Lost in the World of Field Wound Alternators

I have a late 1960s vintage motorcycle built with a motor from East Germany. The alternator is a field wound unit that puts out around 60w. Originally, this motor was designed for a motorcycle that had lights, battery, and a regulator/rectifier. The motor was transplanted into a racing frame and successfully raced. I purchased it a few years ago and it needed some TLC. I've got the machine sorted except for the ignition side.

As a racing machine there's no room for a battery or the big clunky mechanical regulator designed for these engines. Unfortunately a mouse got inside the alternator case and ate all the wires. Also, the previous owner cut out the entire wiring harness, so I have no idea how this machine operated electrically.

I know the alternator does not have permanent magnet, The field coil is around 1.7 ohm and there's a 4.4 ohm "field resistor". In struggling to understand how this motor operated I've concluded it can't run without a regulator/rectifier. In chatting with people who own machines with the same motor I've been told it can't run without a battery. It obviously needs a battery to energize the core to enable ignition, but I don't understand why a start battery couldn't be removed after the engine fired up.

My guess is that this bike was operated with a small battery pack to get things running and a small 6v regulator/rectifier hidden someplace. Once running, the machine competed in long distance desert races with interim stops for fuel. After a long run I'm thinking the core would retain sufficient residual magnetism to support kick starting the bike for the next leg.

Field-wound alternators in cars can run without a battery, although it's not wise on modern autos packed full of sensitive electronics. I'm not sure why folks are telling me this alternator must have a battery. And I have no idea how to source a 6v regulator/rectifier for this alternator and what specs to look for. I'm cautious about zapping the field coil to test for spark because I have no idea how much current would be needed to energize the coil without frying it. I might try sneaking up on it in 1/4 amp increments to see what it takes to get a spark,, but without a regulator I certainly wouldn't run the motor.

I've verified the integrity of the field coil and armature, and I know the ignition side (points, condenser, coils) produced a nice blue spark when hooked to a DC power supply. But I'm full stop until I can figure out the business of finding a regulator/rectifier and confirming that running without a battery is tenable.

I've attached a wiring diagram based on where I am at this point. I am very grateful for any advice on how to proceed - any thanks in advance for taking time to lend a hand.

 

[rjenkinsgb]

Hi,
that circuit shows a Dynamo (DC output) not an alternator.

The D+ and D- are the main outputs to the battery and bike electrical system

You must a regulator.
That will also include a method of disconnecting the dynamo outputs from the battery when the engine is stopped or at low revs and the dynamo is giving less voltage than the battery; otherwise the dynamo armature will draw current, flatten the battery and likely burn itself out...

The first thing is to decide if you want positive earth (as early systems) or negative earth, as most things since the 60s/70s

Then get a regulator to suit.

Examples:
These are a type for negative earth:

Powerdynamo, replacement regulator for diverse stock 6V dynamos

Powerdynamo, power and light to vintage bikes

www.powerdynamo.biz

These are available for either system:

Solid State Dynamo Regulator 6V

Solid State Dynamo Regulator 6V

www.vehiclewiringproducts.co.uk

Once the field coil polepieces are polarised, the dynamo should self-excite, but it may need quite a high speed to do that, rather higher than kickstart speed - car ones generally produced little or no power at tickover revs, which is one of the reasons alternators replaced dynamos in vehicles.

 

[traderyoda]

Super and thank you for this!

Sorry about the confusion. I'm don't speak German and am translating manuals page by page. The motor manuals call this an "alternator" and refer to regulator/recitifier, but more properly this is an old style field wound DC generator that only need a regulator - exactly as you point out.

This is a negative Earth application. Since the engine hasn't run in a long time I zapped it momentarily to ensure it ensure the polarity was correct. You have pointed out my concern about self-excitation. I doubt I'll get much out of it by kicking and will have to supply some current to bootstrap the generator. I'm a little nervous about doing this without knowing how much current is appropriate. And I'd like to settle the regulator issue before trying to run the machine.

My challenge is how to run this generator without a battery. I run another racer with a BSA A65 motor running a Typanium unit, a large capacitor, and no battery, but that's a permanent magnet alternator.

Most of the 6v solid state regulators I've found have battery hookups. What I need is a regulator that will control the generator with the battery out of the system. The link you sent looks like a version of the Podtronics regulator, but will it control the current to field when the only other energy needed is for ignition? This generator is rated at 60W.

Cheers.

 

[MaxHeadRoom78]

DC generators traditionally will self excite due to residual magnetism, Alternators traditionally do not.
Easily anyway.
When called out to a dc generators in the field that had no output due to losing the residual, we would 'zap' the field with a car battery etc,
It was then good to go for a long time usually.

 

[traderyoda]

Yep, that's where I am - ready to zap. I kicked and not gotten anything exciting out of the generator. Once I find the right regulator to fit on this generator (without a permanent battery) I'll be home free.

 

[traderyoda]

Took some time to do some searching... there are not many 6v dynamo regulators to choose from. Most require a battery in the loop. I found two that would operate without a battery. The most promising prospect was the DVR2 regulator from Dynamo Regulators Ltd. However, like others it comes with a stipulation: "...the resistance of the dynamo field winding must be greater that 2.5 ohms." This MZ field coils are exactly 1.7 which they claim is a 'very inefficient' dynamo likely to damage the regulator. Bummer.

Question: since the only things I need this dynamo to do is feed the ignition coil and maintain an energized field why can I not fix it's output? I know the primary resistance in the ignition coil (I have a 6v high-output coil at 1.6ohm and standard coil at 3ohm) which statically sets an upper bound on the current draw needed to saturate the coil. I know the dwell time and can estimate the running current. Let's say I use the standard coil drawing 2A - running it would likely draw around 1.4A at idle and somewhat less (but not much) at higher revs. Sustaining the core can't use much.

I don't need the dynamo to get the motor running, I can just connect my DC power supply. Once I have the motor running, I can measure the actual ignition current draw. Am I wrong in thinking that the regulator, at dynamo outputs above 6v (plus a deadband), sends only a small current through the field coils - enough to maintain 6v? If so, it seems the challenge to running a dynamo without lights or a battery is what the do with the excess energy put out by the dynamo at higher revs. Does that make sense?

 

[Nigel Goodwin]

DC generators traditionally will self excite due to residual magnetism, Alternators traditionally do not.
Easily anyway.
When called out to a dc generators in the field that had no output due to losing the residual, we would 'zap' the field with a car battery etc,
It was then good to go for a long time usually.

Historically, there was a contact in the control unit that supplied power to the field coil - and if you connected the battery the wrong way round (for converting to negative earth for example), you could simply press the contact closed with your finger, and that would re-polarise the field coil the opposite way. Or you could use a jumper wire to do the same thing.

If you didn't do either of those, the car would start perfectly (as starter motors are series wound), but as you revved up the lights got dimmer

Young kids, driving old cars and motorbikes in fields and on moors, easy to make a rookie mistake - and of course we didn't even know if the vehicle was +ve or -ve earth.