Lucas voltage regulator

In the 1930s and subsequently, Lucas supplied a car voltage regulator of the twin bobbin type.

A detailed description of how it operated can be found here, but in summary, one solenoid operated a cut-out to prevent reverse current from the battery discharging through the dynamo, and the other controlled the voltage available to the dynamo field windings via a pair of vibrating contacts.

A pair of so called "series windings" were wound as secondaries on the primary windings controlling the above functions. The winding on the cut-out, which carried the dynamo output and operated through the above mentioned cut-out contacts, was 2mm o/d round enamelled wire of about 14 turns. This was connected to the series winding on the voltage regulator solenoid by means of a soldered joint.

This second winding was made of rectangular section enameled wire of approx 3mm x 0.5 mm. It carried the same current as the winding on the cut-out but had only half the cross-sectional area of the round wire winding.

It is noticeable on old units that the enamel on the cut-out series winding is often perfect whereas the enamel on the rectangular section winding may be dark or burnt. Clearly the power was being preferentially dissipated in this higher resistance winding because of its smaller cross-section.

What I would like to know is why Lucas used a rectangular section wire at all, and having done so, why didn't they select a cross-section the same as the round wire?

There must have been a reason for using the rectangular section wire that outweighed the cost of the extra operation of making the soldered joint and having two different types of wire on hand.

Since the fields produced by the series windings are only dependent on Amp-turns, it is understandable that modern repros of these units use round wire of identical size for both the series windings. Why did Lucas not do this?

  • 2
    \$\begingroup\$ Why? Cost? Availability? manufacturing process time? so many possibilities. Also, please edit your question as formatting is free and a wall of text is hard to read. \$\endgroup\$
    – Solar Mike
    Sep 25 at 7:43
  • 6
    \$\begingroup\$ So, after my first post on the site, my question is closed before hardly anyone has had the chance to answer it on the basis that it is "opinion based". This is utter nonsense and the question has nothing to do with opinion. Engineers create designs on the basis of sound engineering snd electrical principles. I am seeking to understand what those principles that have determined the design are. \$\endgroup\$
    – Gus
    Sep 25 at 9:44
  • 7
    \$\begingroup\$ It is ridiculous that someone can close my question without any explanation of the reasoning behind their action. \$\endgroup\$
    – Gus
    Sep 25 at 9:46
  • 3
    \$\begingroup\$ Unless you can access the records at Lucas to explain their reasoning then any answer will be opinion, so NOT ridiculous... \$\endgroup\$
    – Solar Mike
    Sep 25 at 9:48
  • \$\begingroup\$ I have edited your question to make it more readable, included your photo, and linked to an external source explaining the regulator operation in detail. I may be that there will be a consensus that the question should be reopened. \$\endgroup\$
    – colintd
    Sep 25 at 15:15

3 Answers 3


Rectangular wiring is typically used when you need a higher winding density, as it packs closer than circular wire. Today it is generally only used in very compact, high power motors or coils. However, it is common enough that there are a range of suppliers such as here.

From the picture (now included in your question) I'd say the "tape" style wire is to maximize surface area and associated power dissipation.

As to why they didn't use the same section, I can only assume they wanted a given resistance to turns ratio, which was also capable of dissipating the required heat. This fits with your observation about these windings showing greater effects of heat in vintage units.

  • 1
    \$\begingroup\$ Winding density is not the reason because the winding was only a single layer and there is plenty of space externally surrounding the winding \$\endgroup\$
    – Gus
    Sep 25 at 8:18
  • \$\begingroup\$ Could you include a picture? \$\endgroup\$
    – colintd
    Sep 25 at 8:25
  • \$\begingroup\$ How do I add a picture? \$\endgroup\$
    – Gus
    Sep 25 at 8:38
  • 1
    \$\begingroup\$ @Gus - edit your question post and add the picture, don't add the picture to this answer \$\endgroup\$ Sep 25 at 9:49
  • 3
    \$\begingroup\$ Leaving aside the "opinion based" sniping, I'd suggest that since OP says that the "tape-style" winding was often discoloured due to operating relatively hot an attempt to increase dissipation is an entirely plausible reason. \$\endgroup\$ Sep 25 at 15:55

As colintd identified, the likely reason is that the manufacturer wanted to increase coil resistance on purpose.

We can find some more clues in Lucas MCR2 service manual. It has these diagrams:

Diagrams from service manual

It appears the coil you are asking about is the series coil. The purpose of this coil is described as:

The series or current winding provides a compensation on this system of control, for if the control were arranged entirely on the basis of voltage there would be a risk of very seriously overloading the generator when the battery was in a low state of charge, particularly if the lamps were simultaneously in use.

The main action of the series coil is to open the contacts at a lower operating voltage than normally. But the increased coil resistance would also help to limit the current. The system also has a bimetallic element that lowers the armature operating point when temperature rises.

I haven't found conclusive explanation of the design choices, but all three effects (magnetic field, coil resistance and heating) act to limit the maximum battery charging current.


As far as I understand, this is a fairly simple electromechanical device with variable operating points.

These operating points are adjusted by means of two electromagnets, one which senses the current drawn from the battery, and one which senses the voltage. This "sensing" is just accomplished by countering the pull of the electromagnets by means of adjustable springs.

To answer your question, the wide flat winding is the current sense coil, and it needs to be bigger like that to carry more current without overheating (and to make sure it takes some amount of current before the dynamo is triggered). The geometry of the wire itself was an arbitrary choice as far as function goes. But hand winding that coil would be easier when the wire is flat and round rather than square or some other shape.

  • 1
    \$\begingroup\$ But with the dimensions given, the flat wire cross-section is actually smaller than the round wire (pi * (2mm/2)² = 3.1 mm² vs. 3mm * 0.5mm = 1.5 mm²). I agree that it carries a higher current, which does make the smaller cross section a bit surprising, leading OP to ask this question. \$\endgroup\$
    – jpa
    Sep 26 at 5:09

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