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No generator output below 1000 RPM

PostPosted: Sat Oct 04, 2014 10:13 am
by peg_pilot
My ancient Lucas generator seems to be fine above 1000 RPM, but it puts out nothing below that speed. I know my spiffy microprocessor controlled voltage regulator is applying full field excitation, so I wonder if the problem is demagnetized pole pieces ? Does that ever happen ? Is there a way to re-magnetize them ? Thanks for any ideas.

Re: No generator output below 1000 RPM

PostPosted: Sat Oct 04, 2014 11:20 am
by billwill
It is unlikely to be that, because you say it does generate output above 1000 RPM. The current flowing at that time would have restored the residual magnetism.

Generators simply don't have much output at low revs, which is why many owners switch to an alternator, there may be nothing wrong with your system.

The dynamo regulator ensures that there is no path from dynamo back to the battery until the dynamo is creating more than the battery voltage. This ensures that the current doesn't flow from battery to dynamo which would make your dynamo try to act as a feeble starter motor.

Re: No generator output below 1000 RPM

PostPosted: Sat Oct 04, 2014 12:01 pm
by oldchieft
Look at the brushes and commutator.

Back in the day a skim in a lathe for the commutator, followed by under cutting the mica between the segments was the norm for repairing dynamos.

Brushes need to be a neat fit in the holders so they don't angle and jam, but free to slide,

Jon the Chief

Re: No generator output below 1000 RPM

PostPosted: Sat Oct 04, 2014 2:15 pm
by Plus 2

This may help some understand actually how a dynamo works and why such an action of remagnetising would not be an issue. ... as-dynamo/ It is covered part way through the article so don't assume it is just talking about re-polarisation.

However some say if you point the car direct North South and whack the exhaust with a hammer it may help but could also magnetise the chassis :lol:

Oldchieft has the best solution which may show some improvement but small comms are not always necessary to undercut unless badly worn down and often a clean up of the comm without dismantling and new brushes may again show some improvement.

We would bed new brushes into the comms when I was in the Royal Navy on large DC motors but again with small comms I dont usually bother.

Personally you may be worrying about nothing as I still have a dynamo on the Lotus and it performs about the same as what you say yours is and fitted new brushes around 5000 miles ago.


Re: No generator output below 1000 RPM

PostPosted: Sat Oct 04, 2014 9:05 pm
by Galwaylotus
One of the reasons that alternators replaced dynamos in the automotive industry is that the latter can't tolerate high rotational speeds and so is geared to run at speeds similar to the engine. Cut-in speed on a dynamo is usually around 1000 rpm so there is no charging below that speed. An alternator can, by its design and construction, safely rotate faster than a dynamo so it is geared accordingly and is generating current even at engine idle. This, and the fact that an alternator is more efficient, encouraged Chrysler to introduce alternators in the early 1960s to cope with the increasing electrical demands of cars at that time.
In short, your alternator was probably never designed to charge at idle engine speeds. It's cut-in is probably above that speed! 8)

When my voltage regulator failed due to corrosion, I converted to negative earth and installed a Lucas alternator. It's one of the best changes I've made to the car. I have no hesitation in making changes to improve reliability if it doesn't change the characteristics of the driving experience hence, the alternator, tandem master brake cylinder and electric fuel pump, :wink:

Re: No generator output below 1000 RPM

PostPosted: Tue Nov 18, 2014 2:01 am
by saildrive2001
Can you give details on your spiffy microprocessor controlled voltage regulator? Make, price, where purchased etc.