice. Brushes or brushless. Approximate age, etc. Brand, model, etc.

G