not working C40 dynamos

[Z320]

Hi,

a friend gave me two not working C40 dynamos as a present.

The look like overhoult, D and F terminal fine, coals (?) new and moving free, no bare cable visible.

If I measure F -> ground I have a few Ohm, but changing up to about 15 Ohm when I turn the shaft!

This is what I did not expect!

D -> ground also a few Ohm, also changing, when I spin the shaft.

Must surprising: F -> D also a few Ohm, also changing when I turn the shaft!!!

As told above: no bare cable is visible.

Any idea is welcome, please.

They are too good to scrap them.

Ciao, Marco

[RobH]

The resistances do not change , the variation you see is due to the meter you are using. When you turn the shaft it can generate a small current which confuses the meter and makes it read incorrectly.  

D to case should be a very low resistance - less than 1 Ohm.   Perhaps the new brushes may not be making a good contact as they won't be bedded-in.  The field resistance of a C40 should be 6 Ohms. 

If you measure between F and D it is just the armature and field resistances in series (because they both connect to the case at the other end) so not very meaningful. 

This link is the Lucas workshop manual for the C40:

http://www.liblbc.info/images/Lucas/WorkshopInstructions50/secA-4_issue2.pdf

 

 

Edited August 10, 2025 by RobH

[BlueTR3A-5EKT]

35 minutes ago, RobH said:

The resistances do not change , the variation you see is due to the meter you are using. When you turn the shaft it can generate a small current which confuses the meter and makes it read incorrectly.  

D to case should be a very low resistance - less than 1 Ohm.   Perhaps the new brushes may not be making a good contact as they won't be bedded-in.  The field resistance of a C40 should be 6 Ohms. 

If you measure between F and D it is just the armature and field resistances in series (because they both connect to the case at the other end) so not very meaningful. 

This link is the Lucas workshop manual for the C40:

http://www.liblbc.info/images/Lucas/WorkshopInstructions50/secA-4_issue2.pdf

 

 

You beat me on one item only Rob that I had known to check.   Carbon brushes not bedded properly.  Or a low commutator bar. Ie eccentric commutator.    I still have an abrasive ‘stick’ to work on a rotating commutator to assist in bedding the brushes.  It came from my dad who used to overhaul these things.

[Z320]

Thank both for your advice, I check again and turn the shaft slower.

Indeed a battery symbol appears when I spin the shaft faster.

Both commutators look like new

[BlueTR3A-5EKT]

Are the support bearings at each end in excellent condition too?   Any looseness will allow the armature to deviate slightly and wobble.

Front bearing is a ball bearing race, retained by a plate and 3 rivets.     Rear is a plain phosphor bronze bearing.

Edited August 10, 2025 by BlueTR3A-5EKT

[Z320]

All OK, and I'm still puzzling.

- field coil 7 Ohm (shoud be 6)

- clean commutator with 0.6 Ohm

- bushes new

- rest magnetism: yes

Tomorrow I put all together again and drive it with a drilling machine

[Lebro]

Has the polarity of the dynamo been changed ? if so it will need to be "flashed" to be the new polarity in order to reverse the residual magnetism.

Bob

Edited August 11, 2025 by Lebro

[Z320]

Thank you Bob,

with a wrong polarity the generator produces negativ voltage.

Spinning it with a drilling mashine days ago this was not the case, it was positive 5 V on one, positive 2.5 Volt on the other.

Ciao, Marco

[RobH]

Was that with the F and D terminals connected together Marco? 

[Z320]

yes

[RobH]

You say the dynamos look refurbished - have the field coils been replaced ?  If so, have they been connected correctly ?

If one coil is connected the wrong way round the magnetic poles will be wrong.   There is no guarantee that a replacement field coil has been wound in the same direction as an original ; that doesn't matter though, as long as both coils are wound in the same direction and are connected start-to-finish. (i.e., the start of one winding connects to the finish of the other.)     

 

[Lebro]

I recently had a Lucas dynamo, from an old BSA motorbike, to look at as it was not charging the battery.
I tested it with D & F connected, spiining it with a drill in the correct direction. got no output. The history of the previous polarity was unknown.
I connected a battery directly to D & F & the unit "motored" showing it was basicvaly ok. following that It charged as it should.

All I had done was put 12v on the field winding the right way round, thus reversing the residual magnetism, so it could then generate some initial voltage - enough to pass through the regulator, & re-enforce the field current.

Bob

[Z320]

Hi Rob,

in each dynamo the coils look identical, one wrapped with black textile, to other one white.

So I assume each pair is wound the same direction. For a closer look I must put them out, and this should be a pain?

Before I do that, one question:

if both of a pair are wound the same direktion, for example clockwise, does it really matter how they are connected?

Because by installing them in any way, they remain being clockwise wound?

Or do I understand something wrong?

Ciao, Marco

[RobH]

 

11 minutes ago, Z320 said:

if both of a pair are wound the same direction, for example clockwise, does it really matter how they are connected?

It isn't necessary to take the coils out Marco - and that really is not an easy operation as you can see here:

http://midgetandspriteclub.com/wp-content/uploads/2016/06/C39_dynamo.pdf

Assuming the armature has been removed, I think you could do it this way:

Connect the field to a battery and test each pole in turn with the same end of a small bar magnet.  You should get attraction on one pole and repulsion from the other showing there are N and S poles.  If so the coils are connected correctly.     

 

 

[Z320]

I did some more testing,

this is a view on the commutator, looks OK to me.

The field coils of one of them looks old and like connected together end-by-end and bedinning-by-beginning

With both in S-N orientation and tested with a hiking compass they they both have a lovely north and south pole.

 

Anyway, with F and D connected the volatge is very low now, only 0.2-0.5 V

So I flashed the field again with the TR battery - no result.

The test was at 1,300 revs on the lathe.

Finally I put 12 V from extern on the field (and ground on the housing, of cause) - no result.

But the feild coil seems OK, has 6-7 Ohm and no short circut.

Next test is a combination with parts a working spare C40 

Ciao, Marco 

 

 

Edited August 12, 2025 by Z320

[RobH]

Since there is obviously a decent field,  the problem must be in the armature.   Maybe there is a short between turns or a short from the winding to the shaft. 

[Lebro]

A simple test - put 12V on both F & D connected, with a ground connection as well of course, the armature should spin. like a motor (which it now is)

Bob

[Z320]

Thanks Bob,

I did that already about 2 years ago with both - without result.

Since that I stored them away, now I have time to have a closer look.

Ciao, Marco

[Z320]

Both C40 work with the armature of my spare dynamo.

As the armature is not possible to repair, both are sadly scrap.

No problem at all, I have a working one in the car and a spare in the boot.

This two would have been only 2 spare units on top,

while my C40 not failed in 16 years (ownership since 2009). 

But it was interesting to take a look inside, thanks to Rob and Bob who helped.

Ciao, Marco

[RobH]

Half the price of getting old ones re-wound:

https://www.charnleys.com/parts/37H2985/armature-lucas-c39-c40/18456/

 

[Z320]

Out of stock

[Z320]

I found one more C40 in my workshop!

With broken flange and worn ball bearing.

I try its armature.

[Z320]

Before I forget:

according to the pulley diameters I noticed the need to spin the lathe with 1,800 revs (140/80 x 1.000 rev).

This is the "one more C40" I found in my workshop, a present from another friend (they know me, collecting everything)

 

Its armature is the most accurate made from all I have seen the last days!!!

  

Installed in the C40 with the black field coil above with an old used 2RS ball bearing: it works!

At 1.800 revs about 2 V without D and F connected, 13 V with D and F connected.

Ciao, Marco

 

Edited August 14, 2025 by Z320