EMS wrote: . . . . I stick to what my physics teacher in high school and my electronics professors at university told me.
You're correct of course Mike that a D'Arsonval meter movement pits an electromagnetic field (and what is EMF?) generated by a *current flow* against a permanent magnet so as to make the meter needle move across the scale . . . . you could call that a 'current meter' if you like, but while the change in current is what moves the needle, that current is changing only because the voltage applied to the meter is changing, which is what we're actually measuring - Were there no voltage present, there would also be no current, hence my 'chicken or egg' comparison. If you'll allow me to call that a voltmeter (which is what it was designed to measure) then we can use that voltmeter to measure the voltage drop across a precise shunt of known value and we can then measure current
If your voltmeter really *was* configured as a 'current meter', it would be basically useless because it could only measure such a limited range of current before it blew the meter coil (or wrapped the needle around the peg) whereas we can design that same meter to measure voltages from zero to hundreds of volts, with no need for other circuitry to expand the scale - If it was to function properly as an ammeter, it would need a series of switched shunts for us to measure the (voltage) drop across
In short, I guess you *could say* that one man's voltmeter is an ammeter . . . . even though it does a lousy job of measuring current while it's still an excellent voltmeter ;)
Now, if we're using a DMM to measure things, we'll have to have a completely different discussion
Has anyone had their plates turned by a machine shop like a brake rotor? Does anyone know what a new set of plates measure? I measured mine and I believe one was about 21mm and the other about 13mm. I'd like to turn mine flat and make up the difference with a thrust washer.
The diameter of one is slightly smaller than the other. Mine has worn a slight edge into one plate. I'd also turn them to identical diameters. If they are flat, the slippage and heat generated should be minimal. This should make the alternator last much longer.
A voltmeter creates a parallel circuit to the circuit being measured. The meter resistance is high to reduce the load effect of the meter. The total resistance of the parallel circuits is nearly equal to that of the circuit under measurement. The voltage indicated on the meter will be the voltage across the circuit under measurement in addition to the meter and will be close to the voltage of the circuit under measurement without the meter in place. The meter reacts to current indicating voltage. The only meters that react to voltage are electrostatic meter movements. Oh, it's been a while since I ventured back into those books!
Holy smokes, what a madhouse of posts and counter posts. Some right on and some being contrary (it seems) just for the sake of being contrary.
First off, a simple definition: As one said earlier - VOLTS is like water PRESSURE. You can have Voltage (like a battery sitting on the bench) without having current/amperage flow (like water pressure without water flow). It's just that you cannot determine what the battery's voltage is without measuring it. All modern Voltmeters have such HIGH internal resistance that, for ALL PRACTICAL purposes, it has NO loading affect on the battery. (While it is, technically, true [for the nitpickers] that a tiny/miniscule bit of amperage/current is needed to make the Voltmeter work, for all discussions here - the Voltmeter HAS NO AFFECT ON THE BATTERY/ALTERNATOR CIRCUIT!!!!) It JUST measures VOLTAGE - period.
Now, about that Ammeter. As one also said earlier, the ammeter is just a voltmeter inserted into the circuit, measuring the voltage drop across a shunt. Let me say that a little bit differently to give a little bit of clarity. Think of this circuit: Battery +post wired to one side of a light bulb - other side of the light bulb wired to the -post of the battery (this would be called a COMPLETE circuit). Assuming the battery was charged up, the light bulb would shine brightly (bulb voltage matches the battery voltage). As some have said, the bulb would be called a LOAD on the battery because it would draw/pass amperage/current from the -post to the +post of the battery. If you wanted to know what the amperage was that the circuit was passing, you would INSERT the ammeter into the circuit by taking one of the wires from the bulb and connecting it to one post of the ammeter and placing a new wire from the other post of the ammeter back to the bulb. The bulb would still shine brightly with the ammeter in the circuit. INSIDE the ammeter, as someone called the shunt, is a copper bar that is capable of passing the maximum amperage/current of the ammeter (say 10 amps). This copper bar is extremely precise and calibrated to be about 1 ohm (or less) but thick enough NOT to restrict ANY appreciable current flow through the circuit. This means the ammeter will not, to anyappreciable degree, affect current flow through the circuit. The precise 1 ohm (or less) resistance will create a very tiny bit of voltage drop across the copper bar (end to end). It is this tiny voltage drop that swings the needle of the meter. It should be noted that the voltage drop across the copper bar is a variable according to how much amperage/current flowing THROUGH the copper bar; more current - more voltage drop - more needle swing. It is important to note that the Ammeter MUST be inserted INTO the circuit and carry ALL of the current flowing through ALL of the CBX circuits (this should convince anyone that the CBX voltmeter is NOT an ammeter).
Some/many of us old timers may remember that most of the cars of that time had an ammeter on the dash and it would show both + and - directions; this showed whether the battery was GIVING current -or- RECEIVING current (when the regulator raised the voltage from the generator high enough to force charge into the battery, the ammeter would swing + , when the engine idled, the ammeter would swing - ). I liked that system; it let me see immediately that, at speed, whether the charge circuit was working. Now all you see is a voltmeter but that doesn't tell you if the battery is taking a charge or not. But I digress.
So, while the Voltmeter measures the voltage PRESSURE of the battery and does not require a complete circuit to work, the Ammeter DOES require a complete circuit to work.
Relative to the CBX charging system, it is the job of the Alternator to provide enough Voltage (13.8 - 14.4) to FORCE the Alternator current backwards into the Battery, charging it. Think about it; if the battery is sitting at 12V and the Alternator is creating 12V, the battery will easily resist the charging pressure of the Alternator. So the regulator lets the Alternator voltage to climb higher so it can force a charge back into the battery. [By the way, the regulator is a DUAL FUNCTION device; one part rectifies the AC of the Alternator into DC needed by the Battery and all of the CBX circuits; the other part controls/regulates how high the Alternator voltage climbs.]
While the Alternator rpm is high enough to carry the entire loads of the CBX circuit, the battery becomes, also, a load and takes off some of the current to maintain its charge.
When the Alternator rpm drops below the 12v (or so) level, the battery takes over the CBX circuits and keeps things running. That's why, at idle, you might see the headlight slightly dim, as you transition from the 13.8v of the Alternator to the 12v of the battery.
It has been said many times, in many places, that the battery has TWO functions: 1) to START the engine and 2) carry the 'loads' when at low speed/idle.
As far as the mechanical part of the Alternator, the clutch plates, of course, couple the engine to the rotor. No one has mentioned the fact that as you draw current from the Alternator, there is a little item called "Back EMF". Simply, it is what makes an Alternator need horsepower to keep turning. When current flows through a Rotor, it creates a magnetic field that is OPPOSITE to the magnetic field of the Stator and tries to slow/stop the rotor from turning. It is the engine force, through the clutch, that keeps the rotor turning. In effect, the rotor of the Alternator is a MECHANICAL load on the engine. Thus, if you pulled the Alternator connector OUT of the regulator, there would be no mechanical load on the engine and the clutch would not slip (in effect, the rotor would be 'free wheeling'). With the connector re-inserted into the regulator, any charging current (think maximum) would place a higher and higher load on the clutch connection to the engine. If you had a slipping clutch, it would only slip under high charging current loads; something that, I think, is ofter overlooked in diagnosing alternator problems. Shimming the clutch increases the spring pressure and removes the slippage of a high charging draw on the Alternator.
Sort of (hah) a long description but, hopefully, a little clarity (at a practical level) and recap of what has been said previously.
Oh, yeah, a last minute thought - about turning the clutch plate faces. Anybody who has seen a mechanical breakdown of the alternator drive has seen that the clutch faces are perpendicular to the shaft line and engine drive. I would NEVER turn or trim the clutch face by hand as there would be a great possibility of making the clutch face NOT at right angles to the shaft line. That might not be too bad on the engine plate (A plate) as it kind of 'floats' on the primary shaft, so a small mis-alignment might not hurt (the plate could 'float' into alignment) -BUT- on the Rotor plate (B plate), it is solidly attached to the shaft and if the clutch face is NOT at a perfect right alignment, you won't have a full face-to-face contact and could have a high 'slip' component. So I wouldn't do them by hand, I'd have them chucked up in a lathe and turned true to perpendicular to the shaft line (and I'd only take off enough to get a 'clean' face).
Jim-Jim
