After a period of working OK Lister’s output voltage began to sag horribly under load. The selenium rectifiers are known to be unreliable so I decided to replace them.
The below schematic shows how the Brush 3kVA alternator is wired (Brush connection diagram 9840322)
The generator has both shunt and series fields. A silicon bridge rectifier provides excitation for the shunt field while the series fields are energised by a portion of the load current, through the selenium rectifiers. The armature and series fields are in two identical sections that can be switched in parallel for 115V or in series for 230.
Note that in 230V mode the output is centre tapped to earth. Both live and neutral pins of the outlet have 120V on them. This seems to be a design decision by Brush to reduce the risk of electric shock, however it means that live and neutral both need to be fused.
I took the opportunity to do some 4 wire resistance measurements while the wiring was disconnected. One section had a series field resistance of 0.20 ohms and a diverter resistance of 1.00 ohms. The other had a field DCR of 0.21 and the diverter resistor was set to 0.87. This resistor was burnt from a previous short circuit so I cleaned it and reset to 0.96 ohms using the unburnt end.
I prepared two KBPC3506 silicon bridge rectifiers on a heatsink. These are an inexpensive 35A 600V part available from many distributors.
The new rectifiers are much smaller so I was able to get rid of a lot of wiring, and that sketchy looking woven tube that was probably asbestos.
Of course I saved the original rectifiers, lol no, they went straight in the toxic waste.
I am working on a Lister/Brush ST2 single phase 230v 7kva generator.
It has similar set up for compensating for loading as your generator
Loaded by a 2k2 kw borehole pump motor the output drops to 220v. from 238v
I’ve replaced the selenium bridge rectifier with a 70A 1200v replacement and replaced the old rheostat with an Ohmite 12ohm variable.
Even at minimum diverter current there seems to be little voltage compensation.
I’m interested to try out your AVR.
I’m puzzling over the peak clipping resistor and how this connected within the generator.
Hi Steve, you might notice the rating plate on the generator specifies a power factor of 1, that is because the series field compensation only works properly for resistive loads. With an inductive load like a well pump motor, it undercompensates and the voltage drops under load. You might get better regulation by adding power factor correction capacitors in parallel with your motor.
The peak clipping resistor in my AVR design is a new separate resistor that I added, it’s not part of the original generator circuit. You can see it in the perforated metal box in my photos of the AVR breadboard. I’ve since found out that the waveform of these generators naturally gets more peaky under load, and inductive loads only make the effect worse. So again another reason to try power factor correction.