Alternator W terminal

[whiteoaks7]

Does anyone know what comes squirting out of the W terminal (the one connected to the rev counter)? Is it a pulse train or voltage proportional to the rpm? Also, is the standard rev counter a voltmeter or something more complex? I want to measure rpm with a PIC (fun winter project).

Any answers to any questions gratefully received.

 

[pvb]

The W terminal generally outputs an unrectified AC voltage, the frequency of which is proportional to the engine speed.

 

[VicS]

A bitof very basic stuff here

 

[William_H]

I think the output is AC at a freq 3 times the rotation speed multiplied by half the number of poles on the field coild (rotating part.) (typically again I think 8 or 12 ) It will not necessarily be sine wave as it comes from the centre of the star connected 3 phase output coils. Yes only has an output when the alternator is charging.
The voltage will be something like 7 volts RMS... good luck olewill

 

[wiggy]

On a similar note, what would folks suggest I buy and where from, to get rev info from w terminal on my replacement (non yanmar) 1GM10 alternator?

 

[pvb]

Not always from the star centre, can also be just a single phase tap.

 

[DaveS]

I presume the 3 in your formula is the assumed ratio of pulley diameters. Measuring these would presumably get the actual number to use for a particular set up. However...

Assuming a standard V belt drive, you can't just measure the outside diameters of the pulleys. What is required is the diameters (or radii) to the neutral axis of the belt, which will presumably be closer to the top than the bottom, especially if it is notched. Anyone know how do do this easily? (Or is this being a bit fussy and simply holding a ruler against it and guessing good enough?)

 

[William_H]

High Dave no the 3 in the formular relates to getting output from all 3 phases at once. Now it has been pointed out that it is not always so and that some are connected just to one phase in which case the factor of 3 is not applicable. No i never tried to relate engine speed to alternator speed. olewill

 

[Yngmar]

Just because I had to look at this today and brought my DSO203 (which is great by the way), this is what comes squirting out of the W terminal on our Perkins at about 800 rpm:

 

[VicS]

Just because I had to look at this today and brought my DSO203 (which is great by the way), this is what comes squirting out of the W terminal on our Perkins at about 800 rpm:

View attachment 50727

why is it that shape not more like a sine wave?

 

[cliff]

why is it that shape not more like a sine wave?

Simples, as the Meerkat says, 'tis a crappy, non linear, digi 'scope!

 

[William_H]

Simples, as the Meerkat says, 'tis a crappy, non linear, digi 'scope!

No I am sure it's not a crappy oscilloscope but actually what the wavefrom is. My guess is that the picture was taken while alternator was charging a battery and that the point on each half sine wave where current began to flow into battery is where the load then cut the wave form peak off. But only guessing.
A funny story years back a aircraft mechanic presented me with an alternator and the appropriate paper work where it had just been overhauled. It ould not work and in fact had wiring torn from the pick up outer coils. After some investigation itseems that the original alternator had an earth wire connected. This engineer in fitting the replacement had a spare earth wire left over and fitted it to the w terminal. (by mistake) Now on many small aircraft you actually switch on the alternator after starting the engine. The switch providing field current. The poor alternator on being excited found a short on the outside pick up coils via rectifiers and actually rotated the centre section within the mount until the wires broke off.
I had an old Ford way back (1982) with an auto choke. It had a bimetalic spring to open choke when it got hot. To speed up the heating it had an electric heating element running off the W terminal. Worked well. Only opening the choke after the engine started. W terminals good for all sorts of things like a kind of VSR. olewill

 

[superheat6k]

Sorry to resurrect an old thread, but I am trying to calibrate my new electronic tachos.

Can someone help me determine the Speed ratio I need to set.

I am assuming the alternator is 6 pole, as this seems to be the norm.

The belt ratio is ~ 75 : 150 (alternator pulley OD : engine flywheel pulley)

The FSD of the meter is 4,000 RPM

If someone can help me with the formula I can measure the pulley sizes to get this more accurate.


Or do I simply measure the engine speed with a handheld tacho at idle then adjust the speed ratio setting until the new tacho reads the same ?

Thanks.

 

[rogerthebodger]

The pulley ratio is a 2:1 increasing so if the engine is doing 2000 rpm the alternator is doing 4000 rpm.

I used a tacho on the engine to set my alternator driven tacho. I also have a tacho generator driven from the injector pump which is a 2:1 reduction.

 

[halcyon]

Depends on alt design, but normally 9 pluses a rev.

Brian

 

[Boater Sam]

You can use a protractor on the large pulley and turn the alternator pulley a number of full turns, read the angle on the protractor and do the sum.

I think there is a phone app that works as a tacho by counting a sound,

 

[pvb]

Or do I simply measure the engine speed with a handheld tacho at idle then adjust the speed ratio setting until the new tacho reads the same ?

That's an easy solution.

 

[Ian_Edwards]

I went through all this earlier this year and wrote it up.
This is a quote from the post:
I finally got around to testing the taco, after getting the pulley diameters from Canada.
The crank pulley diameter is 127.86mm
The alternator pulley diameter is 58mm
Ration of 2.2044 to 1
12 pole alternator, 6 pairs of poles
Pulses per rev= number of pairs of poles x the pulley ratio.
6 x 2.2044 = 13.23 pulse per rev, for reference the standard number of pulses per rev for the original alternator is 10.29
This convert to frequencies of:
220 Hz at 1,000 rpm; 440 HZ at 2,000rpm and 660Hz at 3000 rpm
This is what you need to know to test with a waveform generator.
I programmed the pulses per rev into the taco and it seems to work.

The rest of the post is here:
VDO Tachometer- Bulb holder for the instrument lights

The tacho is working and correct, I checked it with a hand-held optical tacho.

 

[superheat6k]

These meters are not very sensitive to changes in the speed ratio and then show a variation after the engine is revved up and then set back to idle. So I finally set the speed ratio to the same level but expect I wil need to adjust this over time.

I did find another version of this meter being sold under the brand name Wema, which suggests using a ballast resistor in series with the pulse input to reduce noise on the pulse wire. It suggested 20k ohms, but for now I have used 3.3 k ohms as that was the highest I had available. The reading has improved so I will likely increase the value, but I wonder if I need another device such as a small capacitor to distinguish the pulse signal from the background noise further.

 

[Ryanmerc]

These meters are not very sensitive to changes in the speed ratio and then show a variation after the engine is revved up and then set back to idle. So I finally set the speed ratio to the same level but expect I wil need to adjust this over time.

I did find another version of this meter being sold under the brand name Wema, which suggests using a ballast resistor in series with the pulse input to reduce noise on the pulse wire. It suggested 20k ohms, but for now I have used 3.3 k ohms as that was the highest I had available. The reading has improved so I will likely increase the value, but I wonder if I need another device such as a small capacitor to distinguish the pulse signal from the background noise further.

These meters are not very sensitive to changes in the speed ratio and then show a variation after the engine is revved up and then set back to idle. So I finally set the speed ratio to the same level but expect I wil need to adjust this over time.

I did find another version of this meter being sold under the brand name Wema, which suggests using a ballast resistor in series with the pulse input to reduce noise on the pulse wire. It suggested 20k ohms, but for now I have used 3.3 k ohms as that was the highest I had available. The reading has improved so I will likely increase the value, but I wonder if I need another device such as a small capacitor to distinguish the pulse signal from the background noise further.

Hi,

I have a 1987 rover diesel and my rev counter is reading about 30% highier. Very curious about you resistor and capacitor fix. My original wire from the w terminal has a built in split with a spade terminal and there is nothing attached to it.

I assume the resistor to be in series with the w line and the capacitor to w terminal to body ground. Also, what wattage resistor is needed?

Ryan