# Why do car alternators need stators and rectifiers with 4 terminals and then use only 3 of them?

#### sarcrileger

Joined Oct 13, 2022
11
i have taken on the project of refurbishing my own car alternators which died on me over the years.
all these alternators are compatible with 1990 mitsubishi eclipse and come in 65/75/90 Amp configurations.

as seen in the attached pictures, some stators come with 3 single wire windings and some with 4 single pair windings;
all rectifiers come with 4 terminals, even if the stator comes with 3 wires;
even though the stators come with 4 wires, some stators dont connect the 4th wire (presumably the earth fault protection circuit) to the rectifier and some others do.

i have the following questions which are related to attached photos:
1/ is there any way by looking at the internals to determine which alternator puts out 65, 75 and 90 amps?
2/ does the neutral point wire connect to a specific terminal on the rectifier or it make no diff to which terminal on the rectifier is the neutral point wire connected to?
3/ what are the characteristics of and how does one identify the neutral point wire?

thank you

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Joined Jul 18, 2013
26,291
It looks in Very bad shape !
You don't have a neutral point connected to the bridge rectifier, The neutral junction is usually the ends of all three windings connected to this star point and sealed off.
You can also have a Delta connected version.

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#### sarcrileger

Joined Oct 13, 2022
11
It looks in Very bad shape !
You don't have a neutral point connected to the bridge rectifier, The neutral junction is usually the ends of all three windings connected to this star point and sealed off.
You can also have a Delta connected version.
yes, i know the parts in the pic are in bad shape. i will be scavenging various parts such as slip rings bearings etc. and build a proper alternator. the pics are for illustration and accompany the description. however, in order to build and configure the refurbished alternator, i am trying to find out what parts and what connections go together, hence, my questions in the original post.

i am presuming that the amount of copper is related to higher output, hence, the single winding would go with the 65amp alternator?

what is the forth wire on the stator for and why some alternators have three wires, some four wires but only three connected to the rectifier while some have all 4 wires connected to the rectifier?

since the name plates with amp rating are no longer on the chassis, what would the 90 amp configuration look like?

performance-wise, what is the functional difference b/w the alternator with three connections and the one with four connections?

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Joined Jul 18, 2013
26,291
Some monitor the star point for indication functions, nothing to do with regulation etc.
The higher current versions are generally Delta connected. i.e. no star neutral point.
There are a few different configurations.

#### sarcrileger

Joined Oct 13, 2022
11
Some monitor the star point for indication functions, nothing to do with regulation etc.
The higher current versions are generally Delta connected. i.e. no star neutral point.
There are a few different configurations.
i am not an electrician by trade and technical terminology such "delta connected" goes over my head. however, having answers to my questions would be very helpful:

1/ is there any way by looking at the internals to determine which alternator puts out 65, 75 and 90 amps?
2/ does the neutral point wire connect to a specific terminal on the rectifier or it make no diff to which terminal on the rectifier is the neutral point wire connected to?
3/ what are the characteristics of and how does one identify the neutral point wire (assuming there is one)?
4/ what is the forth wire on the stator for and why some alternators have three wires, some four wires but only three connected to the rectifier while some have all 4 wires connected to the rectifier?
5/ performance-wise, what is the functional difference b/w the alternator with three connections and the one with four connections?

Joined Jul 18, 2013
26,291
Do a search using "auto alternator stator connections star and delta" you will get a whole multitude of explanations, ignore the 3ph AC induction versions.
This one in particular can answer most questions, https://what-when-how.com/automobile/principle-of-operation-automobile/
Delta is preferred for high current alternators.
Questions:
1/ As far as I know it is not easily possible to determine current capacity by viewing windings.
2/Generally the neutral point is not connected to the rectifier, or at least not one of the 6 used for the main output.
3/The neutral point is a termination point of one end of all three star connected windings
4/Often there is an extra rectifier over and above the 6 required for monitoring.
5/If Delta, higher current version.

Star and delta is the connection configuration options for a 3 phase alternator stator.

#### sarcrileger

Joined Oct 13, 2022
11
Do a search using "auto alternator stator connections star and delta" you will get a whole multitude of explanations, ignore the 3ph AC induction versions.
in addition to the information compiled in the link you provided, i searched further to narrow down relevant data to answer my own question... i have had some success but i am still not entirely clear as to what configurations i am looking at in my alternators.
here;s the relevant information i found at http://www.autoshop101.com/ :

In the Delta arrangement the three windings are connected in the form of the Greek letter ‘A’ and the output is again taken from points A, B and C. Delta wound stators can be identified by having only three stator leads, and each lead will have the same number of wires attached. Delta stators allow for higher current flow being delivered at low RPM. The windings are in parallel rather than series unlike the Wye design.
Wye wound stators have three windings with a common neutral junction. They can be identified because they have 4 stator lead ends. Wye wound stators are used in alternators that require high voltage output at low alternator speed. Two windings are in series at any one time during charge output. Wye style has four stator leads. One of the leads is called the Neutral Junction. The Neutral Junction is common to all the other leads. WYE configuration is a type of Star arrangement with neutral junction.
In the Star arrangements, one end of the three windings is connected together and the output current is supplied from the ends A, B and C.
Voltage output from star winding = 1.732 x voltage output from Delta winding.
Current output from Delta winding = 1.732 x current output from Star winding.
The majority of the alternators for light-cars use the star windings, but the Delta wound stator is preferred for higher current output. On some special designs of heavy-duty alternators, the stator windings can be altered from Star to Delta when a large output current is needed.
Rectification:
The Diode Rectifier Bridge is responsible for for the conversion or rectification of AC voltage to DC voltage. Six or eight diodes are used to rectify the AC stator voltage to DC voltage. Half of these diodes are use on the positive side and the other half are on the negative side. Diodes used in this configuration will redirect both the positive and negative polarity signals of the AC voltage to produce DC voltage. This process is called ‘Full - Wave Rectification’.
A complete circuit is required for the flow of this direct current, therefore an appropriate earth diode (the negative diode in this case) is used to pass current from ‘earth’ to the active winding.
while i have gain some understanding as to what i am looking at in my alternators, i am not clear regarding these observations:

1/ based on the new information i quoted in this post, this image https://forum.allaboutcircuits.com/attachments/img_0457-jpg.278339/ appears to suggest that i am dealing with DELTA arrangement... would that be correct?

2/ based on the new info i quoted above, this image https://forum.allaboutcircuits.com/attachments/photo_2022-10-01_18-03-35-jpg.278341/ would suggest i am dealing with Wye configuration; however, that cannot be correct because while Wye style has four stator leads of which one is the Neutral Junction, the Neutral Junction is common to all the other leads.

based on this^ definition, i would expect one of the leads to have 6 wires because the lead would be common to all the other leads but it is not since i only have 2 wires on each of the 4 leads.

So what exactly am i dealing with? Why dont i see one lead with 6 wires???

3/ furthermore, the image referenced in paragraph^ "2/" shows all 4 leads connected to all 4 rectifier terminals. the quote above said this:
[B]"A complete circuit is required for the flow of this direct current, therefore an appropriate earth diode (the negative diode in this case) is used to pass current from ‘earth’ to the active winding."[/B]

the rectifier in the referenced image has a rectifier with 4 terminals and all 4 terminals are connected to 4 leads coming from the stator.
how is one to know which one of those terminals is 'earth'? if that what it is.... 'earth'

thanks in advance for clarifying these mysteries for me...

Joined Jul 18, 2013
26,291
In some alternators, which are connected in Star, the centre star point is grounded through a NGR Neutral Ground Resistor.
This is used to detect any ground connection defect on any of the windings on the input side to the rectifier.

#### sarcrileger

Joined Oct 13, 2022
11
In some alternators, which are connected in Star, the centre star point is grounded through a NGR Neutral Ground Resistor.
This is used to detect any ground connection defect on any of the windings on the input side to the rectifier.
i do not believe any of my alternators are grounded through a NGR resistor. however, some of my alternators have rectifiers grounded and some dont... my observation is that stators with three leads connected to the rectifier have the rectifier itself grounded and stators which have all 4 leads connected to the rectifier do not ground the rectifier itself. some of my alternators have this type of grounding:

needless to say, all my alternators except one (see pic in OP) have 4 leads just like in this illustration:

however, all my stators with 4 leads have the same number of wires (2) instead of one lead, the neutral line, with 6 wires as highlighted in post #7.

i am making this^ assertion based on the definition which can be found at this link:
https://www.electricalvolt.com/2020/04/why-is-armature-of-alternator-connected-in-star-not-in-delta/

For star connected armature, the neutral is formed where one end of all the three winding are connected. The neutral point is connected to the neutral ground resistor. Under balanced load condition, when current in all the phases are equal, no current flows through the neutral.

is there any way to test which lead on the stator is a phase output line and which one is a neutral line?

Joined Jul 18, 2013
26,291
Three of the conductors will measure the same resistance to the fourth, although you need a meter capable of distinguishing fractions of an ohm as the reading is quite low.
Any other will be two windings in series reading.

#### sarcrileger

Joined Oct 13, 2022
11
Three of the conductors will measure the same resistance to the fourth, although you need a meter capable of distinguishing fractions of an ohm as the reading is quite low.
Any other will be two windings in series reading.
i have desoldered a stator with 4 leads from the rectifier. the stator which i desoldered had 3 leads connected to the rectifier and the 4th lead was insulated and not connected. i took the insulation off the 4th lead, marked it 'N', and started measuring resistance b/w the 3 individual leads and the lead labelled 'N' and then b/w the 3 leads themselves. these are my measurements measured with a digital multimeter:

N -> 1 = 0.2 ohm
N -> 2 = 0.2 ohm
N -> 3 = 0.2 ohm
1 -> 2 = 0.2 ohm
1 -> 3 = 0.2 ohm
2 -> 3 = 0.2 ohm

even though this is (allegedly) a 3 phase stator, there does not appear to be any difference among these leads.
why is there no measurable difference?

what is the purpose of the 4th lead on the stator and why is the 4th lead connected to the rectifier in some cases and in some cases it is not connected to the rectifier? there has to be a logical explanation why the manufacturer configured these stators in this arrangement, would it not?

#### sarcrileger

Joined Oct 13, 2022
11
the stator in the picture below has 4 leads and one of the leads, allegedly the 'neutral junction', is insulated and isolated from the armature. furthermore, the rectifier has a terminal denoted with the letter 'N', presumably the 'ground', and yet the stator's 'neutral junction' is not connected to the 'N' terminal on the rectifier.

question:
what is the advantage of NOT connecting the rectifier 'N' terminal to the stator 'neutral' lead (as shown in these images below)?

Joined Jul 18, 2013
26,291
The output of the rectifier -ve is at ground potential, so any star point cannot be also connected to GND.

#### sarcrileger

Joined Oct 13, 2022
11
The output of the rectifier -ve is at ground potential, so any star point cannot be also connected to GND.
what would be the reasoning in grounding the rectifier vs grounding the stator to the rectifier (see image below)? what is the single deciding factor which determines which way the grounding will be implemented? ... and does it make any significant difference which way the grounding is implemented? would there be a bias which would favor one method of grounding over the other methods?

this^ is not a curiosity based question. i am asking so i can make intelligent choices when i refurbish an alternator using components collected from several alternators. thank you.

Joined Jul 18, 2013
26,291
The automotive system uses a DC -ve ground, therefore the resultant negative terminal of the 3phase bridge rectifier terminal must be connected to chassis
If there is any Direct connection to chassis from the star point, this will cause a short circuit issue.
Not so much if it is used for sensing purposes.

#### sarcrileger

Joined Oct 13, 2022
11
The automotive system uses a DC -ve ground, therefore the resultant negative terminal of the 3phase bridge rectifier terminal must be connected to chassis
If there is any Direct connection to chassis from the star point, this will cause a short circuit issue.
Not so much if it is used for sensing purposes.
i have taken a closer look at the configuration and noticed that the rectifier with an empty 'N' terminal has some kinda (grounding??) red wire which goes to the B terminal instead of some apparent ground terminal.

due to rust and seized bolts, i am not able to take the alternator with all 4 leads connected (see post #14^) apart as to confirm whether the rectifier has a (grounding?) lead attached to the body of the rectifier but based on your answer, the rectifier with all 4 terminals connected would be grounded via one of the leads coming out of the stator? or how exactly would that work if not via stator? what is the general rule of connecting these things which we can apply here?

#### sarcrileger

Joined Oct 13, 2022
11
The automotive system uses a DC -ve ground, therefore the resultant negative terminal of the 3phase bridge rectifier terminal must be connected to chassis
If there is any Direct connection to chassis from the star point, this will cause a short circuit issue.
Not so much if it is used for sensing purposes.
based on the discussion regarding grounding and short circuit issues, i took a closer look at the alternators again and how they are wired/grounded. upon closer examination of the image in the OP, https://forum.allaboutcircuits.com/attachments/photo_2022-10-01_18-03-35-jpg.278341/

and subsequent examination of where the black wire came from, i discovered that the black lead is a ground wire attached to the body of the rectifier and then connected to B terminal. due to the rust etc., i could not see where the black grounding wire is attached; however, i verified with ohm meter that i have connectivity b/w the rectifier body and the black wire/lead.

even though this type of wiring should have caused short circuit issues, it did not. i tested all components and this alternator failed due to totally worn out brushes. in other words, a stator which connected it's neutral lead to the rectifier and then the same rectifier was grounded to a B terminal did not cause any short circuit issues and failed due to worn out brushes:

#### sarcrileger

Joined Oct 13, 2022
11
The output of the rectifier -ve is at ground potential, so any star point cannot be also connected to GND.
here's another odd situation: the stator in the pic below has what we believe (allegedly) is a neutral junction lead insulated and isolated from the armature instead of it being connected to the 'N' terminal on the rectifier. furthermore, as the pic below shows, the rectifier itself is not grounded, i.e. there is not grounding wire which would connect to the B terminal as it was the case shown in previous post. the pic below shows (pointed to by a screwdriver) the neutral junction lead as well as the lower part of the rectifier to show there is no ground wire attached to it:

and here's the other side of the rectifier (already separated from the stator) to show there is no grounding wire to be seen.
so, ... this alternator operated without any issues even though the stator or rectifier was not grounded until it failed due to worn out brushes:

so there you have it^ . i have no explanation for how the connections work but i certainly would like to know what the proper way of connecting stators and rectifiers is because what i see makes little sense to me... and that is the reason why i am on this forum