Alternator from a boat motor

[wbd]

Ok, I hope this is allowed. I am an EE but primarily power circuits and not generators. This is a small alternator for a small diesel motor on a sailboat. It has not been charging the battery so the alternator and voltage regulator were removed.

When I spin the alternator by hand it is very jerky. I measured the resistance of the alternator and got 0.8 ohms. The meter leads plus the alligator clips was 0.6 ohms so the coil is 0.2 ohms. There are only 2 leads coming off the alternator. When I spin the alternator by hand as fast as possible I only get about 1 volt out but not at sync speed of 900 rpm but would have expected more.

So, my questions are:
1. Why is it so jerky when spun?
2. Is this alternator ok?

On my scooters (Vespa) the alternator is 3 leads and generates ~60V so I was expecting the same here.

 

[crshears]

An alternator will have diodes in it, so the resistance measured in one direction should be quite low but very high if the polarity is reversed; if not, a diode could be fused closed.

Jerky motion is likely due to cogging, if I'm not mistaken, meaning additional mechanical effort is required to overcome the magnetic forces of the stator and rotor poles, therefore the mechanical resistance to rotation through one revolution will not be smooth. Either that, or there's a bad bearing...

Only two wires: possibly a permanent magnet alternator that uses some manner of external regulation?


CR

"As iron sharpens iron, so one person sharpens another." [Proverbs 27:17, NIV]

 

[waross]

Do you have a picture of the alternator and regulator?
There are a couple of standard formats for automobile alternators.
For a purpose built alternator and regulator the sky is the limit for configuration.
The original automotive alternators had two wires.
One wire was the output and the other was the control from the regulator.
These had an internal, three phase bridge rectifier.
Later types had an internal regulator.
Some needed external power from the battery to start charging.
Automotive alternators had a wound field.
The cogging suggests either a permanent magnet field or really bad bearings.
Regulators: Automotive regulators control the voltage by varying the current through the rotating field coil.
Some automotive alternators had an extra terminal that was used to light the idiot light.
Motorcycle alternators on smaller bikes had a PM alternator and a bridge rectifier.
Regulation was by a Zener diode to ground.
Excess voltage and energy was dissipated as heat by the Zener heat sink.

Jerky motion suggests permanent magnets.
That would also suggest 1 or 3 wires out of the alternator, not two wires.
I am sure that it works when it works but there are too many possibilities for more than a WAG with the information given.
What size diesel engine?
What amperage alternator?
The smaller alternators tend to be the most "interesting".

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[wbd]

Waross - there is a pic attached.

 

[waross]

OK, after two cups of coffee. grin
What that tells us is that it is not three phase. (8 poles)
Do you have continuity between the two leads?
Typically, one end of the windings are grounded, but it is possible that for marine use, both ends of the winding are brought out for an isolated ground system.
Are there windings or permanent magnets in the part not shown?
Did you check the voltage on AC or DC?
From what we see in the picture, we would expect AC out.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[wbd]

Waross, I have continuity (about 0.2 ohms) and there are only 2 wires that go to wires wrapped around the pole pieces. When spin it with an electric drill I get varying voltages depending on the drill speed. The voltages are AC as expected as the two leads go to a Voltage Regulator. I suspect the voltage regulator is shot.