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Lichtmaschine Trouble? - The dreaded red light stays on!

Here we go again; you have been working hard on restoring your DKW and then the charging issue hits you. On the original NZ 250 and NZ 350 the red light on the fuel tank should go OFF when the engine is running and the dynamo is charging if not get ready for some brain exercise!

Before you start a full strip down you should of course check the obvious things like loose wires, battery connections and the usual things. I'm going to assume that you have done all that and you have a lovely NZ 350 that just will not charge.

Testing NZ 350 stator resistance
Basic Testing Procedure

Is it wired correctly and battery charged? - check the diagram and check again!

Is the Flywheel rubbing the wires in the stator housing? - it is a tight fit? they often rub the flywheel and short the armature.

Does it generate electricity? - When it is running you should have a higher voltage at the D+ wire / connection.

Check the brushes - replace if half worn - check also that the brush lead is not shorting out.

Does it regulate and switch (shalter) - try touching a positive wire for a split second to the metal bridge joining the schalter and regulator.

Apply 6V to the stator - disconnect the wires from the regulator box and apply 6v to the DF wire and ground. Then a multi meter to the D+ wire set to MilliAmps - when you kick the bike you should see a small output increase proportional to how hard you kick it - open the decompressor valve.

If this all fails - Move on below

lichtmaschine dynamo wiring diagram for NZ 350
How Does The Lichtmaschine Alternator Work?

Okay, so this is how it works in English....

First up; why should the red light go out when the dynamo is charging? well when the engine is not running a +ve voltage flows through the light to ground via the regulator and the bulb lights- basically in reverse though the regulator. When the engine is running the dynamo outputs a voltage though the regulator and the path to ground is lost as the regulator becomes +ve. So both sides of the bulb are now positive, then a current cannot flow and the bulb will go out.

The schalter or switch disconnects the dynamo when the battery voltage reaches a certain level and the regulator shorts the dynamo to reset it and stop it producing an over voltage of say 20v which you will notice by your battery frying and the armature flywheel getting warm.

Picture a cheap toy motor and you have a rotor in the middle, some permanent magnets in the case and some brushes that transfer electricity to the rotor. The DKW alternator is basically an inside out basic motor and rather than have fixed strength permanent magnets, it has a coil of wire that it uses to alter the magnetism. It does this so that it can alter the amount of electricity being generated; for instance when the battery is full.

The stator is fixed in the middle and used to generate a magnetic field by passing a current through it. Spinning around the outside is the flywheel armature and the two are connected just like a motor using carbon brushes. The engine spins the flywheel and a current from the battery flows through the stator to magnetise it. The flywheel is full of copper wire and as this spins though the stators magnetic field a current is induced in the flywheel windings and a voltage of course. If you did not know, as you pass a copper wire though a magnetic field a current flows in that wire.

As the engine spins faster more current is produced along with a higher voltage. Some of that voltage is fed back to the stator via the carbon brushes which increases the magnetic field in the stator and some out to the regulator and to charge the battery. The regulator monitors the voltage being produced on the D+ brush and if it gets too high, it short circuits the dynamo for a split second to reduce the magnet field in the stator and thus the output of generator is reduced. This happens super fast and the key is that the initial current to the stator from the battery only creates an initial magnetic field; the dynamo is then self generating by way of positive voltage feedback via the link from the D+ brush to the stator coil.

Once you understand this concept you are half way there!

The Stator

The stator is the star shaped lump of iron that grips a coil of wire. This is equivalent to the permanent magnets in a cheap little motor. It uses a coil of wire wrapped around an iron core to create a magnetic field in the same way that wrapping wire around a nail makes it magnetic.

Having analysed the DKW NZ 350 lichtmaschine the stator consists of a 95mm nominal diameter iron ring made up two windings of 0.5mm diameter wire with a total resistance of 3.5 to 5 ohms and a much smaller outer winding made from 0.3mm diameter wire with a total resistance of 10 ohms. The two are connected in series as per the diagram below and separated by an insulated layer of paper and shellac.

DKW NZ350 stator wiring diagram

To check your stator you can measure the resistance between the leads on the coil. If there is much deviation then there is a problem. The problem could be a short inside, or as in my case some numpty soldering the wrong leads together leading to burning of the outer winding. I fixed this with a quick epoxy coating. See the diagram for the resistance values. Once your resistance values are within range, you can move on. If you want to test it further, put it back in its iron keeper and apply + 6v and 1 Amp from a PSU to leads 1 and 20 and you will feel the magnetic field if you hold a nail near it.

DKW and NSU stator coil

You can see above the thicker coil and the thinner outer coil. They are separated by a wax cardboard layer. The outer layer of 0.3mm diameter copper has only about 4 or 5 turns. You can see also where it has overheated and burnt due to incorrect wiring. Note that this was not visible without taking the coil out of the iron cores - so remove yours and check! The outer winding is the feedback link between the D+ brush and the main field coil DF. Be careful when you reassemble the stator as often the copper winding will short onto the iron stator. Wrap some black insulation tape around it first, but not too much as it is a tight fit.

Stator is good but still no charge?

If you suspect that your shalter or regulator is messing about you can fit a 6v solid state regulator and disconnect the original cartridges. Before you do, try touching a live on the to of the schalter and listen for a click? if you do the fault is probably the regulator.

lichtmaschine 6v solid state regulator NZ 350 In the diagram above notice how the D+ brush wire is no longer connected to the field - essentially breaking the positive voltage feedback. Having the field go straight to ground only is Lucas style. This is a more modern approach as it makes sense to control the magnetic field from the regulator - and thus dynamo output - by an external controller or solid state regulator. The two methods of connection - with or without feedback - field only to ground - Lucas and feedback via brush is Bosch. This applies to almost all 1930s 40s and 50s DC generators ans is often called shunt, parallel winding etc

The simplest way to adapt the stator is just to cut the wire from the stator to the brush then connect the output field DF from the regulator to the DF on the stator. look for a regulator that then works this way such as the DVR2 or cheaper ones on ebay. lucas style field winding

More to follow....

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