Alternator query...

[McGuinn]

Hi, long time no post!

I have a question about a car electrical system.

The alternator has two functions... #1, to provide power to the car's electical system (fans, spark plugs, injectors, ECU...) and #2 to charge the car battery.

What I would like to know is the following:
When an additional high load is placed on the alternator, the engine note changes somewhat to a rumble.

For what reason does this occur? Is it:
1. Because the power to turn the alternator places a higher power load on the engine (due to additional loading on the engine).
2. Because the ECU compensates for a lower charge voltage by increasing the RPM to spin-up the alternator.

In addition to this... are these statements correct:
- The torque required to turn a fully UNLOADED alternator is greater than that required to turn a moderately loaded alternator.
- The torque required to turn a fully LOADED alternator is greater than that required to turn a moderately loaded alternator.

It's for a discussion I'm having on a car website about power savings.
It's said that the engine efficiency drops 6% when the lights are left on during a drive. I'm trying to figure out whether this is a consequence of the ECU compensating for the load, or the alternator causing more resistance.

 

[stevez]

It would be reasonable to assume that the torque required to turn the alternator is proportional to the power output of the alternator. It's not likley to be exactly linear because there are friction or windage losses (the rotor turning) and belt losses that are related more to RPM than power. I would expect that a fully loaded alternator would present the greatest load - and a fully unloaded alternator to present the least load.

The change in torque demand of the alternator (as lights are turned on/off) is sufficient to change the engine idle RPM - and the ECU is likely to respond. I don't know if the ECU would necessarily try to hold a fixed RPM but watching a tach might reveal the answer.

Headlights might require 100 watts of power - if the net efficiency of the alternator is 33% then the HP difference is 300/746 or somewhat less than 1/2 HP. I could see where that could be 6% at idle but while driving that seems like a rather large percentage. I am not saying your or they are wrong - it just seems awfully high.

What about cooling fan losses? I have been considering replacing my belt driven fan with electrics - noise being one of the primary reasons.

 

[Nigel Goodwin]

I'm trying to figure out whether this is a consequence of the ECU compensating for the load, or the alternator causing more resistance.

It's the alternator causing more resistance, the exact same effect occurs on old non-ECU engines.

I can remember years ago there used to be special car rallies, based on who could go the greatest distance on a measured amount of fuel. The cars had to be standard, but a popular modification was to strip the insides out of the alternator - just leaving the rotating shaft. This left the car looking standard, but with no alternator load you got more miles from the amount of fuel provided.

The tests were performed in daylight, so you didn't need lights, and the battery was more than enough to feed the ignition for the duration of the tests.

 

[McGuinn]

I would expect that a fully loaded alternator would present the greatest load - and a fully unloaded alternator to present the least load.

Correct, but when run unloaded, back-EMF causes increased loading on the alternator, doesn't it?

 

[williB]

It's said that the engine efficiency drops 6% when the lights are left on during a drive. I'm trying to figure out whether this is a consequence of the ECU compensating for the load, or the alternator causing more resistance.

i agree , its the alternator draining more power from the engine..

 

[Juglenaut]

I would expect that a fully loaded alternator would present the greatest load - and a fully unloaded alternator to present the least load.

Correct, but when run unloaded, back-EMF causes increased loading on the alternator, doesn't it?

That is why they are regulated. An unloaded alt has a nearly 100% charged core while a loaded core drops almost linear with applied loads.

This rotor field is energized by battery power, so that it takes a small amount of electrical power input to the alternator to get it to generate a lot of output power.
By rectifying the three-phase AC power produced, it is possible to have the alternator power its own field coil with DC voltage, eliminating the need for a battery. However, some independent source of DC voltage will still be necessary for start-up, as the field coil must be energized before any AC power can be produced.

This is not EMF feeback as the core will always be energized while in a set range of alt rpm, only a small amount is needed to start-up the alt.

 

[stevez]

So, what some say is that the alternator torque doesn't drop as I'd expect but help me untangle things just a little.

If one were to plot the torque vs output on a graph, would they see the highest torque at full output? At what point is the torque at a minimum? If the minimum torque is not close to the lowest output then how significant or large is the increase as it approaches the lowest output? Now, do the regulators or controls, if working properly, impact this?

Thanks. The insight is interesting - one of the great things about this forum.

 

[Nigel Goodwin]

If one were to plot the torque vs output on a graph, would they see the highest torque at full output? At what point is the torque at a minimum? If the minimum torque is not close to the lowest output then how significant or large is the increase as it approaches the lowest output? Now, do the regulators or controls, if working properly, impact this?

I'm not quite sure if 'torque' can be applied to an alternator?, but I presume you mean the force required to keep it spinning under load?.

I would suggest it will be a fairly straight line, assuming the alternator is providing maximum power all the time (regulator full ON), with the minimum at no load, and the maximum at full load.

The regulator will have a massive affect on the power required, simply because it changes the power output of the alternator. All the regulator does is reduce the power to the rotor (which is the magnet in an alternator), by reducing the power to the magnet it becomes a less powerful magnet and the output is reduced accordingly - likewise the mechanical power will be reduced as well.

 

[stevez]

Nigel-you are correct in understanding that I use the word torque to describe the effort it takes to turn the alternator shaft. A more correct word would have been shaft horsepower which would address the effects of rotational speed.

It sounds like some are saying that the minimum torque or shaft horsepower is not at low or zero output.

 

[Juglenaut]

The regulator makes alts stronger infact, as the load increases the overhead so to speak is drained of the core or field, this voltage that can be used form an everyday alt can supply four 12V light bulbs with a steady pace form a bike generator unregulated AC.

given that there is a certain amount of voltage and overall current before it is rectified, most alts are 3 phase operational, you really must understand 3 phase operational of a AC to DC rectified source. If it supplies 14.4 volts then the unrectifed field has to be greater than that to supply enough current to charge to battery and run various electronics I am assuming 28.8 volts RMS.
3 phase, every 60 degrees there is a peak. Rectifing such a phase relationship yields great current capabilities, also requires less filtering as the ripple would be minimal.

Most believe that alts don't see the loads untill the battery is maxed out on current, that is wrong all electronic equipment the alt sees as resistance loads, yes even the battery that is the most major reason why the alt sees these loads. As the loads discharge the battery the alt charges it so in effect the alt is driving the load, the battery acts like no more than a huge capacity capacitor stiffinening the voltage constant.

Torque required to spin the alt at the shaft increase with load. As RPM increases the torque required eventually drops as the field generates more than enough to drive a given heavy load. In a sense the alt actally acts like an amp as the more current in the field is produced the more regulated current it can drive to a point of a max limited by the components.

 

[Phasor]

3 phase, every 90 degrees there is a peak.

I think you mean every 60 degrees?

 

[Juglenaut]

3 phase, every 90 degrees there is a peak.

I think you mean every 60 degrees?

yeah sorry I was drinking and keyboarding again