Re: bench test a generator 6v


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Posted by D. Sherman on Sunday, January 27, 2008 at 22:17:53 :

In Reply to: bench test a generator 6v posted by Dave in N Vt on Sunday, January 27, 2008 at 20:41:41 :

An old-fashioned generator, if it's been polarized, should build up voltage without being connected to a battery. An alternator won't, because the diodes don't conduct until it gets a certain amount of voltage going, which it won't do without a battery to power the field. The harder problem is to rig up a way to spin it, meters to measure the output voltage and current, and a load bank to load it down.

For the mechanical end of it, I've been using my lathe as a bench test set up. I chucked an old crankshaft pulley in the lathe chuck, and welded up a very simple bracket to hold the alternator to the tool post. With this method, the mounting is very solid and it's easy to get the pulleys lined up and the belt properly tensioned. The lathe also lets me try various speeds. I'm using a strobotach to measure the speed of the alternator pulley, but you could easily just go by the ratio of pulley sizes and the lathe RPM off its speed plate.

The trouble is that this can all get fairly elaborate fairly quickly. For practical purposes, it's usually enough to test a generator or alternator while it's in the vehicle by hooking a volt-meter across the battery and monitoring it while increasing the engine speed. At idle, you'd expect about 6.3 volts across a charged 6 volt battery. As you rev up the engine, that should go up to 7 to 7.2 volts above 1000 RPM or so. The details are less important than determining that the voltage does indeed rise when you rev up the engine. If you're not sure about a mechanical regulator, you can finesse it by taking off the cover and pulling up the armature of the voltage and current coils by hand to force full voltage onto the field. That's if it's a 3-coil regulator. If it's a 2-coil version, there's only one combined current/voltage coil to futz with. If that brings the battery voltage up, then the generator is good and the regulator is suspect. Sometimes that will actually fix the problem, if a rig has sat unused for a long time and the regulator contacts got corroded.

Another way to determine if it's the regulator or the generator is to simply jump from the ungrounded terminal of the battery to the generator field terminal while keeping the engine RPM up. If that brings up the battery voltage to 7+ volts, the generator is good. Briefly applying full voltage to the field like this won't hurt an old-fashioned generator or an electromechanical regulator.

If you determine the generator isn't generating, even with full voltage on the field, the most likely problem is worn out brushes, which you can replace yourself or have a good auto electric shop do. If the brushes are good, the problem is more serious, probably shorted or open turns in the windings, at which point the generator is probably junk unless it's valuable enough to have it rewound. There are shops that still do this, but not many and they're not cheap. If you do decide to work on the innards yourself, don't go overboard with sanding the commutator. If it doesn't have scratches deep enough to catch a fingernail, a little polishing with crocus cloth is enough. If it's badly pitted, it's usually a sign of arcing caused by shorted windings, so merely smoothing out the commutator won't solve the problem.



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