How to calculate the necessary power of an alternator?

[dauphin95]

Hello,

How to calculate the necessary power of a 12V car alternator, to operate at 100%, 9 headlights of 130W.

Knowing that it will be a second alternator in addition to the one that is mounted.

I have two different calculations !!

The first 9x130 = 1170W output of 0.75 (1170x0.75 = 1560w: 12 = 130A

Le second 9x130=1170:12=97.5A

What is the correct calculation ???
I don't want to go wrong to order this second alternator!

Thank you to enlighten me!!!

ANSWER:


Your calculations are far too optimistic.



An alternator has an efficiency close to 50% and its belt transmission has one of the order of 90%.



Electric power to be supplied: 130W x 9 = 1 W.



Power absorbed by the alternator: 1 / 170 / 0.5 = 0.9 W.



Alternator rating to be provided: 2 / 600 = 14 A.



Pay attention to the power taken from the engine:

1 CV = 736 W -> 2600/736 = 3.5 CV.

By taking a security of 15% which takes account of any misalignments and mechanical friction, the real practical power will be close to 4 CV.



The most powerful model we can offer you is a 12V 160A - see attached data sheet. It should suit you because its priming speed is quite low (1 rpm).



Public price incl.VAT = 522.65 EUR. 1 piece available to date. Delivered without pulley.



Rotation speeds of the engine and the alternator:

Minimum alternator speed> ignition point (1 rpm) = engine at idle speed.

Maximum alternator speed (approximately 15 rpm) <manufacturer data = maximum engine.



Diesel engine: 2000 rpm <cruising speed <4000 rpm.

800 rpm <Idle speed <1200 rpm.



Taking these considerations into account, the pulley ratio allowing to load correctly when the engine is idling and to obtain a maximum load at cruising speed brings us to a ratio between 2 and 3.

Advice: do not exceed a pulley ratio of 3.5 (premature wear of the belts).

Reminder: pulley ratio = diam. engine pulley / alternator pulley diam



Sincerely,



Frederic DESMONS

Electricity Product Manager for PROSHIP

[Forhorse]

In principle when we announce the power of a generator, it is about the electric power, therefore second calculation.
(the efficiency is to be taken into account only to know which mechanical power it will be necessary to provide to the generator so that it delivers its nominal current)

[chatelot16]

I confirm !

the efficiency is not there: if your lamps consume 97A the alternator must supply 97A!

the output will calculate the mechanical power requested

on the other hand, there is something else to check if it has to run for a long time at 97 A and there is a battery to maintain. 97A should not be the maximum alternator current: it would have to do a little more to have room and be sure it does not drain the battery

[Rabbit]

Are these 2 alternators connected to the same battery?
Isn't that likely to burn out the weakest alternator?

[dedeleco]

Attention two alternators connected in series (or in parallel uh) must first be synchronous therefore on the same axis and calibrated in the same way and then they cannot give the same voltage and therefore in parallel the strongest will flow in the most low, so danger with around 100A, to make a nice smoke !!!
It is better to rectify with controller chargers independently, to supply two batteries with their respective lamps, 4 and 5 completely separate is safer.
Personally, so as not to lose money, I would try with alternators and regulators taken from cheap scrap cars !!!
Given the armada of cars for scrapping it is possible, and less expensive if error and cramage !!
For alternators with 50A or 600W, are they more suitable for trucks?
Without suitable regulator and clearly higher power capacity of the alternators, the batteries will suffer discharged or overcharged if poor control?
For use on a car or small truck, different from normal use, rapid intermittent charging rarely as much as possible, there will be problems, such as death of the alternator or batteries.
It is necessary to know the typical characteristics of use, to respect them by measuring the voltages and current of the alternators and the regulators and to be aware of the traps over a long enough time to see arriving the excessive heating (even the wires) !!!

[Alain G]

There is no problem in coupling 2 alternators of different capacity, each alternator is designed to absorb all of its available load otherwise we would change alternators each time we boost!

The internal resistance of the windings makes them balance at 14.5 volts!

If you are afraid replace the existing for the same type as the new!

[dedeleco]

There is no problem in coupling 2 alternators of different capacity, each alternator is designed to absorb all of its available load otherwise we would change alternators each time we boost!

If this were true, EDF with its large thermal or nuclear power plant alternators would not tire of synchronizing its alternators, before coupling them to the network !!!!
It is not a question of overvoltage, but to avoid that the highest voltage (without overvoltage), does not flow into the other full pot alternator with alternating currents between the two, which heat up and burn !!

Weird, who said weird !!

[Alain G]

There is no problem in coupling 2 alternators of different capacity, each alternator is designed to absorb all of its available load otherwise we would change alternators each time we boost!

If this were true, EDF with its large thermal or nuclear power plant alternators would not tire of synchronizing its alternators, before coupling them to the network !!!!
It is not a question of overvoltage, but to avoid that the highest voltage (without overvoltage), does not flow into the other full pot alternator with alternating currents between the two, which heat up and burn !!

Weird, who said weird !!

An alternator that produces DC through a regulator has nothing to do with synchronous generator groups in power plants!

Each automotive alternator is designed to operate at its full efficiency or full capacity if this word annoys you, whether it is 10 or 100 amps does not change anything!

Naturally, a margin of 25 or 30% of extra amps is required if it is used for a long time!

The power stations use systems which gradually engage to synchronize the alternators and in the new systems they operate in DC converted to AC to keep the frequency perfect!

[Rabbit]

We can therefore connect 2 alternators to a battery.

It is really fascinating. I will be able to add some
spots to my tractor.

[Alain G]

Rabbit

Yes we can add several alternators like this example or we use 3 x 350 amps and 1 x 250 amps:

http://www.youtube.com/watch?v=yti-GQa5n0I