# Alternator and poles

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#### martino

##### Member
I need to know the number of poles inside an alternator (Lundell type of course).
Is there an instrument or procedure that can help me to get this number ?
Thank You

#### shortbus=

##### Well-Known Member
The only way to know for sure would be to take it apart. Different manufacturers use different designs. Are you asking about the rotor or the stator? One thing that is common to most designs is the stator is wound three phase.

#### tcmtech

##### Banned
10, 12, 14, and 16 are the most common.

#### martino

##### Member
Interesting, thank you Tcmtech
but how can i be sure ?

#### crutschow

##### Well-Known Member
If you rotate it at a known speed and measure the frequency of the output ripple voltage, you can calculate the number of poles.

#### tcmtech

##### Banned
Interesting, thank you Tcmtech
but how can i be sure ?
Look in the end or side and count them.

#### martino

##### Member
Shortbus, i talk about the ROTOR (sorry my mistake).

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#### martino

##### Member
Crutshow:

frequency (Hz) = cp * RPM/60

cp=couples of poles
it is correct ?

#### martino

##### Member
The following image is a 6 poles or 12 poles rotor ?

#### shortbus=

##### Well-Known Member
I'd call it a 12 pole. 6 north and 6 south magnetic poles.

#### crutschow

##### Well-Known Member
Crutshow:

frequency (Hz) = cp * RPM/60

cp=couples of poles
it is correct ?
That would be the sine-wave frequency for a single-phase generator.
The ripple frequency of a full-wave rectified signal is twice that and you increase that further by a factor of 3 for a 3-phase full-wave rectifier.
This gives the ripple frequency for a 3-phase vehicle alternator full-wave rectified output of
cp*RPM/10.

#### spec

##### Well-Known Member
Issue 2 of 2016_10_28

Shouldn't that be: (RPM * number of pole pairs * number of phases * 2 [full wave rec])/60 to give an answer in Hz?

When:
Pole pairs = 6
Phases = 3
Full wave rectification multiplier = 2
The ripple frequency would be calculated by:

(RPM * 6 * 3 * 2)/60 = RPM/1.67 in Hz

spec

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#### spec

##### Well-Known Member
The following image is a 6 poles or 12 poles rotor ?

View attachment 101809
Hi Martino,

Do you know, I have never bothered to figure out how you get N poles from a single winding on an automobile alternator rotor. Also, the winding direction seemed to be wrong. Your picture makes the technique crystal clear.

That design of rotor probably explains why automobile alternators are so inefficient.

You lean something new every day on ETO.

spec

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#### martino

##### Member
Thank you all,
the frequency shown by Crutshow seems (to me) the right one for full wave rectifier (6 diodes) as usually found in the car's alternator.
The frequency shown by Spec seems (to me) the right one for half wave rectifier (3 diodes): there are some alternator, in automotivee area, with this ? I don't know.
Any feedback is welcome.

#### spec

##### Well-Known Member
Thank you all,
the frequency shown by Crutshow seems (to me) the right one for full wave rectifier (6 diodes) as usually found in the car's alternator.
The frequency shown by Spec seems (to me) the right one for half wave rectifier (3 diodes): there are some alternator, in automotive area, with this ? I don't know.
Any feedback is welcome.
Yes, I would like to resolve this too.

The formula in post#12 is for half wave wave rectification, as you say, so it needs another 2 x on the top line to account for full wave rectification but that makes the ripple an even higher frequency. I have updated the formula in post #12 to include full wave rectification.

Crutchow's formula, in post #11, does not include the contribution to the ripple by the number of poles, just the contribution of the three phase (x3) and the full wave rectification factor (x2).

spec

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#### crutschow

##### Well-Known Member
Do you know, I have never bothered to figure out how you get N poles from a single winding on an automobile alternator rotor. Also, the winding direction seemed to be wrong. Your picture makes the technique crystal clear.
The winding direction is such as to make one side of the rotor (one claw) one magnetic pole and the opposite side the opposite pole.
How is that backwards?
Crutchow's formula, in post #11, does not include the contribution to the ripple by the number of poles, just the contribution of the three phase (x3) and the full wave rectification factor (x2).
There is the factor "cp" (pole pairs) in my formula.

#### spec

##### Well-Known Member
There is the factor "cp" (pole pairs) in my formula.
Sorry Cruts,

You were correct from the start. I missed your 'cp' variable.

Since I corrected my formula, in view of Martino's post #14 comment, it now agrees with your formula. For an alternator with a six-pole-pair rotor and three phase stator, the formula boils down to: Ripple (in Hz) = RPM/1.67

spec

#### spec

##### Well-Known Member
The winding direction is such as to make one side of the rotor (one claw) one magnetic pole and the opposite side the opposite pole.
How is that backwards?
I didn't mean that the single rotor winding was backwards. The rotor winding is in the wrong direction (orientation) and, without the magnet ears, the rotor coil magnetic field would not cut the stator coils at 90 Deg. If I haven't explained that correctly, I hope I have conveyed the principle.

spec

#### crutschow

##### Well-Known Member
I didn't mean that the single rotor winding was backwards. The rotor winding is in the wrong direction (orientation) and, without the magnet ears, the rotor coil magnetic field would not cut the stator coils at 90 Deg. If I haven't explained that correctly, I hope I have conveyed the principle.
I believe you misunderstand how the rotor works.
The magnetic ears are indeed integral to its operation.
One claw is magnetized with one magnetic polarity and the other claw with the opposite polarity by the coil between them.
It is the magnetic ears of the two claws that generate the field between the claws that cut the stator coils.
(The field direction is not exactly 90 degrees to the stator but I don't think that has a significant effect on the operation).

#### spec

##### Well-Known Member
I believe you misunderstand how the rotor works.
The magnetic ears are indeed integral to its operation.
One claw is magnetized with one magnetic polarity and the other claw with the opposite polarity by the coil between them.
It is the magnetic ears of the two claws that generate the field between the claws that cut the stator coils.
(The field direction is not exactly 90 degrees to the stator but I don't think that has a significant effect on the operation).
I did get that Cruts. But thanks anyway.

spec

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