Alternator charge

[itchyfeet]

Only tests will prove it but I'd say the terminals and wire in the crimp must be corroded and you can't really clean them up as they are inside a plastic housing, I'd cut it off, you can always use it to reconnect if you leave a bit of wire.

[CJH]

I'm not so sure, but I'm not certain I'm doing the right sums. The dashboard LED only draws around 20mA I believe, so we're talking very low current. And I think a lot of that could find it's way to earth through three relays. I wonder if the alternator is relying on the current or the voltage. As you say - tests needed.

[MidLifeCrisis]

I'm not so sure, but I'm not certain I'm doing the right sums. The dashboard LED only draws around 20mA I believe, so we're talking very low current. And I think a lot of that could find it's way to earth through three relays. I wonder if the alternator is relying on the current or the voltage. As you say - tests needed.

That's why there is that other resistor in parallel to the LED - the majority of the current for the alternator will pass through the resistor rather than the LED.

[itchyfeet]

I'm not so sure, but I'm not certain I'm doing the right sums. The dashboard LED only draws around 20mA I believe, so we're talking very low current. And I think a lot of that could find it's way to earth through three relays. I wonder if the alternator is relying on the current or the voltage. As you say - tests needed.

The parallel resistor at 100 ohm is about 100mA, the relay coils can vary alot in resistance, some modern ones are quite high, high power relays have high power lower resistance coils, enough to power the light for sure but who knows if it affects excitation.

bad contacts can stop current flow even if this resistance is unmeasureable with a multimeter because multimeters measure with tiny currents. Voltage may appear ok also with high resistance, need to measure current, but that will be hard to do in a dynamic situation without an osciloscope and current probe, so empirical tests are all you can do I think.

[MidLifeCrisis]

Maybe if you disconnect all the relays that hang off that circuit and see how/if it changes the behaviour of the warning light when the engine is started?
That would at least suggest if the relays were having an effect?

[Oldiebut goodie]

My van and my boat both need a little rev to kickstart the alternator. No relays involved on either of the exciter wires.

Also, both of my warning lights are bulbs not leds and resistors.

[itchyfeet]

Bet those bulbs are around 100 ohms

[Oldiebut goodie]

Without bothering to research it I seem to recall that the voltage required is around 4v to begin excitation. Once initial excitation has taken place the alternator supplies its own excitation current.
More modern alternators are self exciting, both my van and boat are 1980's era technology. Simple!

For those that are having trouble following this little piece may help with following what happens.

the stator is the fixed/non-rotating part. It consists of a laminated iron frame with 3 windings distributed around it. Electrically, these windings are 120 degrees apart.

The rotor is an electro-magnet which is energized by current fed thru the slip rings and brushes. As the rotor magnetic field sweeps past the stator windings, it induces an alternating current in the windings. There are actually three voltages; each winding has a voltage 120 degrees out of phase with its neighbor.

To convert the alternating current to DC to charge the battery, a full-wave bridge rectifier is used. This consists of the 6 large rectifier diodes shown in the drawing. Any time a winding has positive polarity, the diode on the right conducts, connecting it to battery. In the next half cycle, when the winding is negative polarity, the diode on the left conducts. A "full-wave" circuit means that all the windings are used all the time.

The output voltage is a combination of the waveforms from the 3 windings. This results in an output which is pretty close to DC.

When the alternator is not turning, the diodes prevent battery current from flowing back into the alternator. Thus, no cut-out relay is required as with generators.

FIELD EXCITATION To control the output voltage, and maintain the correct battery charge, the field winding current is varied. The regulator is a solid-state sensor which monitors battery voltage. When the battery voltage is low, more field current is supplied.

Excitation voltage is supplied by the alternator windings, rectified by the Trio diodes (shown as the smaller diodes on the drawing). These diodes are quite small since maximum field current is only about 2 Amps. The regulator acts as a rheostat, controlling the current from the Trio diodes to the field.

Unlike a generator, the alternator is not self-exciting at start-up. A generator has field poles made of soft iron which hold a residual magnetism. The alternator field structure has little residual and thus has almost no output unless field current is supplied.

To get the alternator going, a tiny field current must be supplied. In most designs, this current is initially provided by the dash warning light. With the Ignition switch closed, current flows thru the Lamp to the regulator and into the field winding. If the dash warning light is burned out or disconnected, the alternator probably won't begin charging.

(Note: The drawing shows an externally regulated alternator. Other designs, and internally regulated units may place the regulator either in the DF lead or in the D- lead. Operation is the same though.)

As the alternator speeds up, stator voltage increases until the Trio diode voltage is sufficient to provide field excitation. As the voltage approaches 12 Volts, the dash Lamp goes off because it has the same potential on both sides.

REGULATION How does the regulator sense the battery voltage when it's not even connected to the battery? The answer is that the voltage at D+ almost exactly follows the voltage at B+ because the voltage drop in the Trio diodes is almost the same as that in the larger rectifier diodes.

If battery voltage drops, the regulator circuitry senses that fact (at D+) and increases the current flowing into DF until battery voltage is restored. Most regulators also include some form of temperature compensation. A cold battery requires slightly higher voltage to fully charge. A temperature sensing element in the regulator increases output voltage at low temperatures.

Current limiting is not provided in alternator regulators since the alternator magnetic structure inherently limits the maximum current that can be produced.

[itchyfeet]

You forgot to cut and paste the drawing

[Oldiebut goodie]

It was an old article and the drawings were missing!
All the guff that I can find refer to voltage rather than current - the current required appears to be so small as to be irrelevant, the voltage being more important. I suppose that the potential difference across the blue wire and the alternator forming the earth can be affected at start of excitation to cause the lack of immediate rise in output. With the dodgy earths that we all experience at some point this could be a factor in delaying excitation. But then a bulb wouldn't illuminate fully, I dunno!

[Oldiebut goodie]

Extract from Pelicanparts.com (good source of info even though from over the water ) page here http://www.pelicanparts.com/techarticle ... eshoot.htm

The Bosch alternator is incapable of self-excitation, or "boot-strapping" itself to an operating condition. Older DC generators and some U.S. alternators have residual magnetism retained in the core, or some other scheme to get enough field current to get themselves up and running. The Bosch alternator uses a different scheme. The charge warning lamp is connected between the ignition switch and the D+ terminal. When the car is first started, there is no output from the alternator at either the B+ or D+ terminals. The voltage regulator, sensing no output, is attempting to command maximum field current... it effectively shorts the D+ and DF terminals together. This places the D+ terminal close to ground potential, because the resistance of the field winding is not large. This means that there is +12 volts on one side of the charge warning lamp, and the other side of the lamp is grounded through the alternator field winding. Current thus flows through the lamp, lighting it. This same current, however, also flows through the alternator field winding, producing a magnetic field. This magnetic field is what the alternator needs to start up, and if everything is working correctly, that's exactly what happens. The alternator now begins to develop identical voltages at the D+ and B+ terminals. The D+ terminal is connected to one end of the charge warning lamp, while the other end of the lamp is connected to the battery via the ignition switch. Since the B+ terminal is hard-wired to the battery, and since both the D+ and B+ diodes are fed from the same set of windings in the alternator, no voltage difference can exist between these two points. The warning lamp goes out.

Goes on to say that Bosch state that a 2w minimum bulb is required to enable excitation.

[bigherb]

Unlike a generator, the alternator is not self-exciting at start-up. A generator has field poles made of soft iron which hold a residual magnetism. The alternator field structure has little residual and thus has almost no output unless field current is supplied.

Really!

[itchyfeet]

This means that there is +12 volts on one side of the charge warning lamp, and the other side of the lamp is grounded through the alternator field winding. Current thus flows through the lamp, lighting it. This same current, however, also flows through the alternator field winding, producing a magnetic field. This magnetic field is what the alternator needs to start up

If the field winding is low resistance then there will be little voltage on the blue wire ( it will be close to ground) but a current will flow limited by the resistor or lamp resistance, lets say 100mA
If the blue has a bad connection it will reduce the current (may still be plenty to light an LED 10mA is enough)
A relay coil to ground will also reduce current as it will have two paths to ground from the 100 R resistor or lamp
voltage alone can't create a magnetic field ( I think) a current is needed

[CJH]

Extract from Pelicanparts.com (good source of info even though from over the water ) page here http://www.pelicanparts.com/techarticle ... eshoot.htm

The Bosch alternator is ...

That's the best explanation I've read so far. And if the description covers my 90A Bosch alternator it confirms what I encountered when I tested without the blue wire yesterday - I couldn't get it to self-excite.

[CovKid]

Drop your trousers next time - it may get excited then