What we're calling an alternator here is just a certain type of generator. In the UK they are all just "dynamos."
The old "generators" on tractors made AC just like the modern day "alternators." The difference is - the old "generators" use brush placement to select just a fraction of the electrical pulses made and send it through the brushes as DC current. Much of the power generator is wasted. Thus the reason why they are big for the low output the make.
A modern day "alternator" type of generator uses all the AC it makes and sends it through full-wave rectifiers so it's all converted into DC. Also the brushes do not carry any of the output power.
The old cars that did not run when the battery got unhooked did so because of the way the mechanical regulators were wired up. It had nothing to do with the old brushed generator itself. The old brushed generators were better able to make DC power all on their own due to residual magnetism. The newer tech "alternators" mostly rely on 12 volts of "exciter" current fed to them to create enough field magnetism to charge. That being said - alternators also have a bit of residual magnetism and some can self-start just like the old generators if spun fast enough.
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Today's Featured Article - Oil Bath Air Filters - by Chris Pratt. Some of us grew up thinking that an air filter was a paper thing that allowed air to pass while trapping dirt particles of a particles of a certain size. What a surprise to open up your first old tractor's air filter case and find a can that appears to be filled with the scrap metal swept from around a machine shop metal lathe. To top that off, you have a cup with oil in it ("why would you want to lubricate your carburetor?"). On closer examination (and some reading in a AC D-14 service manual), I found out that this is a pretty ingenious method of cleaning the air in the tractor's intake tract.
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