30 replies to this topic

[manthony121]

In some power generation systems what you describe is more true. It is NOT true for an automotive alternator. Instead of generating a constant amount of power and "throwing away" the excess, its never generated in the first place. The stator is an electromagnet. The current through the stator controls the strength of that magnet. So, if you reduce the current through the electromagnet, you decrease the strength of the magnetic field, which in-turn reduces the amount of electro-motive-force induced into the alternator coils, and thus the required torque on the alternator pulley.

Now you've got me confused again.  For a "simple" generator: let's say a bundle of wires wrapped around a stick, being turned inside a fixed magnetic field, does the current drawn from the output effect how much torque it takes to turn the rotor?  IOW, is the stick harder to spin if the generated electricity is being used across some load?

 

I realize that the alternator in a car does not use a fixed magnet, but an electromagnet, the strength of which depends on how much power is needed to charge the battery and other such electrical housekeeping.  In that setting, it makes sense that turning up the power of the electromagnet will make the rotor harder to turn, and will generate more power.

[Bench Racer]

This should transfer the requested info. About the 2 1/2 minute mark is the explanation how more or less is generated.

 

[Tom Hampton]

Now you've got me confused again.  For a "simple" generator: let's say a bundle of wires wrapped around a stick, being turned inside a fixed magnetic field, does the current drawn from the output effect how much torque it takes to turn the rotor?  IOW, is the stick harder to spin if the generated electricity is being used across some load?

Sorry for confusing you. But, as you may imagine the more we deviate from talking about a specific case (miata alternators), to a more general discussion the more variations there are. Its hard to put several textbooks' worth of information into a couple forum posts. But, to answer your question directly: yes. More specifically, the more electrical POWER (Current * voltage) that is drawn from the alternator the more mechanical POWER (RPM * Torque) is required to turn it.

 

This fact is a basic fact of physics.  Its the first law of thermodynamics.  it is independent of any method of power generation. 

 

Simply putting a load across the output terminals of a field-coil-alternator or a permanent magnet generator will both make the rotor harder to turn---as a direct result of the above law. 

 

Since electronics are hard to design that operate on wildly varying voltages, the generator output has to be regulated.  This is the tricky part. There are lots  of methods for regulating voltage.  Rather than confuse matters further, I'll just say that voltage regulators can either be more wasteful (where they discard excess voltage---convert it to heat), or they can be less wasteful where they transform the volts.   The more wasteful methods would see a more constant load on the rotor shaft and the excess power would be burned off as heat.  The energy always has to come from somewhere and it always has to go somewhere. 

[Ron Alan]

Good stuff Dave! So i have another question for you EE/ME guru's. It seems our alternators are pretty simple...when we do get failures...what on the charging/voltage output side fails most often? On the mechanical side my guess would be bearings but i'm asking to be educated! 

[Tom Hampton]

Not much rocket science in the answer.  Its the parts that move or carry most of the current.  The ones that do both tend to go first.  That said, alternators tend to either go bad right away or last "forever".  Bad parts, bad manufacturing, etc.  Once these are eliminated through infant failures...they tend to last to their design age. 

 

Mechanical:

1. Rotor Brushes/slip-rings.

2. Bearings.

 

Electrical:

1. Power switching transistor(s)---the primary power carrying component in the regulator.

2. Stator Windings. 

 

Note the NA and NB have different regulator designs.  The NA alternator has the regulator completely contained within the alternator.  Whereas the NB moves the regulator "control" from inside the alternator to the ECU, and only leaves the power-transistor(s) inside the alternator. 

[SaulSpeedwell]

FWIW...I have never seen anywhere near what many are posting on HP loss on a dyno from the alt.    

 

Yeah, we tested and calculated this moons ago?  Maximum alternator output in watts, converted by the pulley ratio of the "belt train", converted to "horsepower" came out to some tiny fraction of a HP?

 

For those that are worried about this, worry about brake drag or the aerodynamics of your exhaust system or your alignment toe in angstroms.  Zero toe is never the lowest drag for a decambered wheel.  Strain gauge the tie rods and figure out how much toe-out is optimal!

[ECOBRAP]

Yeah, we tested and calculated this moons ago?  Maximum alternator output in watts, converted by the pulley ratio of the "belt train", converted to "horsepower" came out to some tiny fraction of a HP?

 

For those that are worried about this, worry about brake drag or the aerodynamics of your exhaust system or your alignment toe in angstroms.  Zero toe is never the lowest drag for a decambered wheel.  Strain gauge the tie rods and figure out how much toe-out is optimal!

 

Yes, alternator tested fine anyways, so the main question is how much hp can a weak battery cause, if any.

 

New battery is in so I will find out this weekend.

[FTodaro]

Not much rocket science in the answer.  Its the parts that move or carry most of the current.  The ones that do both tend to go first.  That said, alternators tend to either go bad right away or last "forever".  Bad parts, bad manufacturing, etc.  Once these are eliminated through infant failures...they tend to last to their design age. 

 

Mechanical:

1. Rotor Brushes/slip-rings.

2. Bearings.

 

Electrical:

1. Power switching transistor(s)---the primary power carrying component in the regulator.

2. Stator Windings. 

 

Note the NA and NB have different regulator designs.  The NA alternator has the regulator completely contained within the alternator.  Whereas the NB moves the regulator "control" from inside the alternator to the ECU, and only leaves the power-transistor(s) inside the alternator. 

I do know what will kill an alternator is a battery that is not adequately anchored. I lost two alternators when the positive terminal grounded out from impact. Now I always run a volt meter and have a modified bracket holding the battery down.

[davew]

try one of these

 

[Jim Drago]

try one of these

 

That looks nice dave. Are you drilling holes in car to hold or are your holes aligned with existing? i dont remeber any and too lazy to go look

[davew]

Uses existing holes in the chassis. We include the bolts/washers and nylock nuts. Latest version is powder coated.