I am trying to repair a 5.5 KW portable generator for a friend. It has only 8 hours running time since new. The problem is in the exciter circuit.
The exciter winding feeds a bridge rectifier circuit that is made up with 8 discrete 2.5 amp 1000 piv diodes. The diodes are paralleled which is the reason for it using eight. There is also a 460 mfd. 200 wvdc. capacitor.
The field has a resistance of approx. 40 ohms and when flashing it with a 12 volt battery it draws about 250 ma. However, when running the current draw starts at approx. 6.5 amps cold and goes up to 9.5~10 amps as the temperature rises. The diodes get hot enough to melt the solder and fall off the circuit board or will short out sending AC to the capacitor causing it to swell up and burn open.
I spent a lot of time rigging up a 35 amp 100 piv. rectifier module and 2 1000 mfd 200wvdc capacitors wired in series to half the capacitance and double the working voltage. The circuit was placed in an enclosure with a heat sink in an attempt to keep it cool. I wired it to the circuit board that previously held the discrete components and brought it out so I could monitor the operation. After about 20 minutes of operating at approx. 15% load the output failed. This failure was due to the AC input connector on the rectifier board melting the solder.
I have worked with electrical equipment, both AC and DC including HF, VHF and UHF, for over 50 years but this problem has me stumped. Anyone have any ideas?
Joe