Thread: Dual Voltage Perenties. Do you use the 24V side?

Originally Posted by Dervish

Has anyone measured their fuel consumption with the 24V generator spinning and then again with the belt off? That would be interesting.

I do not know about fuel consumption but the 24v alternator does draw a lot more power than a 12v alternator.

On the 101 the power output of the 12v V8 is 120 BHP where the 24v V8 is 115 BHP so the 12v alternator probably draws a couple of HP where the 24v alternator is probably around 7hp - so that probably equates up to 1l/100km on the Perentie.

garry

natural belt drag is about half a horse and the alternator with no electrical load on it is negligible (mostly its fan load which is coverd by the natural belt drag)
when its charging the batteries its (V*A/650)*1.3 for the power required to provide a given out put in HP.

so against flat batteries in the 80 amp setting its

28.8*80/650 *1.3=4.6 hp drive the shaft and very very roughly you need about 1/3HP per belt per HP to deal with belt loss

so near enough 4.5hp and add 1/3HP per hp belt drag plus the natural gives you 1.8 HP drag or near enough to 6.8 HP drag at an 80 amp push into the batteries


IF you dont load the alternators they only have their bearings and fan do drive

Hi Dave, we noted the absence of a clutch on the unit, yet you are saying that power generation and thus engine loading are based on battery demand?

Can you please explain how the engine loading variability works?

Further, if not drawing on the 24/28v system, removing the belt/s to the CAV (?) will give negligible improvement in fuel consumption. Is this correct?

If so, my proposed experiment above hardly seems worthwhile.

Thanks in advance for your advice.

Originally Posted by DBT

Hi Dave, we noted the absence of a clutch on the unit, yet you are saying that power generation and thus engine loading are based on battery demand?

Can you please explain how that works?

Of course the load on an alternator or generator has to be proportional to the power being generated. Otherwise you would be violating the laws of thermodynamics. Which is the reason i commented on Diana's poor choice of words referring to an excess of power.

As dave says, spinning an alternator or generator when there is no electrical power demand doesn't require much engine power.

Originally Posted by isuzurover

Of course the load on an alternator or generator has to be proportional to the power being generated. Otherwise you would be violating the laws of thermodynamics. Which is the reason i commented on Diana's poor choice of words referring to an excess of power.

Ya, I get that bit, I wasn't asleep in most of my physics classes.

Originally Posted by isuzurover

As dave says, spinning an alternator or generator when there is no electrical power demand doesn't require much engine power.

This is the bit I think I slept through. Alternators are variable in their power generation? Based on demand? Is this where they differ from "generators" in not having a fixed power production (assuming a fixed input RPM), which is then "forced" into the circuit/battery/batteries?

Please educate me.

Originally Posted by DBT

Alternators are variable in their power generation? Based on demand? Is this where they differ from "generators" in not having a fixed power production (assuming a fixed input RPM), which is then "forced" into the circuit/battery/batteries?

Please educate me.

Automotive alternators all run with a voltage regulator. When the output of the alternator drives the voltage input of the regulator to it's design limited voltage (e.g. 14.5V), the regulator then reduces the current to the rotor via the slip rings. This reduction of current reduces the magnetic field strength around the rotor and therefore induces less current in the stator. If the load on the alternator is very small the current to the rotor will be regulated to a very low value, only enough to cover the magnetic losses and a tiny bit over to keep the output voltage high. The rotational speed of the rotor multiplied by the magnetic field strength around it gives you the power required to turn the shaft. The electrical output power is the shaft power less the losses inherent in the windings' resistance and magnetic eddies in the iron field cores. These losses are what heat up a working alternator.

Oh and automotive generators normally have a regulator too, it works in a similar way.

Just tightening my hat so my head doesn't explode, but I think I get the gist of it.

So basically there's a regulator that creates some kind of feedback loop to control the output of the alternator, based on "demand" of the circuit it's feeding.

I'm guessing this demand would be sensed as a voltage drop in the circuit, which would prompt the alternator to ramp up output 'till voltage stabilises?

Power production increases rotational resistance, which is then transferred as mechanical load to the engine!

Whoah!

Think of the regulator as a governor that always tries to ensure the voltage is at a set limit.
Think of the two forces in the alternator as being a pair of magnets held north to north that try to repel each other. (and at the end of some reallynl complicated physics that's what it is)

Here's the rules the regulator senses the amount of potential force coming off the magnet that represents the electrical out put and tries to keep that constant. It does this by controlling the size of the magnet that represents the mechanical force .

So. If the voltage potential is already high because there is nondemand being made on the alternator then the regulator makes the magnet on the rotor small. A small magnet is easy to move so doesn't place Mich load on the shaft.

If the voltage potential is low then the regulator ales the magnet on the shaft big and a big magnet is hard to to spin so outs a lot of load on the shaft.


A simple proof of this is on a car with an old belt, on start up while the alternator is trying to put all the spent energy back into the battery the belt slips and squeals, as the load comes off because the battery is charged the driving effort required of the belt drops off and so it doesn't slip anymore.