Alternator running sensing

I have an ongoing project which involves a generator where I need to detect and confirm that the generator is running.
So the simplest a easiest that I have done so far is do a simple voltage divider circuit and monitor the increased voltage ( caused by the alternator producing a charging voltage). Therefore the generator must be running..

However, I am looking for a better approach that will allow me to perhaps monitor the rotation from the alternator and sense the cranking rpm.

Does anyone have any ideas or input on what components and methods to add an external sensing devices to an alternator? (not all my generators have an integrated RPM sensor,, so i'm trying to develop a universal application here)

I'm thinking of a hall effect sensor and some magnets on the alternator fan/pulley,,, or any other suggestions or ideas someone might have?

thanks

A "prox sensor" (proximity) can detect a bolt head on a rotating shaft. No need to add a magnet.

They are mostly used in industrial automation so prices can be high. But keep looking to find the cheaper ones.

A reflective optical sensor such as a QRE1113 may be suitable unless there is a risk of the sensing area getting dirty. All it would need would be a blob of white paint on a dark coloured rotating part.

However if you are talking about a 12v automotive alternator I think I would be quite content to monitor the voltage.

...R

If one adds a magnet to the back side of a hall effect, they can be used to pick up the metal bolt head.
i.e. no need for a "travelling magnet".

Standard practice to check the engine is running on dual fuel cars is to sense the alternator voltage before the regulator, i.e open it up and put a wire in. I don't see how you could possibly detect direction of rotation with the alternator, and I'm sure you need to look more closely at whatever is rotating it, which I assume is not a motor

You could try AC-coupling the output of the alternator and using an ADC pin to see if you can detect ripple, an artifact most 3-phase alternators produce in detectable quantities.

You'd probably want to protect the pin with a pair of Schottky clamping diodes to keep the pin one diode drop away from either rail.

MorganS:
A "prox sensor" (proximity) can detect a bolt head on a rotating shaft. No need to add a magnet.

They are mostly used in industrial automation so prices can be high. But keep looking to find the cheaper ones.

excellent idea,, I will dig into that, hopefully there is an affordable version available

Robin2:
A reflective optical sensor such as a QRE1113 may be suitable unless there is a risk of the sensing area getting dirty. All it would need would be a blob of white paint on a dark coloured rotating part.

However if you are talking about a 12v automotive alternator I think I would be quite content to monitor the voltage.

...R

the dirt aspect would be my concern also. In addition, the operating environment may not always be exposed to sufficient light to ensure the reliability of picking up the passes. But never the less,,, I do appreciate the input and ideas.
Yes,,although not a strictly "automotive" environment ( variety of 8KW to 30 Kw diesel generators), the alternators are operating either in a 12VDC or 24 VDC application.
Any ideas on how to measure and react to adjustable threshold voltages to apply to the arduino analog inputs (max 5 v)?
My idea is to have an adjustable setpoint that (for example) says "anything above 13.5 volts,, it must be running), below that,,,its not.

Nick_Pyner:
Standard practice the engine is running on dual fuel cars is to sense the alternator voltage before the regulator, i.e open it up and put a wire in. I don't see how you could possibly detect direction of rotation with the alternator, and I'm sure you need to look more closely at whatever is rotating it, which I assume is not a motor

I don't think I described my project as detecting "direction" of rotation,, but rather,, just the fact of rotation to compare the rate of observed to the rate of expected to make a determination if its going faster than the minimum (while cranking,, attempting to start). If its faster than the minimum,, it must be running,, and I can stop engaging the starter.
I also mentioned i believe that this was in a generator,, so yes,,, it is a motor (diesel) that is turning it.

RobG3987:
excellent idea,, I will dig into that, hopefully there is an affordable version available

I already described an "affordable" version i.e. hall switch with a 3mm maget on the back, couldn't be more than a couple of dollars.

OK, I read too much into rotation. The alternator wire I mentioned should be quite enough, or an oil presusre switch. I thought a starter lockout was common on diesel generators anyway, as they are often operated unattended.

Alternator has 3 external diodes that get pulsed once each per rev. The pulse should be detectable by a Hall Sensor.

bluejets:
If one adds a magnet to the back side of a hall effect, they can be used to pick up the metal bolt head.
i.e. no need for a "travelling magnet".

The DIY gear tooth counter.

RobG3987:
the operating environment may not always be exposed to sufficient light to ensure the reliability of picking up the passes.

Sensors like the QRE1113 provide their own light.

...R