CAV DEL type dynamo question

[Mark McKibbin]

I am working on my new to me Chummy and the dynamo is a CAV and it looks like a type DEL,

1.  it produces virtually no voltage (0.5v)  
   
2. the brushes are good,
   
3. it will motor although it does not seem to go that fast,
   
4. I have used a growler and it finds no short circuited windings
   
5. The field coils measure 1.8 ohms
   
6. moving the 3 brush seems to make little difference
   
7. I have no idea if the dynamo ever worked as previous owner is dead 
   

The armature looks to have been rewound in modern times, I'm wondering as this type of dynamo has the brushes at 90 degrees is it different to those with brushes at 180 degrees and if so has it been rewound properly?

Picture of armature attached

[Dave Mann]

Are the main and third brush boxes insulated from the end plate?

[John Mason]

Hi Mark, when I look at your photo it looks like the gaps between the segments of the commutator where the windings are soldered to it are not clean. This could cause short circuits within the windings. it would do no harm to clean these out (and the gaps on the rest of the com) with a narrow sharp tool such as a pointed knife used upside down so that the sharp edge is facing upwards.

John Mason

[Bob Culver]

The two pole armatures are wound 1 to 2  to 3 etc and much can be fathomed with a digital meter which reads very low ohm. The 4 pole advances in steps of several segments and requires more thought to check .

[Mark McKibbin]

Are the main and third brush boxes insulated from the end plate?

Yes I have tested, they are open cct to the frame

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Hi Mark, when I look at your photo  it looks like the gaps between the segments of the commutator where the windings are soldered to it are not clean. This could cause short circuits within the windings.  it would do no harm to clean these out (and the gaps on the rest of the com) with a narrow sharp  tool such as a pointed knife used upside down so that the sharp edge is facing upwards.

John Mason

I have checked resistance between segments and there is some although not much at about 0.9 ohm

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The two pole armatures are wound 1 to 2  to 3 etc and much can be fathomed with a digital meter which reads very low ohm. The 4 pole advances in steps of several segments and requires more thought to check .

Thanks Bob,

With that in mind I will have another look with the multimeter.

[John H]

Correct me if I'm wrong, but I suspect it won't generate unless you supply current to the field winding from a battery via a "warning light" bulb, like you get with the dynamo installed in the car. If you simply spin it on the bench (with nothing else connected), I don't think it will do very much, even if everything is okay?

[Mark McKibbin]

Correct me if I'm wrong, but I suspect it won't generate unless you supply current to the field winding from a battery via a "warning light" bulb, like you get with the dynamo installed in the car. If you simply spin it on the bench (with nothing else connected), I don't think it will do very much, even if everything is okay?

Yes I have been running it in the car with the lamp connected, on the bench it has been run as a motor with the F and D terminals linked

[John Mason]

Mark, Have you another dynamo that you know is good that you can swap over armatures over with. That way ar least you’re will know if the problem is with the armature.

John Mason

[Bob Culver]

Digital voltmeters detect all sorts of stray voltages. Seven dynamos do seem prone to lose magnetisation. Re polarisation  is often helpful.
It is a long shot but if the field is connected backwards the dynamo generates an output which cancels the field so it will never get going.
It is very unlikely the armature is wound wrong . Even if given a bare armature and the wrong one as a pattern I think the comm section numbers is different, and one has  armature poles in a slight helix form..

[Mark McKibbin]

Digital voltmeters detect all sorts of stray voltages. Seven dynamos do seem prone to lose magnetisation. Re polarisation  is often helpful.
It is a long shot but if the field is connected backwards the dynamo generates an output which cancels the field so it will never get going.
It is very unlikely the armature is wound wrong . Even if given a bare armature and the wrong one as a pattern I think the comm section numbers is different, and one has  armature poles in a slight helix form..

I have found that as well until I bought a good one (Fluke) it is rock solid in an electrically noisy environment like a 7's charging system, I have re polarised it and I assume as I have run it as a motor that would have re polarised it again.

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Mark, Have you another dynamo that you know is good that you can swap over armatures over with. That way ar least you’re will know if the problem is with the armature.

John Mason

Good Idea John,
That will be on today's list.