Alternator and poles

[MaxHeadRoom78]

Input 3 phase to the stator and you have a synchronous motor.
Max.

 

[crutschow]

I did get that Cruts. But thanks anyway.

Okay.
Then I'm still confused about your statement.

 

[shortbus=]

The 'claws' are on the ends of the coil, they transfer the magnetic lines from the coil. Winding "direction" is irrelevant.

 

[martino]

At this point, i could write:
cp (couples of poles) = 10*Fripple/RPM
where RPM are referred to the alternator (not to the engine).
So with a scope (to read the Fripple) and an RPM counter i should get the poles of an automotive alternator.

 

[spec]

The 'claws' are on the ends of the coil, they transfer the magnetic lines from the coil. Winding "direction" is irrelevant.

That is the magnetic field for a coil which I am quite happy with. And as you say winding direction is irrelevant - I didn't say otherwise. My issue is with the direction that the lines of magnetic force from the rotor coil cut the stator coils. If you include the stator windings in the diagram you will see what I mean. In a conventional motor/generator the rotor coils are 90 degrees rotated compared to in an automobile alternator.

My contention is that because of this, an automobile alternator is not very efficient because there is poor magnetic coupling between the rotor field and stator field. Of course as Cruts implies I may be missing something.

I must admit that I haven't learn't much about motors and generators, so what I say is pure speculation; after all, automobile alternators do work, just not very well in terms of efficiency, but they are reliable and cheap which are the most important characteristics.

spec

 

[crutschow]

............
In a conventional motor/generator the rotor coils are 90 degrees rotated compared to in an automobile alternator.

My contention is that because of this, an automobile alternator is not very efficient because there is poor magnetic coupling between the rotor field and stator field. Of course as Cruts implies I may be missing something.

It isn't the direction of the magnetic field from the coil that's important, it's the direction of the field between the claws and that's nearly 90 degrees to the stator coils. It doesn't make the field less efficient because it changes direction as it travels to and across the claw teeth.
The only reduction in magnetic efficiency that I can see is that the claw gaps are not exactly at 90 degrees.

According to this white paper the largest source of losses in a vehicle alternator is resistive losses in the stator and rectifier diodes.
The magnetic coupling efficiency would show up in losses from the required field current, which would increase with poorer coupling.

 

[spec]

It isn't the direction of the magnetic field from the coil that's important, it's the direction of the field between the claws and that's nearly 90 degrees to the stator coils. It doesn't make the field less efficient because it changes direction as it travels to and across the claw teeth.
The only reduction in magnetic efficiency that I can see is that the claw gaps are not exactly at 90 degrees.

According to this white paper the largest source of losses in a vehicle alternator is resistive losses in the stator and rectifier diodes.
The magnetic coupling efficiency would show up in losses from the required field current, which would increase with poorer coupling.

Thanks a lot Cruts: very informative.

I will have to learn a bit more about automobile alternators:

spec

 

[shortbus=]

My issue is with the direction that the lines of magnetic force from the rotor coil cut the stator coils. If you include the stator windings in the diagram you will see what I mean. In a conventional motor/generator the rotor coils are 90 degrees rotated compared to in an automobile alternator.

I guess I'm not understanding what you mean. The stator of an automotive alternator and an induction motor are wound alike. Even the lamination cores are the same. If you would see one of each laying on the bench you would be hard pressed to tell which was which.

The "claws" are the shape they are to make a sine wave in the stator winding's as they rotate in them. Any other shape would make DC not AC. Making it a generator not an alternator.

 

[MaxHeadRoom78]

A BLDC motor is a three phase generator when the rotor is turned.
Rotor consists of a series of permanent magnets.
Max.

 

[spec]

I guess I'm not understanding what you mean. The stator of an automotive alternator and an induction motor are wound alike. Even the lamination cores are the same. If you would see one of each laying on the bench you would be hard pressed to tell which was which.

The "claws" are the shape they are to make a sine wave in the stator winding's as they rotate in them. Any other shape would make DC not AC. Making it a generator not an alternator.

I am investigating the relationship between the rotor and stator fields and hope hope to do s sketch and some words to illustrate the dilemma. I suspect there is a bit more to the claws than simply shaping the magnetic field to produce a sine wave.

But bear in mind that my only objective is to establish in my mind the exact interaction of the rotor and stator fields. And I do appreciate that some of you have a lot of experience in this area so don't think my objective is to contradict anyone.

In the little research I have done so far, all I can find is simplified descriptions of how automobile alternators work or academic but arcane books that immediately leap into vectors and phasors etc, without specifically dealing with the actual motors/generators. I had a look at Wikipedia too, but that didn't even get the output frequency of an automobile alternator right.

spec

 

[crutschow]

Actually the automotive alternator rotor is designed to give more of a square-wave magnetic field in the stator as opposed to a sine-wave since that is more efficient and produces less ripple in the rectified output.

 

[MaxHeadRoom78]

This is the AC output waveform of a back fed rotated BLDC motor with alternating PM poles on the rotor, I don't see a auto alternator being much different. (Ignore the Hall pulses).
I have an old one on the bench, I may do some test on the rotor and stator.
Max.

 

[spec]

The sketch below, shows, the relationship between an automobile alternator rotor and stator winding; the claws are not shown. The dilemma is that this physical relationship gives the minimum coupling between the two coils. In EMC work it is the very orientation that you would choose if you were minimizing the interaction between the two coils.

If the rotor winding was then rotated around the axis shown on the sketch, there would still be no induced voltage in the stator coil.

In fact, it would be easy to do a simple experiment and rotate a permanent rod magnet (polarized axially, rather than radially) in the relative position to a coil shown in the sketch.

Theory says that no voltage would be induced in the coil.

That, I hope, illustrates my point.

spec

 

[MaxHeadRoom78]

The way I see it the rotor lines of force are crossing the stator at right angles according to a dismantled alternator.??
Max.

 

[shortbus=]

A BLDC motor is a three phase generator when the rotor is turned.
Rotor consists of a series of permanent magnets.
Max.

But is it 3 phase AC? Was always told it was thee separate DC phases.

 

[shortbus=]

The sketch below, shows, the relationship between an automobile alternator rotor and stator winding; the claws are not shown. The dilemma is that this physical relationship gives the minimum coupling between the two coils. In EMC work it is the very orientation that you would choose if you were minimizing the interaction between the two coils.

If the rotor winding was then rotated around the axis shown on the sketch, there would still be no induced voltage in the stator coil.

In fact, it would be easy to do a simple experiment and rotate a permanent rod magnet (polarized axially, rather than radially) in the relative position to a coil shown in the sketch.

Theory says that no voltage would be induced in the coil.

That, I hope, illustrates my point.

spec

View attachment 101863​

In the drawing your correct, nothing produced in the stator. But that does not show the 'claw' poles in an automotive alternator, it's the 'claws' that makes each alternate claw a north or south magnetic pole, and allows them to make electricity in the stator.

Now if instead of rotating like in your drawing, the center electromagnet reciprocated, moving in and out of the stator, it would generate a charge.

 

[MaxHeadRoom78]

But is it 3 phase AC? Was always told it was thee separate DC phases.

Not true, BLDC normally wound in delta, but if wound in star you do not have access to the star point, so what do you use as a reference point to decide it is varying DC?
If you set up a virtual zero (Y) point by connecting three resistors, say 1k to 10k from each phase and connect the other ends and use this as a virtual zero, you will measure 3 distinct AC phases WRT zero, as shown on the PDF I posted.
Max.

 

[tcmtech]

In fact, it would be easy to do a simple experiment and rotate a permanent rod magnet (polarized axially, rather than radially) in the relative position to a coil shown in the sketch.

Theory says that no voltage would be induced in the coil.

That, I hope, illustrates my point.

In a two dimensional world that would be correct but alternators run their magnetic fields in a three dimensional flow circuit pattern.

The rotor end plates and claws take the magnetic field coming out of the rotor coil and bends it around 90 degrees to put it on the rotors circumference in an alternating N/S configuration.

From there those alternating N/S pole lines of force loop up into the stator and back down thus completing the magnetic field loop through the stator windings in the correct 90 degree direction flow to the actual coils physical layout.

As for alternator efficiency that has far more to do with the physical components and tolerances than anything. Many low end and remanufactured alternators have terrible mechanical tolerances resulting in huge gaps between the rotor and stator plus very bad winding configurations as well like having only 6 of the 12 poles of each phase circuit wound so the cores are running at near saturation to make any output power at all.

Higher end well made commercial application units however can often times have efficiencies comparable or even better than many common portable power generators and induction motors.

I've seen cheap reman Delco and Motorola alternators with rotor to stator gaps near 3/32" (~.090+) an inch with the absolute bare minimum gauge and number of windings to work many times that represent the lowest of the low end units.
I have also seen many higher quality units like the commercial Prestolite and Leece Neville high continuous duty output units made to very tight tolerances of a few thousandths of an inch rotor to stator gaps along with massively over wound stator coils behind that.

 

[spec]

Nicely put TCM.

You have cleared the matter up for me and, to be fair, have confirmed what I said in post #13. That is that the claws do in fact rotate the magnetic field from the single rotor coil.

Some informative stuff about automobile efficiency too. It is also my contention that the design of automobile rotors has knock on effects that reduce the efficiency, mainly by copper loss. But that is another story.

Thanks to you all for your posts.

spec

 

[spec]

It isn't the direction of the magnetic field from the coil that's important, it's the direction of the field between the claws and that's nearly 90 degrees to the stator coils. It doesn't make the field less efficient because it changes direction as it travels to and across the claw teeth.
The only reduction in magnetic efficiency that I can see is that the claw gaps are not exactly at 90 degrees.

According to this white paper the largest source of losses in a vehicle alternator is resistive losses in the stator and rectifier diodes.
The magnetic coupling efficiency would show up in losses from the required field current, which would increase with poorer coupling.

Hi Cruts,

Reading back over the posts I see now that you did mention the 90 Deg change in flux direction caused by the claws, but I missed it as you were discussing efficiency.

spec