Dynamos - Reversing the polarity

A debate is raging locally: when changing a car-type dynamo from positive to negative earth, what needs to be altered?

1. One view is that the output depends on how the dynamo is polarised (ie which way round the residual magnetism is) - so, no change to the wiring, just connect up to the reversed battery and close the cut-out contacts briefly (ie pulling them apart again).

2. The other view is that either the field winding or the armature brush connections should be reversed (not both) as well as changing polarity as above.

My opinion is that a shunt dynamo is the same as a motor, and a shunt motor will go the same way round whichever way the battery is connected, so I incline to view 1 above. And that this applies only to the dynamo, and only to one relying entirely on electromagnets, not permanent ones. In my opinion, too, changing one of the windings round is what you do to reverse the rotation, not the polarity. But I have been wrong before ...

[The whole problem is simpler, of course, if the body of the dynamo is not earthed or grounded - but those on vehicles generally are, dammit.]

Comments, please - which view is correct?

Regards, Tim

I always understood the the dynamo to be self-energising based on its residual magnetism. Many years since I have done it, but this seems to agree with what I dimly remember about touching a wire to the terminal to polarise a new dynamo.

**LINK**

Brian

I’ve flashed the field coils on countless dynamos, over the years. Simple and works.

I would have thought changing the magnet polarity would work, or, changing the armature or coil polarity(but not both).

Theoretically, could Fleming's Left-hand or Right-hand rules be used to figure this out? We didn't do much with motor theory in college, so I'm not sure if the same rules apply to dynamo's.

Ed.

There's no magnet in a dynamo as such, just the residual magnetism that arises from the hysteresis of the rotor (and stator), aka "remanence".

I suspect there's truth in both arguments but it's evidently not necessary to do any rewiring in practice, when it seems perfectly feasible to simply repolarise the rotor. I guess if you didn't, you might get a bit of an event when the regulator first connected the output up to the battery. I'm guessing that what happens - unlike an alternator, there are no diodes to prevent it trying to operate as a (starter) motor if it were left powered up, so one of the relay things inside the regulator must make the battery connection when the dynamo is up and running.

You can certainly use a dynamo as a motor, which is how I powered my first electric vehicle as a 9 year old. 12V car battery and a Lucas dynamo from a scrapped MG Magnette ZB, with a **** off big manual switch.

As you vary the current in the rotor, the voltage constant (Kv) varies which makes it possible to regulate the output current at a given speed.

Murray

Both are correct but most if not all will flash the field coil. If when pressing the contacts together I always slacken off the drive belt and you can see/hear the dynamo pulley revolve as a motor.For Ed, left hand rule for motors and right one for generators. All to do with residual magnetism, hysteresis is for transformer energy loss loop. Same with car tyres. Incidentally a 12 volt dynamo make a good powerful DC motor when run on 24 volts, we used one for years lifting in/out heavy objects from a paraffin de greasing tank, the field was wired via a double pole double throw switch for up /down. Don't suppose it would be allowed today.John

Muzzer is quite right about a 'relay thing' to connect the dynamo only when it is charging. It is normally called a Cut-out, and is on its own in many pre-war cars (and some post-war cheapos) and in more modern dynamo systems it is one end of the Regulator coils.

Analysing the comments, it seems I am likely to be right, but some of you have doubts. So, I'll try suck-it-&-see.

Thanks everyone

Tim

Some car dynamos don't have their brushes perpendicular to the axis of the commutator and if reversed, do not sit properly on the commutator.

Paris Rhone dynamos as used on 2CV cars can't be "flashed" by holding the regulator contacts shut. You need to disconnect the field windings and flash the dynamo by connecting the field coils directly to the battery with a bulb in series.

Can three brush dynamos (where the position of the third brush controls the output) be flashed in the same way as those with a regulator?

Brian

You'll have to think about that and explain what you think is the difference. All magnetic materials exhibit hysteresis and hence some degree of remanence. Can't have one without the other.

"Remanence or remanent magnetization or residual magnetism is the magnetization left behind in a ferromagnetic material (such as iron) after an external magnetic field is removed. It is also the measure of that magnetization. Colloquially, when a magnet is "magnetized" it has remanence."

Are we debating a dynamo on a car? Or are we debating a dynamo on a small generating plant. Say 32 Volts DC? I have worked on both. Car genererator / dynamo - reverse the battery & flash the field before starting the car, otherwise you will weld one set of the contacts of the voltage regulator together. Problems: the car radio may blow up, and the ampmeter will read backwards. Depends on the age of the car. Proceed with caution!

Just an afterthought......

I have had old dynamos that lose their residual magnetism, so they do not excite when started. Had this with a 1954 Holden motor (engine) fittd with a dynamo that was installed in a forklift. It would not self excite if it had not been used for a week or so. Solution: fit an alternator. Also in the 80's I had the same problem with small AC generating sets (3 to 5 KVa or so) made in the late 40's. They would sit on stand-by for a month, and when started the exciter would not! Basically the iron had lost it's residual magnetism.

On the old fashioned Lucas "standard" setup as fitted to series Land - Rovers we used to swap the vehicles from positive earth to negative earth by :-

A. Swap the battery leads

B, Swap the low tension connectors on the coil

C, Reverse the connections to the ammeter

D. Remove the D and F terminals from the regulator and take the F wire and brush it against the A terminal then replace the D and F wires.

I've always reversed dynamo polarity by "flashing" the field coil, with the dynamo removed from the vehicle. Always worked, at the time I did not really know what I was doing, but always worked. If in a car reverse the ammeter connections and coil. That's it, if uprating to 12v from 6v change the bulbs and coil, the starter will work even better on 12v. Also works on 3 brush and moving brush types.

Off the topic, but after reading Gordon W above it brought back a few memories when my mates had Ford POP car fitted side valve engines. One mod was to fit a cylinder head from another model increasing compression ratio. making the car to go a bit faster Next do a full12 volt conversion, change polarity, but leaving the 6 volt starter in place. Now that 6 volt starter when on 12 volts really whipped the engine around, it couldn't fail to start. Happy days

John,

That is fairly standard practice for vintage tractors, etc. Yes, the engine will usually start quite quickly. But if it does not, for some reason, the starter might get too hot that much quicker.

Many don’t even need to change polarity - magnetos didn’t need an external power supply! Several of mine don’t even have provision for a starter motor.