3 Phase Bridge Rectifier Specification with Alternator Question

tcmtech said:

A typical automotive alternator is a 12 pole unit so if you know you're working RPM's you can figure out the base frequency from there. At 3600 RPM you would have a 720 Hz base frequency which if its a commercial unit rated for continuous duty would be rated at 3600 RPM Vs the typical 6000+ RPM that most automotive units are.

Now as for the rest of the system and components there are some common misconceptions regarding rectifiers three phase power and general wiring.

First of any decent rectifier is rated for average amps which means that you don't need a huge amperage rating for rectifying three phase power. To be honest the diodes in your original alternator despite it being a 120 amp rated unit were likely not rated for much over 40 - 50 amps each tops. I have never found any alternators that used huge current rated diodes for their relevant amperage ratings.
Reason being for practical purposes to get 120 amps DC out of three phase AC they average current per phase would be about 34 amps which for our application a 50 amp three phase bridge would be plenty enough.

Next is realistic overhead voltage rating which for a 12 volt systems any diode with a 50 or more volt rating is way more than you need so the 1600 volt rating is just wasting power due to it inherent higher forward voltage drop which is likely around 1 volt Vs what a common germanium type diode which was commonly used due to it low forward voltage drop of ~ .3 - .4 volts at full load hence how they get so much power through a bridge rectifier with such a small heat sink even with air cooling.

So given that, by realistic and past personal experimenting with modified alternators of all makes and ages your alternator using 6 Ga wire for each phase should be able to supply a good 100+ amps at the 12 - 15 volt range it's rated to work at a good 50+ feet away! Odds are if it was a quality commercial unit that has a fair amount of reserve capacity that could be used to compensate for a bit of voltage drop in the lines even 10 ga wire for a 50 foot run would be more than big enough.

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Puzzeled said:

As you can see I did a frequency calc above but with half the result of yours so I might have made a mistake, noting my RPM's are rotor, not engine, whereas yours might have been gas engine revs with a 2:1 pulley ratio allowed?

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Puzzeled said:

However your comment that it's 1600v rating is just wasting power concerns me. Should I be looking for say a 50v spec, if indeed such a thing is available?

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tcmtech said:

It's at 3600 RPM alternator speed. A standard 2 pole alternating current generator puts out 60 HZ so a 12 pole at that RPM would be be 6 times that at 720 HZ.

I agree with your standard 2 pole at 3600 RPM putting out is 60Hz OR using f = PN/120 or 2 x 3600/120 = 60Hz

Same equation for 12 pole at 3600 RPM is 12 X 3600/120 = 360Hz or 6 X 60Hz. However that is NOT 12 X 60Hz = 720Hz as you suggest. Maybe a 12 should be 6 and typo slip up your end or am I missing something?

If power loss in the rectifiers is an issue you would want a Germanium type which has a low forward drop. Voltage rating is less critical other than being somewhat higher than the maximum voltage the unit can put out which with most automotive units a 50 - 100 volt Germanium diode is a common item in them. Sorry I didn't say power loss in rectifiers was as issue, you said it would be by saying in your post above, quote "so the 1600 volt rating is just wasting power due to it inherent higher forward voltage drop" All rectifiers generate power loss, so why is a 1600 v rated one practically any different to say a 50v one for this application?

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Puzzeled said:

Should I be looking for a lessor voltage spec, say a 50v instead of 1600v, to mitigate the power loss attached to 1600v (that you indicate will occur), may or may not be of significant in terms of this application, and if significant, is such a thing commercially and readily available?

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tcmtech said:

It depends on the individual diodes forward drop voltages and whether or not you can live with the power losses.

Most high voltage diodes have a ~ 1 volt forward drop and being there will always be two in series in conduction at any time that ~ 2 volt drop at 100 amps is a fair amount of lost power. ~200 watts.

With low forward drop Germanium diodes with a .3 - .4 volt forward drop that's only .6 - .8 volts or ~ 60 - 80 watts loss.

If your heat sinks can handle 200 watts of thermal load and the power sources can handle the added wasted energy being lost then yes, they 1600 volt rectifiers are fine.

Mostly this just points out what happens when time is not taken to properly research and design a system. You end up wasting more money on component capacity overkill that serves no purpose while reducing the overall system efficiency for it.

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