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

In the case of the RM21, this single-phase stator has two wires and the Lucas ones have six poles (three parallel pairs of series connected coils laid out in opposing poles)

These are coils of lacquered copper wire.

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.

12 replies

  1. Hi, I am stuck on wiring a 3 phase Lucas rec/reg.the bike is positive ground .I do not understand the instructions on were the black and red wires go.Can you please in simple terms advise were the red and black wires from the rec/ reg go.Than You.

    • Hi Mike,

      There are five wires to connect:

      Wire ColourDescription
      Yellow (x 3)these are the AC input and pick up on the three wires coming out of the three phase alternator stator (connection can be any way round, as this is the AC side of the circuit)
      Redthis is the Positive output and will join to one of the red wires in the harness

      alternatively you may choose to wire this directly to the battery positive (+) terminal

      Blackthis is the Negative output (known as the hot wire) – it will be wired to the NU (brown/blue) that goes back to the battery negative terminal via a fuse

      alternatively you may choose to wire this directly to the battery negative (-) terminal

      • Will be doing this myself soon due to Zener failure on my T140. Negative earth in my case and I’m planning to connect direct to the battery. Just wondered if you thought it may be worthwhile to provide separate inline fuse protection for the regulator/rectifier, and, if so, whether to put it on the +ve or -ve terminal. Bike currently has one fuse covering everything, including the electronic ignition.

        • Hi Graham,


          I am personally a big fan of using a separate fuse for the reg/rec.
          I like the automotive blade fuses (they stand up well to vibration and are much less prone to failure)
          Usually, I will spec a 15 amp for a standard bike or a 20 amp for a bike with electric start.

          Having a separate fuse between the reg/rec affords you lots of second chances.

        • You stand a chance of getting home on the reg/rec if the battery fails.
        • You stand a chance of getting home on battery only if anything fails in the charging circuit.
        • …a worthwhile mod!


          • Hi Grant,

            Many thanks for the reply, I shall include a fuse. Would you recommend attaching this to the -ve or +ve terminal on a negative earth bike?

            Thanks again,

          • Hi Graham,

            I always like to fuse the hot wire not the earth (although, in truth it makes no difference)

            So with a negative earth bike, let your black reg/rec cable go uninterrupted to the battery negative terminal.

            Then fuse the red cable, and connect it direct to the battery positive terminal.

  2. The service manuals confusingly show a RM21 single phase stator as having three separate coils all connected in parallel with three wires to give a two wire output as a single phase.
    Your single phase diagram shows all six coils arranged in series, very logical but at odds with the BSA etc service manuals.
    What is the coil arrangement of a RM21stator?

  3. I understand your diagram is made for simplicity, that’s fine.
    The diagram shown in your RM21 link just shows three windings connected in parallel superimposed over a circle which does not give any clue as to how the actual windings are arranged on the stator.
    Is this information available anywhere?

    • Hi Rod – I am sorry that you don’t find my diagrams clear.

      They are in the same format as Lucas drew originally, and are as used in the various factory workshop manuals at the time.

      In the case of the RM21 it is three parallel wired sets of series connected pairs.

      The coil bobbin pairs are arranged about the laminated frame in pole opposites.

      The first bobbin of each pair is 60 degrees apart from it’s neighbour, which makes the second bobbin of each pair 180 degrees from it’s series connected partner.

      You can see the layout and connections here:

Leave a Reply

Your email address will not be published. Required fields are marked *