Monday, July 20, 2009


24V GeneratorThe generator I removed some while ago has been cluttering up the place.

It's a remarkably heavy device, which has just been getting in the way since it was taken out.

I finally got round to stripping it, ready to fit the replacement bearings it needs.

What really surprised me was that the rotor, the spinney bit from the middle you can see, is not magnetic in any way.

Anyway, the main thing is getting replacement bearings which I shall pursue during this week.


  1. Anonymous10:37 PM

    I would have assumed those square inset areas in the armature were magnets -- like the flywheel magnet in a magneto.

    An alternator design like this is a new one on me. I can't read what the label on the side says to see if there's anything googlable.

    Was it generating 24V okay before you pulled it?

  2. Hi Anonymous

    Strange isn't it.

    Yes it was producing 28.8 volts just fine when the main engine was running.

    There is not even anywhere that I can see that current can be induced onto the armature, to produce magnetism that way.

    I'll photo the label when I'm next on board, though I recall thinking 'hmm this is a pretty good one' once I'd cleaned it up a bit, as it produces 10 - 100 amps at 800 to 4200 rpm.



  3. Anonymous3:14 PM

    Is any part of the stator permanently magnetized?

  4. Hi Anonymous

    By stator, I presume you mean the middle bit?

    If so, none of it is magnetic.



  5. Anonymous10:12 PM

    No, the stator is the outside bit that doesn't spin. If some of it was magnetic, then the spinning iron bits (rotor) would "stir up" the fields on their way around.

  6. Hi Anonymous

    The learning never stops...

    I'll check when I'm on board again, tomorrow.



  7. Anonymous3:16 PM

    After reading up on induction generators I am inclined to think that this is what you have here.

    In these animals the magnetic fields are created by the current flowing and vice versa. When it first starts spinning, only a small magnetic field is needed to get the current flow going, then after that the currents and field strengths build on one another.

  8. Hi Anonymous

    Is this good or bad do you think?

    Just curious.


  9. Anonymous3:31 PM

    Neither good nor bad, I wouldn't say.

    On the up side an induction generator has no brushes or commutator to worry about. The design is pretty simple and low maintenance.

    On the downside, this unit weighs a good deal and is huge.

    An automotive/lorry alternator would be smaller, and probably more easily serviced whilst in the back of beyond. But it would also be much more likely to need servicing or replacement.

  10. It is very, very, heavy!

    In Lady Jane, weight is not an issue though. So I'll be grateful for the low maintenance.



  11. Hi Tim I posted on MBY "Practical boat owner forum this morning under "Generator" and got sever semi informed answers1 You could always ask Downing and mills the question? I seem to remember something about exciting the coil and then it all will start to generate electricity But I`m not sure! should be some more answers tonight on MBY

  12. Hi Rob


    I have the new bearings for it nw, so it should all go back together this weekend.

    Not sure when I'll get to run it up again. Hopefully soon!



  13. Sorry the answers weren`t more informative but I bet it will work when you get it back togeather!
    Take care

  14. Hi Tim Latest answer from MBY forum seems to say what you believed!

    A whole lot of answers. It seems to me that we can't answer the question without more detail of the DC generator.
    The typical car generatr is often called an alternator because indeed it generates AC which is rectified to DC output. The magnetic field required for all generators is produced by DC being fed via slip rings and brushes to the centre rotating component. They usually have a coil mounted with an axis in line with the shaft. The mag field is directed from the ends of the coil via pole pieces, steel bent around so that the N and S appear at the circumference of the component. There are multiple pole pieces ( at least 4)
    In this case there is no residual magnetic field (the pole pieces are soft iron) it requires some power from the battery to energise the field initially.

    Old style care generators had the magnetic field produced by coils around the inside of the casing with pole pieces being just a stub to bring the field close to the rotating armature. Usually one coil divided in half (each side) so just one N and one S pole. These pole pieces did retain residual magnetism and relied on that residual magnetism to produce some output which then fed the field coils to reinforce the mag field.
    The current is actually generated by the rotating armature and the AC is converted to DC by the switching and collecting action of the commutator and brushes. Hence all output current goes through the brushes hence maintenance problems. This Dc generator is very similar to a DC motor so is the type used in a combined starter generator (Dyna start)

    Then occasionally you will see generators DC or AC with or without rectifiers that rely on a permanent magnet to provide the magnetic field. Old bicycle light generators or wind generators. Because the field is permanent you lose the ability to control the power generated by controlling the field coil current. But you save wasted current.

    Then there is the large aircraft generators. AC. They use a brushless generator (alternator in this case)
    Here the magnetic field is first produced by current from a battery ( or residual + from the output and controlled) in a coil around the outside of a small armature (rotor) this armature produces AC which is immediately rectified and fed to a larger rotating field coil. This rotates inside collecting coild on the outside which produce the output power. There are no slip rings commutator or brushes so with just bearings to support the rotor that may wear they are extremely reliable .
    This type involving 2 generators are usually only for high power ie many Kilowatts.

    Bottom line does a gen require a residual magnetism some do some don't. The old car type used top have to generate some voltage from residual to get an output to reinforce the field so bringing it up to full voltage which then enabled a relay whci connected it to the battery. good luck olewill