Alternator for racing?

[2013cossj]

I need to buy a racing alternator and would like input on how many amps I need. Do alternators that output more amps take more HP to drive? My car is a typical drag only car with elect. fan, water pump, fuel pump, msd, etc. It is a class car so I am concerned with not loosing HP from installing it. Any input would be appreciated.

[BCjohnny]

I need to buy a racing alternator and would like input on how many amps I need

You need to calculate up all you running loads and pick one about +50% higher output

Do alternators that output more amps take more HP to drive?

Yes, from my own testbed observations, and assuming normal efficiencies, each 30 amps outputted consumes roughly about 1 hp to drive @ 14.4v

You'll likely get any hp 'losses' made up, or bettered, by improved ignition output

My car is a typical drag only car with elect. fan, water pump, fuel pump, msd, etc. It is a class car so I am concerned with not loosing HP from installing it

Again, as in reply one above, tot up your amp requirement, add about 50% margin, and calculate the alternator output needed

The alternator will only output the amperage demanded, a 50 amp alternator will only output 30 amps if required, so going slightly bigger is fine assuming the MOI of the rotor isn't vastly greater (but that's splitting hairs, really)

There are lots of small Denso copies about, 40-50 amp ...... the OE application for these being mostly small Kubota two & three cyl diesels

Get a genuine Denso one and be happy, small & easy to mount, pulleys are interchangeable to keep the rpm from centrifuging the winding

denso_alt.jpg

[golden_commando]

An alternator has the efficiency around 50%, so 30amps times 14v divided by .50 is 840watts thats close to the 1hp mentioned above, it can vary a ittle with temperature as well as belt tension and efficiency but it is a good estimative

[BCjohnny]

Typically alternators have an efficiency of 40% - 45% over much of their operating range ...... they generally get less efficient as rotational speed rises

The '1hp per 30 amps' rule of thumb is simply that, a 'worst case scenario' ...... it's actually in practice a little higher

My test bench has a 3hp motor and the majority of tested 14v alternators would begin to stall the prime mover at around 100-105 amps loaded @ circa 8k rpm, a speed where most alternators are down at 40% efficiency, hot

So it's a direct reading, if a little clumsy, not an extrapolated one

You might reasonably expect 35-40 amps under better circumstances

[LotusElise]

Typically alternators have an efficiency of 40% - 45% over much of their operating range ...... they generally get less efficient as rotational speed rises

I've worked in alternator projects at an well known Stuttgart located company and from that I would always overdesign it because the efficiency declines hard with shaft speed and load, if rotational inertia isn't the primary goal of your application. Peak efficiency is around 55-70 % on 16 V-Basis depending on the design, the efficiency at 70 % of max allowed shaft speed way below 45 %, talking about 30-35 % more likely.

[BCjohnny]

I've only ever worked for myself ...... self taught too ....... but nice to see our figures are comparable

What I should have also mentioned was that 'hot' output on a test bench, mostly sucking in ambient air, isn't the same as 'stinking hot' inside an engine bay, where efficiencies suffer further .......

So I'm happy with '1hp per 30 amps' generally as a rule of thumb

[BCjohnny]

Just dug this up for anyone who might be interested ......

Alternator_efficiency_curve.png

[LotusElise]

Just dug this up for anyone who might be interested ......

Thanks for sharing. What alternator is this specifically? I am really curious now...

I was thinking of a 1.5:1 belt ratio in a 10,000 rpm engine of an alternator with 18,000 rpm maximum shaft speed. At 15,000 rpm the efficiency was around 30-35 % typically. Normally the alternator gear ratios are specified for idle demand at full electrical activity, therefore a 1:1 ratio is not typical. One may need to adapt this.

[BCjohnny]

TBH I don't know, just an efficiency graph I pulled off the net, all mine are on dead trees, so it was easier

At a punt I'd say maybe an old ext fan type, the characteristics look similar to a Bosch N1, either 14v or 28v, but maybe not, most curves are much of a muchness, unless you're squeezing a quart out of a pint pot

Speccing an alternator for a road application vs 'race' are two different things, not confined just to pulley ratio ....... but as you can see the 'sweet spot' in the curves needs to be 'geared' to the rpm range you expect to use, at the very least

[This thread got way more involved than I thought it might, which I guess is no bad thing ....... at least there's some commonality of opinion, which for ST is kinda rare, lol]

[LotusElise]

At a punt I'd say maybe an old ext fan type, the characteristics look similar to a Bosch N1, either 14v or 28v, but maybe not, most curves are much of a muchness, unless you're squeezing a quart out of a pint pot

I can't say by heart, I would have to check in the SAP data base. I was developing an alternator for a CV application on 28 V for harshest environment of an construction site machine for up to 110 °C, very aggressive (because edgy) dust with only best components regarding bearings, dust filtering housing, but still top shaft speed of 18000 rpm and 6000 h lifetime requirement.

Speccing an alternator for a road application vs 'race' are two different things, not confined just to pulley ratio ....... but as you can see the 'sweet spot' in the curves needs to be 'geared' to the rpm range you expect to use, at the very least

Yeap, application depended pully ratio is one point on the list to applicate an alternator right. A race engine is pretty tough stuff on the shaft speed bandwidth and efficiency field requirements. For my 10,500 rpm race engine I have to reduce the pully ratio pretty much to get into a better efficiency field but to have still to have enough low speed power supply. This is also a system application of battery, aimed voltage control over engine speed and shaft speed bandwidth. But also the forces on that up to 2 kg rotator need to be accounted, any speed change create nutition forces which pull hard on the mount base. This need to be addressed on custom brackets.

[BCjohnny]

I was developing an alternator for a CV application on 28 V for harshest environment of an construction site machine for up to 110 °C, very aggressive (because edgy) dust with only best components regarding bearings, dust filtering housing, but still top shaft speed of 18000 rpm and 6000 h lifetime requirement

Back in the early/mid nineties when I was designing a lot of high powered geared starter motor stuff I was asked ..... by a very 'promising customer' ..... if I knew anything about alternators ......

It was a 'Paris Dakar' type deal, and the chief project engineer hadn't been particularly impressed with what 'their people' had come up with

Having been given a vague overview of what they'd arrived at, I ended up in a very different place, however when it quickly became clear it was just a 'brain picking exercise' I left it alone, but as that sort of nonsense interested me at the time I came up with some solid conclusions :

The electronics/solid state couldn't be mounted on the machine, slip rings were a no-no, and air cooling was a bust ...... and it wasn't as unaffordable as it sounds, especially when compared to the cost of failure

[LotusElise]

Back in the early/mid nineties when I was designing a lot of high powered geared starter motor stuff I was asked ..... by a very 'promising customer' ..... if I knew anything about alternators ......

Very nice. Which company?

[BCjohnny]

PM'd

[LotusElise]

Thanks

[BOOT]

There are basic amp calculators, here is one but I've seen/used others https://www.qualitypowerauto.com/pages/ ... ulator.php