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A dynamo question: rotation direction?

I'm stuck - help !

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Tim Stevens11/05/2019 15:02:13
1142 forum posts

I am restoring a dynamo from an Austin 7 about 1930. The owners needs 12v so I have rewound the 4 field coils with new thinner wire, carefully (I thought) checking the winding directions LRLR. Connect it all up and test with the Field and Output connected together to live, and the body to earth. It motors round as expected, but the wrong way round. So, I rearrange the connections of the field winding set so their current is reversed, leaving the brush connections the same. And once more, it goes the wrong way round. Where am I going wrong?

Conventional shunt dynamo, four field coils, brushes at right angles. No third brush or other complications.

PS: yes, you guessed, this is a follow on from my Empire Tape query for which you were all very helpful - thanks.

Regards, Tim

Edited By Tim Stevens on 11/05/2019 15:03:47

Phil Whitley11/05/2019 18:48:52
1068 forum posts
137 photos

Tim, long time since I did a dynamo, so don't have the circuit in my head, but to reverse a universal or DC brush motor, you swap the wires connected to the brushes over. You have proved it as a motor, if you polarise it by sparking a battery lead onto the field terminal, then spin it in the right direction and see if it charges, or you get some output. I assume the car is positive earth, so flash the battery negative to the field to polarise it in the right direction and it should be fine. We used to swap over from + eart to - earth to install a stereo, which were mainly -earth, just swap the battery and coil terminals, repolarise the dynamo, and off we go! can you post up a circuit for the dynamo internal connections?

hope this helps


not done it yet11/05/2019 19:52:40
4163 forum posts
15 photos

Yes, direction for motoring makes no difference to generation - it just needs the field flashing to magnetise the field magnets to set the charging polarity. The armature might need to rotate in one particular direction for installations with off-set brush gear - but not this one.

Jon Lawes11/05/2019 19:58:52
352 forum posts

Agreed, I converted my Morris 1000 to negative earth and had to flash the dynamo to achieve it. It wasn't onerous.

vintage engineer11/05/2019 20:21:18
232 forum posts
1 photos

To convert an Austin 7 dynamo to 12 you just need to change the voltage regulator.

Speedy Builder511/05/2019 20:30:53
1928 forum posts
132 photos

+1 for change the voltage regulator. Loads of info on Cornwall Austin 7 site.

Tim Stevens11/05/2019 23:20:11
1142 forum posts

There is a certain amount of confusion - sorry. I start with a few facts in clarification:

1. The existing field coils were duff, so needed rewinding anyway. Therefore 'Just fit a regulator' does not address the issue.

2. I am not (yet) trying to test whether it generates. I know it motors and I need it to motor in the other sense. Motoring does not rely on a small amount of residual magnetism, as the field coils are energised by the applied voltage. The residual magnetism is only relevant when I test it as a dynamo. And in any event, I have tried flashing and while I did pass current, it made no difference.

3. The reason we have a cut-out on a traditional dynamo circuit is to stop the dynamo, when the engine stops, trying to turn as a motor. It is trying to turn in the direction it normally runs, so I am trying to test my dynamo to see if it will motor - but it does so the wrong way round.

4. Swapping the brush connections won't help in this case as one of the brush holders is earthed, or, more accurately, only one holder is insulated from earth.

5. Swapping the field winding end to end is surely the same effect as swapping the brush connections? This is what I have done and it makes no difference - that is my problem.

6. The problem is not to do with positive or negative earth wiring. A good dynamo will motor in the right direction whichever system is applied (but doing this the 'wrong way round' is likely to set up the residual magnetism the wrong way for charging the 'right way round'[close parenthesis].

So, thanks, everyone for your helpful replies, but we're not there yet.

Regards, Tim


Edited By Tim Stevens on 11/05/2019 23:22:28

Nimble12/05/2019 00:15:33
33 forum posts

Hi Tim,

would Fleming's Left hand, and Right hand rules explain why it is motoring in a different direction?

Regards, Neil.

Nicholas Farr12/05/2019 07:45:21
2118 forum posts
1025 photos

Hi Tim, long time since I've tested a dynamo as a motor and I don't know which way it would run, or even if I took any notice. However have you tried spinning it in the direction you want it to go immediately before juicing it up? just a thought.

Regards Nick.

Edited By Nicholas Farr on 12/05/2019 07:46:22

Trevor Johnson 112/05/2019 09:08:14
4 forum posts

Hello Tim, not sure which dynamo you got, Lucas no doubt. As far as I can remember on the C39, C40,C42, C45 type dynamos you will have two wires exiting the field coils after they are joined, one wire goes to earth the other to the field terminal, to reverse direction of charge just change those two connections. From what I remember looking down into any of the above numbered dynamo's the field coil earth lead comes from the left coil and the field terminal from the right, just reverse them. An Idea of the type/number of your dynamo would be helpful.



vintage engineer12/05/2019 10:05:25
232 forum posts
1 photos
  • If you put the field coils in the casing the other way round will not run the way you want?
Robert Atkinson 212/05/2019 11:14:25
547 forum posts
21 photos

Reversing either the field OR the armature (brush) connections will reverse a "universal" motor which is what a Dynamo in motor mode is. Are you sure you actually reversed only the field? Does the dynamo produce plenty of torque when motoring? you might have only reversed on section of the field winding or wound / connected one or more section in the wrong direction. "Flashing the Field" produces a residual field of fixed polarity that makes the dynamo self starting in the correct polarity. If for instance there are two field coils and you would one in the wrong direction or connected one backwards the two electic fields would cancel out but the dynamo would still draw a large current and possibly turn on the residual permanent magnetic field. The torque would be much lower though. If a field s open circuit the ame would apply but current would be lower.

Direction of rotation does make a difference in two ways:
1/ Field distortion. The rotating and fixed fields interact changing the neutral position of the brushes and causing sparking at the commutator. Often the brushes are offset to compensate. Reversing the direction of rotation increases the offset and makes sparking worse. Compensating windings, or on old desigins moving brush holders, are also used on big machines to acheive the same effect over a range of loads.
2/ Cooling fan will run the wrong way reducing efficency.
These may not apply to a small Austin 7 dynamo of course.

Dynamos are still used as starter-generators on small turboprop and turboshaft engines on aircraft.

Robert G8RPI.

Russell Eberhardt12/05/2019 11:20:40
2564 forum posts
85 photos

Hi Tim,

It's a long time since I worked on my Austin Sevens but:-

I guess that, as there is no third brush, the dynamo is an early one with the field windings earthed to the case at one end and coming out to a terminal from the other end. The commutator also has one brush earthed and the other coming out to a second terminal. The field connection went to a dashboard switch to give a choice of high or low charge rate there being no regulator. Reversing the field windings must reverse the direction all other things being the same so I can't understand what could be happening in your case.

The correct procedure for flashing the field residual magnetism is to disconnect the commutator connection (D on the wiring diagram) and then momentarily close the cut-out contacts - with the dynamo in the car of course.


Tim Stevens12/05/2019 11:52:26
1142 forum posts

Nimble: Yes, it would but changing the field current direction should change the movement, and it doesn't.

Nicholas: Yes, tried that (along with all the other tricks I know about) - makes no difference.

Trevor J: It is a CAV dynamo - but they were owned by Lucas at about the same time ?1930. My problem is that I have reversed the field connections and it made no difference. I guess that the dynamo number will not be much help as I am not clear how much of it is original, etc. For certain the barrel with the field windings is not the same as the original (but seems identical in appearance, dimensions etc.)

Vintage engineer: They cannot be reversed end to end as the connections won't reach; nor back to front as the curved shape is wrong; nor moved round one place as the slots for the through bolts won't line up. So, I swapped the field connections (which should do the same thing as your suggestions) and it made no difference.

Russel: There was a third brush, but it is not used. The field windings were 'earthed' at the third brush, in effect, but I have connected this end (of the new thinner windings) to the earthed armature brush. And I agree - I can't understand what is happening either.

So - I am going to spend some time with a plotting compass and a low-voltage supply, to see what exactly happens to the field magnetism in various wiring conditions. I'm hoping I don't have to start again with another rewind ...

Regards, Tim

Robert Atkinson 212/05/2019 12:19:12
547 forum posts
21 photos

Some info from contemporary literature




Images in my Album.

Robert G8RPI.

Edited By Robert Atkinson 2 on 12/05/2019 12:21:23

Tim Stevens12/05/2019 14:29:24
1142 forum posts

Robert G8RPI: A very helpful (1960s*) summary which is a reminder of stuff I already have at my fingertips. [See latest edition of the Vintage Minor Register newspaper.] So, unless I am missing something, I am no further forward. Thanks, though - the answer ought to be in there somewhere so I conclude that my fault(s) is/are fairly non-standard. Perhaps I have inadvertently fitted two N and then two S coils, rather than NSNS, or even three of one and one of the other. But I don't see how that would give the symptoms I have - so perhaps I have got two things wrong at the same time.

* For interest - I suggest this date for these reasons:
1. only as late as 1960 was the third brush system on the way out:
2. The diagrams show the mainly post-WWII positive earth wiring;
3. The typeface only became generally available in about 1960 - before that it was Times property.

Thanks anyway. If all else fails can I parcel the thing up for you to look at ... no, I thought not

Regards, Tim

Robert Atkinson 212/05/2019 15:11:02
547 forum posts
21 photos

Where are you located? I could come over - seriously.

I like a challenge. One motor problem I had was a machine with a geared squirrel cage induction motor, The gearbox output was running backwards after somene "fixed" it. After spending time studying the gear box I finally reallised they had flipped the coil and stator over (not the whole motor, think about it) wich caused the shaded pole to be on the other side and the motor to run in reverse

Robert G8RPI.

Tim Stevens12/05/2019 15:55:43
1142 forum posts

Aha - I think I have discovered my problem - or at least one of my problems. When connecting the coils I fitted the steel centres to each coil, and used a small current and an old plotting compass to check the polarity. Now, though, I have checked again with a new compass, and I have four poles the same way round. All I can imagine is that I am getting old and dafter (true) or the the old compass had lost any magnetism it ever had (quite likely, I think it came from a cracker about 4 moves ago ie 30+ years), and simply was attracted to the first magnetism it saw.

So, I will now reverse the winding connections of two opposite poles and try again.

But this does not answer vthe question fully - it should have declined to rotate altogether, surely?

Robert - I am in Knighton, Mid-Wales, and you are welcome to come if you wish, but give me a day or two to change the connections, re-tape etc, and rebuild, ready for a further test session.

Regards, Tim

PS for Sale - vintage plotting compass, only used by two old ladies to go to church.

Robert Atkinson 212/05/2019 16:33:36
547 forum posts
21 photos

Mid Wales is a bit too far I'm afraid.
Any imbalance or residual magnetism will cause it to rotate. Not much torque though.

Robert G8RPI.

Tim Stevens13/05/2019 17:23:28
1142 forum posts

Windings correctly connected (using a truly magnetic compass) and away it goes, the right way round.

I still wonder why it motored the wrong way with field coils alternately the right, and the wrong way, round. Ho hum.

Cheers, everyone - helpful as always


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