Thread: alternator's w wire & digital multimeter?

roughly speaking..

ignoring things like bearings pullies fans housings and slip rings...

your alternator consists of 2 sections of 2 parts each.

the front half has the stator and the rotor and the back half has the rectifier and the regulator


the recitifier pack and the regulator are in parallel to each other. the rectifier is attached to the stator (the outside part of the alternator) and the regulator is attached to the rotor (the inside spinny bit of the alternator)

the rectifier turns AC into DC with just a little voltage drop across it because its just a bunch of diodes doing their thing.

The regulator sits along side of this and reads the voltage coming out of the rectifier then feeds in power from outside the alternator/rectifier (which is why you need to have excitation/existing power for an alternator to work as opposed to a generator)

as the rotor spins the power from the regulator creates a magnetic field in the rotor, this then via the 2 wedges of interleaved triangles makes a series of interlocking N/S poles as the alternator turns these pass by the coils that make up the stator and as they pass they create current, positive then negaitve which is why they are called alternators, because the unfiltered electickery coming out of them changes directions and thats where the rectifier comes in. it turns AC into DC. and DC charges batteries.

Because the output voltage of the alternator is controlled as part of the initial generation and not after generation you can calculate the theoretical maximum voltage off of the windings, the simple way is to take the maximum regultated output voltage and add 3v (most diodes drop about 2v)

to work out engine RPMS from a single winding you need to know a couple of things

1. how many "teeth" on the rotor
2. where are you tapping the winding (will you get both the positive and negative sides of the current or just one side)
3. whats the MA ratio from the crank pully to the alternator pully

once you have that.

you can work out from #3 how many times the alternator will turn for each turn of the crank and once you know that you simply multiply it by the number of teeth on the rotor. (both north and south poles)

that gives you the total number of cycles you will get that go past zero volts which is both sides of the sine wave (which is important because some cycle counters count both -ve and +ve as one each with 0v being the trigger point and this gives you much finer resolution on your RPM count)

IF you are using a simple 0/+5v logic counter all you need to do is drop the voltage down to 5V (and DONT use a simple resistor here bad things happen, I would be using an optic coupler or at least a transformer) and divide the above number by 2 (since you wont see the negative sides of the wave)

what would be the purpose of that zenner? should all these be in serial or in parallel? what if I just use couple of 1n4007 before and after the LED (the second one to get rid of the negative halfwave? or if it is ground there shouldn't be any negative half waves coming from the other side?) and a current limiting resistor for that led?