W terminal on alternator

[heliosuk]

Odd one this but know someone here has the answer. Have an alternator on a narrow boat that has failed for various reasons. Seems like it was fitted incorrectly in that it was fitted without a W terminal so no rev counter. Does anyone know which fiels connector the W terminal would normally come off. Working in the dark here to a certain extent. Frankly don't like the set up but friend short of cash big time so need to find a way around the issue.

Any advice in relation to how to make the rev counter work greatly received. Charging system is now working correctly.

Thanks peeps.

 

[ericmark]

Different makes use different methods of rectifying the AC to DC with a delta wound only option is connected to one phase wire but star wound often connects to star point there are also single phase units and there are 4 and 6 pole stators even with same make like the CAV units all which will effect how a rev counter will work.
The voltage does not vary that much. Around 9 volt from star point to 12 volt on phase leg and likely the rev counter can connect to either but it may not register correct speed. (For 12v unit)
Most likely is phase winding which is what I would use if designing alternator. I would guess the alternator is quite a high output around 70A or more I know son has two on his both 70A and to get that high output it is common to rectify the star point as it reduces eddy currents within the unit. So even if designed for star point and you connect it correctly it still can give odd readings. Also regulators on boats are often of step charge type where to start with max output until high volt limit is reached then charges at high volts until amps drop below set figure then switches to low volt float charge. This can result in alternator completely switching off during part of cycle. This would mean rev counter would fail.
There are rev counters that work by painting white mark on front pulley or flywheel and work with a infer-red light and sensor that is aimed at reflective spot. At speed of prop I would guess you could use a push bike speedo to measure prop shaft speed. Out of interest why does it have rev counter? My multi-meter will measure Hz at most voltages and if rev counter is to get Hz correct then cheap multi-meter may be way forward.

If an old alternator I may have some details but stopped auto electrics back in 1994 so well out of date now. Although with son having narrow boat still have interest.

 

[heliosuk]

Hi Ericmark,

Thanks for this very, very informative reply. I grasp the principles of what you say but don't fully understand. Is there anyway I can contact you to discuss off forum as there are some other issues with it where your obvious expertise on the subject and affiliation to narrow boats would be most helpfull and appreciated. It's a long and sorry tale but making exceptional progress so far in rectifying appaling build quality and engineering design on a narrow boat. I'm a professional engineer who just happened to get involved in an emergency situation and now feel obliged to correct commercial failings so to speak!!

The post was very usefull to me but I like to get the best options and document it with the record of the boat.

Kindest regards.