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| Not true. Until about 1960 the SAE standard for vehicle and tractor starting and electrical systems did not specify voltage or polarity. Fact is either polarity works with a generator, ignition coil is a bit picky about polarity and the ammeter fully picky about polarity. In the 30s some automotive engineers thought positive ground was better about erosion at connections to the frame and others thought the opposite, so both were used. Actually if direction of current a ground connections was a problem the different polarities would move the corrosion between load connections and the main battery ground cable connection. There still would be corrosion. Some of the arguments came from corrosion in DC trolley systems of underground water and gas pipes. And whether a particular ground connection corroded a pipe depended on whether the pipe was near the power substation or distant from the power substation.
Radios in the old days were sometimes polarity sensitive, but some simply (like Motorola, GE, and RCA two way) had a vibrator that could be turned around to adjust for polarity. That was tube radios. Reversed polarity kept the radio from working but didn't damage it. Solid state radios have no tolerance for reversed polarity, because it causes instant destruction. It is possible to protect but few makers have ever bothered. So about 1955 the SAE standard took out 6 volts as an option leaving 12 and 24 volts, but still didn't specify a ground. About 1960 that standard was modified to specify only negative ground with preference for 12 volts. A few years later it was modified again to only allow 24 volts for situations where 12 volts wouldn't crank a big engine.
Generators will charge with either polarity once flashed to set the building polarity. Alternators generally have to be rewired. I have a big Leece-Neville that can be wired for either polarity. Most Delco were negative ground though a manual I have shows positive ground alternators were a possibility. Just depends on the connections to the diodes inside the alternator. But the alternator is also as intolerant of reversed voltage as the solid state radio.
Then there were JD 24 volt diesel tractors in the 50s and 60 that actually had one each 12 volt positive and negative ground system with the charger and starter using the 24 volts, but all the other circuits for gauges and lights were 12 volts. Had to balance the positive and negative circuits to keep the battery charging even. For neophytes to electricity, these are difficult to understand, but not really different from the first 110/220 DC circuits used by the Edison Electric company in the 1890s.
Gerald J. | |
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Positive or negative ground? How to tell?
