Introduction to repolarising a Lucas dynamoRepolarising a dynamo (sometimes also referred to as ‘flashing’ or ‘remagnetising’) is a way to ‘wake up’ generators that may not have been used for a long time, been incorrectly wired up in the past or are not producing any volts for some other reason. This guide describes how a Lucas dynamo works, why and when you might need to repolarise it, and how to go about this. It is also (probably) applicable to other makes and models of dynamo, but it is based upon the Lucas type “E3NL” dynamo fitted to my Matchless motorbike.
This article covers the following topics:
- Introduction to repolarising a Lucas dynamo
- How a Lucas dynamo works
- When to repolarise a dynamo
- Getting ready to repolarise the dynamo
- Repolarising (or ‘flashing’) the dynamo
- Conclusions and your comments
How a Lucas dynamo works
A Lucas motorcycle dynamo can be thought of as a simple DC motor running in reverse. Rather than applying a DC current and getting out rotational movement as in a motor, you supply rotational movement from the engine and (all being well!) get out an electrical current. It should be noted that dynamos are DC (direct current) devices and are therefore very different to more modern alternators which, as their names suggests, are AC (alternating current) devices.
Let’s first give some thought to exactly how dynamos work as this will hopefully make what we are going to do later make much more sense. A basic dynamo (or motor) consists of two main components: a magnet and a coil. In a dynamo, the coil is rotated within the magnetic field of the magnet which induces a current to flow. In a motor, current is supplied to the coil which induces a magnetic field which is opposed by that from the magnet, causing the coil to move (rotate). This is why some people recommend testing a dynamo by “motoring” it – basically rewiring it to the battery as if it were a motor and seeing whether it spins. But that’s for another day…
The various components inside a Lucas E3 dynamo are shown in the exploded diagram below – click to view a larger version.
Where a Lucas dynamo differs from a simple DC motor is that there are actually two sources of magnetic field within the unit. The first is a soft-iron core which is what retains the magnetism when the engine is not running. However, this is not a particularly good magnet and the amount of magnetism it retains is relatively small and certainly not enough to generate the 45-60 Watts required by a classic motorbike. But it is just about enough to get a small current flowing in the coil when the engine is first started.
This is where the ‘field coil’ (a secondary coil within the dynamo) comes in to play. Some of the output from the generator coil is fed back into the dynamo through the field coil which then generates a secondary magnetic field (hence the name) of its own. This magnetic field is stronger than the residual field from the soft-iron core and therefore induces a bigger current to flow in the coil. More current is therefore supplied back to the field coil, giving an even greater field strength, more induced current in the coil, and so on until an equilibrium is reached for a given engine speed. Simple hey!
The following guide describes the relatively simple task of repolarising (sometimes referred to as “flashing”) a Lucas dynamo and is applicable to models such as the E3N, E3M, E3NL, E3L (etc) as fitted to many classic bikes. It may also be applicable to alternative makes and types of dynamos fitted to other classic bikes and cars too, but you’ll need to work out for yourself which are the correct terminals etc.
When to repolarise a dynamo
So why would we need to repolarise a dynamo? Well, there are two reasons for repolarising: Firstly the residual magnetic field in the soft-iron core may not be strong enough to get things started, or secondly it might be the wrong way around (i.e. the north and south poles are reversed) causing the induced current to flow the wrong way to the battery.
The first condition may occur if the motorbike hasn’t been used for some time as the residual magnetism will gradually fade over time. Alternatively the dynamo may have been knocked or damaged which can sometimes cause the magnetism to be instantaneously lost.
An incorrectly polarised dynamo may have been caused by incorrect wiring (e.g. connecting the battery the wrong way round) or the unit could have been previously fitted to another bike which required the opposite polarity. The original Lucas service data sheets specified that all replacement dynamos would be supplied pre-polarised for positive earth motorbikes, and that they would therefore need to be repolarised for negative earth machines. What we are about to do is therefore a very standard procedure and requires very little technical equipment or knowledge.
The main indication that a dynamo will need to be repolarised is when everything else seems to be in order with the charging system (see my General System Checks guide for more info) but the dynamo output is still very low, usually after not being used for a while. A negative dynamo output voltage (see my Testing a Lucas Dynamo guide) usually indicates an incorrectly polarised dynamo.
Getting ready to repolarise the dynamo
So that’s the ‘why’ taken care of, now comes the ‘how’ bit for repolarising your dynamo. All you’ll need is a long (say about 1m) length of wire – that’s about it! The wire needs to be reasonably thick as you’re going to use it to pass quite a big current (albeit it only momentarily) through the field winding of the dynamo. Something like multicore mains wire or battery cable is ideal. Anything too thin might get hot and melt if the current is too large for it.
I would advise reading right through this article first as this will take you through disconnecting the battery and dynamo, identifying the relevant connections and checking the existing output voltages. After ‘flashing’ the dynamo, you will also need to repeat the measurements described in Steps 3 to 5 of this guide so it’s worth becoming familiar with them anyway.
Repolarising (or ‘flashing’) the dynamo
The first thing to check is which way round your bike is wired, or in other words, which battery terminal is connected to the frame (earth) of your motorbike. If the positive terminal of the battery is wired to the frame then your bike is positive earth – the normal configuration for bikes up until around the 1950’s. Later bikes and nearly all modern vehicles are wired negative earth, that is to say that the negative terminal of the battery is connected to the frame of the motorbike. It is important to get this right as it will determine which way round you need to polarise the dynamo unit.
To try to make this clear, I am going to refer to the ‘Live’ and ‘Earth’ terminals of the battery. The ‘Live’ terminal is the one wired to the ammeter and the ‘Earth’ the one connected to the frame of the motorbike.
Ensure the Earth terminal of the battery is still connected to the bike (i.e. it is earthed) but disconnect the Live terminal from the bike. In it’s place connect the length of temporary jumper wire. Take care to not let the other end of this temporary jumper touch the bike frame or engine as this will create a short circuit and allow the full discharge current of the battery to flow through the wire which could potentially be quite dangerous to both you and the bike!
On my bike, I have a 15A fuse fitted in the earth connection between the battery and the frame (my bike is positive earth and the fuse connection I had was red, so this seemed a more logical way to connect it, although normally a fuse would be fitted to the live side of the battery). The high current drawn when flashing the dynamo will probably blow the fuse so I temporarily swapped it for a higher rated fuse (say 30 Amp). The alternative would have been to swap the fused connection for a direct earth connection, but this would have taken longer and anyway, I liked the reassurance of having a fuse there in case I did something really silly! If you have a fuse fitted to the live side of your battery, then just remove this to effectively disconnect the battery whilst leaving the earth connection in place.
Next, carefully identify the Field terminal on the dynamo unit. This should be the one on the left (click on the photo at the top of this article for a close-up view) and is normally marked with the letter “F”.
Carefully touch the Live end of the jumper wire to the Field terminal of the dynamo for just a second or so, no longer. You should see sparks as you touch the wire to the terminal, but this is ok (it just shows that current is flowing from the battery). If you don’t see any sparks, check that the earth terminal is still connected to the battery which will form the return path for the current via the dynamos metal case. Also make sure the battery is well charged. Touch the Live wire to the dynamo in this way a further couple of times to complete the ‘flashing’. Immediately disconnect the temporary jumper wire from the battery before it causes an accidental short.
That’s it – you have now repolarised your dynamo! Not at all difficult was it? Now go back and recheck the dynamo output using a multimeter as described in my other article and you should hopefully see a nice healthy positive output voltage. If not, please refer to back to the guide for further trouble-shooting ideas.
I hope you found this article useful. Please feel free to leave me a comment below to let me know how you get on, if you have any further tips or suggestions, or maybe if you have spotted a mistake.