Alternator/Drives Question

[richie00boy]

I have the opportunity to change my old cars alternator for one of the same physical size but increased current ability from a much newer vehicle. Obviously the increase in capability comes from better magnets and windings.

My question is that for a given current draw well within the range of both old and new alternators, will the new alternator actually be more of a load on my engine due to the stronger magnets? Or will it be more efficient and thus less of a load? Or even the same load and just has the ability to put out more current?

[OzMikeH]

Alternators dont have magnets, at least until you switch the ignition on...

Most likely about the same load.

It isn't enough power to worry about. Just get the bigger one if they cost the same or you plan to install some big amplifiers.
An alternator doesn't take much horsepower to run. They usually run a single belt, an airconditioner runs two belts on a large diameter pulley and still slips if it isnt very tight.
Drive along with fixed throttle position and switch A/C on and off to get a feel for it. Your alternator at full load would be much less than half of this.

If you're doing it for better performance there are about a thousand other things you can do that make more difference.

[richie00boy]

Thanks for the response.

My car does not have aircon (I did say it was old!). I have a choice of alternators to use, performance is important and I don't have a big stereo. However, I am planning on fitting a fully active system that will have 5 amp channels.

[theAnonymous1]

A 150A high current alternator is capable of roughly 2kW.

2kW = ~2.7HP

The efficiency of an automotive alternator is something like 50-60%, so at full output power the alternator is using somewhere around 5-6HP.

Since the alternator is only capable of a percentage of its output power at idle the load on the engine will be even less, maybe 1-2HP.

Nothing to really worry about.

[richie00boy]

It's not idle that I'm particularly bothered about.

I'm just really trying to establish if the newer higher current alternator will be more efficient in it's engine power requirements for a given load. I appreciate the engine load is nothing to worry about, but I'd just like to know.

[dangus]

I have heard that some high-output alternators don't get going until high RPMs, and may put out lower amps at idle. That could mean lower efficiency at idle, if the field current is the same.

Whether that's true or not, having an accurate digital voltmeter on the dash will tell you a lot about what's happening, and possibly help you avoid standing on the shoulder waving jumper cables at passing cars.

[Richard Ellis]

Watch the ratio drive on the pully set-up...you'll notice they are considerably overdriven......A lower ratio ( Slower.... RPMs for Alt.) helps immensely, freeing up more horsepower.
___________________________________________Rick... ..........

[AndrewT]

Hi,
the load that the alternator puts on the drive belt is ~friction+output.
The friction at any particular speed stays much the same.
The load changes with output. As the demand from the battery charging and ancillaries changes, the alternator load will change.

A bigger alternator is capable of putting a bigger load on the drive belt if the demand is there.

High demand could lead to drive belt slip.