standby generator problem

The generator is nearly new - only 48 hours show on the Hobbs meter. It had been working fine in standby service in my brother shop for several years.

The engine starts/runs fine, and spins at 3,650 RPM no load. Open circuit voltage (no loads connected) is about 28 VAC on both of the 120 VAC outlets, and 56 VAC on the 240 VAC outlets. If I plug in a single 300 watt resistive load (quartz floodlamp) into one of the 120 outlest the voltage increases to about 60 VAC. Adding additional resistive loads (more floodlamps and a couple of 1,500 waty hair dryers) causes output voltage to INCREASE with each load addition, peaking at about 88 volts. At this point RPM is about 3,630.

Unfortunately I do not have a suitable 240 VAC load and so cannot test with additional load.

I've checked all wiring between the generator head and the panel. Both stator windings both show good continuity - a couple tenths of an ohm. Both 60 amp breakers are closed and show 0 ohms across them.

Inside of the generator head is pristine - no evidence whatsoever of overheated windings, no melted/broken solder joints, etc.

The generator is brushless so there's no possibility of broken brushes, slip rings, etc. The rotor does however have a pair of diodes connected to the rotor windings and stud-mounted to a shaft-mounted heat sink (diodes spin with the armature). I can't get at the diodes to check them without removing the generator end bell and outboard bearing. However the diodes look fine and the solder connections are solid.

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At this point I am puzzled. Seems as though the generator is suffering from a low field current condtion, but that field current somehow increases with increasing load.

Any wisdom as to the nature of the problem and possible solutions is appreciated!!

Check all the diodes on the heat sink of the rotating exciter as one short will allow part of the energy to reflect back and if there is a capacitor to smooth the rectified waveform the resulting reverse flow to the capacitor will discharge it completly thus requiring another cycle to charge it again leaving a little energy left to develop our desired output at the sockets. Also polarised electrolytic capacitors do not like AC across the terminals and if high enough they will explode or vent from heat build-up. Diodes themselves being stud mounts show no indication of being shorted or damaged, they are still available through Digi-Key and other vendors. The capacitor is also available through Digi-Key and Im willing to bet its around 3300uF and 25 volts or greater, all said and done you will have under $40 in parts to repair and this includes shipping.
Digi-Key is at>Link

Good Luck
Brandon
Deere-R-Diesel

"We install 25 Amp diodes at 3200 Volts and therefore it is virtually impossible to destroy the diodes in a Gillette built generator. For this reason, WE WARRANTY OUR DIODES FOR LIFE."

"We use a 370 VAC capacitor which allows 33% more capacitor film and dielectric fluid insulator. This heavier duty capacitor can take the abuses of it's circuitry and for this reason, WE WARRANTY OUR CAPACITORS FOR LIFE."

So the problem has been narrowed down to a failed diode on the rotor or an out of tolerance capacitor. Next step is to isolate these parts and determine which has gone bad.

Thanks everyone for your input!

Just because it is brushless does not mean that it still has to be flashed. And there has to be a voltage regulator somewhere or the voltage would vary with speed. The rotor has a dc generator (using diodes) to drive the AC section of the rotor for the generator. There is a seperate exciter field in the stator that is controlled by the voltage regulator. The regulator may be buried somewhere buy I bet it is there.

As it is a "brushless" generator (field wiring is totally enclosed and isolated within the rotor) there are no field connections to flash. And since the unit does generate SOME power it appears the rotor has sufficient residual magnetism to cause the field to build at startup.

The unit has no internal voltage regulator - electronic or otherwise. Power wiring runs direct from the stator windings, out to a pair of circuit breakers then to several different syles of outlets in the front panel. Aside from a small flywheel alternator to charge the starting battery (completely independent from the main alternator) there are no other electrical/electronic components on the unit.

The wiring behind the panel has never been disturbed. (I was the first to remove the panel cover). In service load connection was accomplished via a cord and 4-pole twistlock plug to a transfer switch next to the shop's breaker panel.

JimN: I will check online for diagram/specs (thanks for the suggestion!).

Also lightning damage is a plausible cause. My brother purchased the generator AFTER a lightning strike destroyed the service drop to his shop (and nearly burned it down) a few years ago. But that not to say a more recent strike didn't somehow damage the generator.