3 phase bridge rectifier

[mark forrester]

Evening all,

Firstly let me introduce myself abit. My trade is High voltage substations so i do abit of mechanical and Low voltage maintenance but more mechanically biased.

My hobby is using my car on track days (Honda S2000) but in the past 12 months i have managed to cook 2 alternators and each time its been the rectifier built into the alternator thats packed in. The engine bay gets hot and the alternator is sitting at about 90c using a infra red thermometer. A new rectifier is £180

Now i have seen in the USA they manufacturer a external rectifier kit that eases these heat issues

http://alternatorparts.com/quicktifier-external-bridge-rectifier.html

Its quite simple in design and i understand how it works. From what i have read thou is that these kits are cheap to produce and inside you basically have a 3 phase bridge rectifier with 6 diodes converting the AC into DC.

If i was to buy something like the link below would it do the same job?

http://uk.rs-online.com/web/p/bridge-rectifiers/0192528/

Obviously there are a few considerations

1- Fan cooled to increase its lifespan and thats not an issue i can make a box to place it in and use a 12v fan to force air through
2- Car alternators generate AC in various frequencies. Would this sort of diode rectifier be able to handle those frequencies? I dont want to fit it and then find that it burns out after a couple of days.
3. I would still use the existing alternator voltage regulator so if i understand it correctly the regulator will still control the voltage regardless of what the diode rectifier is doing. ( from what i understand when the regulator senses the correct voltage it collapses the alternators field and it stops generating )

Thanks for any advice you can provide.

Like i said im more a hammers and spanners type guy but i have a fair understanding of 12v circuits and just needs some pointing in the right direction regarding diode choice etc.

 

[(*steve*)]

Those devices certainly have a generous voltage rating. Assuming that their current rating well exceeds your maximum requirements then you are left with temperature, switching speed and voltage drop.

I'm not certain what the maximum frequency they would face, but the datasheet will contain information to enable you to determine how fast the diodes switch.

The next issue is temperature. You need to consider both the ambient temperature and the heat generated internally to calculate if the junction temperature stays within spec.

The last issue is the voltage drop. This is a major contributor to heating issues as well as representing the efficiency of the device.

If you can do in the blanks with regard to frequency and maximum current then we can take a closer look at the datasheets and answer some of these questions for you.

 

[mark forrester]

The standard alternator outputs 105 amps but im replacing it with a heavy duty item thats designed to output 220amps. I think that might be on the optimistic side of the manufacturers spec and i cant think of a time when its going to output anywhere near 220 amps but i wanted abit of redundancy built into it. The main reason i have it is that after a long session on track i tend to stick the cooling fans on and this current draw seems to cause the alternator diodes to burn out. I have measured it and its only 30amps draw. The electric power steering on its own can draw 80 amps so i think after a long session the alternator is pretty much red hot just from the current required as well as the engine bay heat.

The data sheet for the diodes i linked too is here

http://docs-europe.electrocomponents.com/webdocs/0af0/0900766b80af04fd.pdf

I dont really understand it to be honest.I also cant see any specs regarding frequency.I did work out that if the crank spins at 1000rpm then the alternator is at 2200rpm so with a 9000 rpm redline the alternator is spinning at 19800rpm. I wouldnt know how to calculate that in AC frequency thou.

Im tempted to maybe go for a 300amp version and bin the whole alternator rectifier as it doesn't look too difficult to do. I would just have to ensure i connect the voltage regulator up correctly so that it can still do its job.

 

[(*steve*)]

Those rectifiers are designed for mains rectification (400Hz is the most you'll normally face) and are probably not going to function well at 20kHz (approx frequency at red line).