Choosing a New Alternator

So, you have trawled through my Alternator Issues article and have made your mind up that you are going for a brand-new alternator stator and rotor – not a bad idea, the rotor will lose some of it’s magnetism after fifty or so years on a bike, which will certainly lower the effective output from the stator.

In my opinion, it is best to stick with Lucas – although technically speaking, these are Wassell (Wassell buy a license that gives them the right to use the Lucas name and logo) they are still decent quality.

I understand from staff at Wassell that the Lucas branded items go through several additional stages of finishing and quality control versus the Wassell labelled counterpart – well worth the small additional price premium in my opinion.

Alternatives include Sparx – this is Andy “Tricor” Gregory’s brand – I certainly don’t rate the quality of these. There were reports of poor quality and several of these going up in smoke in the past, plus from my own personal experience, I didn’t like the fact that by design the rotor was an interference fit on the crankshaft, and you are supposed to broach it on. I feel sorry for the next guy that tries to get it off again.

Another alternative is LAP – but these are fast becoming few and far between. LAP was formed when a few ex-Lucas employees did a management buyout a few years back. They were producing Lucas alternator stators and rotors, and the quality was superb. These later became branded as “Made In England” before they packed in alternator production in order to concentrate on the lighting side of the business. If you can find an LAP product buy it, as it will be excellent.

The quoted output of an alternator is the amount of current it is capable of producing at a specific rotational speed. And confusingly, the speed varies from model to model. It means that you are not getting an apples for apples comparison.

So, assuming you proceed with Lucas, there are four choices available to you:

Lucas PartDescriptionWassell PartOutput
LU47205Lucas RM21
Single Phase Stator
2 lead
WW1010910 amp
LU47239Lucas RM23
Single Phase Stator
2 lead
(High Output)
WW1019016 amp
LU47252Lucas RM24
3 Phase Stator
3 lead
WW10193L10.5 amp
LU47244Lucas RM24
3 Phase Stator
3 lead
(High Output)
WW10192L14.5 amp

Confusingly, with the three-phase stator, both the standard and high-output versions are called RM24 – just keep an eye on that one.

All of these units are served by the same rotor:

Lucas PartDescriptionWassell PartInfo
LU54202299Lucas RM20
WW101056 pole
Welded Centre

You should note that the centre of the rotor is not welded at all – another brit bike misnomer.

This was just a marketing term used by Lucas back in the day after the centre of their rotors worked loose, one resulting in a very public and explosive failure on the Isle of Man.

On these newer style rotors though, the magnets are retained by covers (keepers) that are cast in, so everything is held much more safe and secure.

Here is what the insides of a “welded” rotor look like:

Grant Tiller
Grant Tiller

Thanks to PeterJoe on Access Norton who sacrificed a perfectly good rotor to provide us with this insight!

There are two types of stator available – single-phase and three-phase.

Grant Tiller

The single-phase stator has two wires and the Lucas ones have six poles (six coils of lacquered copper wire which are all joined together to form one continuous piece).

The AC output looks like this:

Grant Tiller

When this is converted into DC by the rectifier, the output will end up looking something like this:

Grant Tiller

Compare that to a three-phase stator:

Grant Tiller

The Lucas three-phase stators have nine poles and are wired in star formation – they are floating and are not grounded at all.

The AC output looks like this:

Grant Tiller

When this is converted into DC by the rectifier, it will look something like this:

Grant Tiller

You can see that the output is great quality, as the peaks are so close together!

When you are choosing an alternator stator, as I mentioned before, try and match it’s output to the amount of power your bike is consuming.

A Commando with electronic ignition is using around 3 amps during a daytime ride, and under 10 amps with the lights on – so the standard stators will be a great fit. The single-phase 10-amp output rating is at 6,667 rpm. The three-phase 10.5-amp unit though makes similar power at only 2,824 rpm, so this unit is much better suited for riding in today’s modern conditions – slower speeds and stop-go traffic.

If you have an electric start, additional high-power lighting (not low consumption LEDs) or heated gloves/jackets then you can consider a high output alternator. The single phase 16-amp output rating is once again at 6,667 rpm. The three-phase 14.5-amp stator is at 2,824 rpm

RPMs and output ratings were quoted by someone from LAP, and I have no reason to believe they are any different to the original Lucas units or the modern Wassell/Lucas ones. LAP (and Lucas before them) measure against a calibrated 1 ohm load (which is also well documented in the Lucas service notes, and most of the brit bike workshop manuals of the time. The Norton Commando even walks you through the process of making your own for testing purposes.

There was a chart that Mistral Engineering showed in a late 70s advert for their Lucas Powerbase kit (as covered in my other article) which showed a much more curved plot (less amps at higher rpm)

I have been assured by several that this is not accurate and should be disregarded.

In reality, the two plots are not actually too different – the main difference being that Mistral chart incorrectly shows the RM21 reaching it’s 10 amp output at around 3,700rpm vs the LAP supplied figures showing 10 amps at nearer to 6,700rpm.

From my own testing, I have certainly never seen an RM21 hit anything near 10 amps at 3,700rpm!

Here is a chart (using the LAP figures) that gives you an indication of the output at different RPMs of all the available stators:

Grant Tiller

In reality, the output is not totally linear, due to losses, resistance, efficiency, quality of the copper, thickness of the lacquer etc… – but it’s a pretty close likeness to actuality and should give you a good idea as to what is right for you.

Choosing an output based on the power consumers on the bike gets you off to a great start. Also bear in mind though that the second you fit an electric starter, you will be recharging a heavily depleted battery for the majority of your ride – so this is a consideration that should be facotred it.

It is not a good idea to fit the highest output alternator stator, because you are intending on fitting an electric starter kit at somepoint in the future. You’ll be doing damage in the meanwhile, so you should spec out and fit based on your setup today.

Hopefully this article has helped you make an informed decision about what to buy. We are conditioned in life to believe that bigger is better. This is certainly one of those occasions where that is not at all the case.

This piece is part of a series that covers the charging system in general.

Hopefully these will fill in some of the gaps and questions you may have, but as always don’t hesitate to reach out if you need any help or advice.

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