Another Dynamo Conumdrum

geechr · Post by **geechr** » Wed Jun 13, 2007 11:13 pm

I have been having a problem with a dynamo from my 1957 twin - it will not produce any charge. I have done the following: -

Tested the field coil - 3.1 Ohm.
Checked that it is not shorted to the body.
Cleaned the commutator with petrol.
Carefully cleaned the gaps between the commutator with a ground down hacksaw blade.
Tested the resistance between adjacent poles of the commutator - all about 0.4 Ohm
Cleaned the brushes.
Made sure that the brushes move in their holders.
Flashed the field coils.
Connected "F" and "D" together and connected a battery so the unit became a motor. The armature rotated smoothly and went in the direction of the arrow on the body.
Connected "F" and "D" together and checked for a voltage when I span it up using my lathe. I only get about 0.1V. I tried connecting a bulb to add a load but the bulb did not light up.

How can it work as a motor but not as a generator?
Is there any other tests I can perform?
What does a growler do? I have access to some fairly sophisticated measuring equipment, oscilloscope etc so I might be able to simulate a growler.

I'm pretty confused and frustrated!

Thanks in anticipation.

Chris Gee

Don Madden · Post by **Don Madden** » Thu Jun 14, 2007 3:14 am

It sounds like you have shorted turns inside your armature, which a growler is designed to detect. It induces a current in the windings, which will cause a metal strip held on it to vibrate.

Even with a minor short in the armature it will motor, but not generate.

Typically with a shorted turn, applying the battery to the two connected terminals while the engine is running above 1000RPM, then removing the battery but leave the two terminals connected, it will charge until stopped, then not produce until the process is repeated.

A growler is an iron, open frame coil with the pole piece ends shaped like a V. The armature is placed in the V & it completes the magnetic loop. As it has alternating current appied to the windings the resultant oscillating field will induce similar current in the windings of the armature. In short, they form a transformer.

Cheers, Don.

GDPR · Post by **GDPR** » Thu Jun 14, 2007 12:17 pm

GDPR · Post by **GDPR** » Thu Jun 14, 2007 12:21 pm

Hi I had the same problems which all the usual tests failed to resolve even had it "professionally " worked on. Had some problems with oil leaking onto the commutator which blacked it up which after cleaning seemed to work however within short space of time same problem.

GDPR · Post by **GDPR** » Thu Jun 14, 2007 12:25 pm

Sorry having problems with this now ,I was going to say in desperation took it up to see my friend John Bolton at Kettering and all he did was set it up on the bench and give it a high speed run with a hand drill and guess what touch wood it seems to work a treat. Well worth a try seems they need a bit of a high speed run sometimes.

56G80S · Post by **56G80S** » Mon Jun 18, 2007 3:14 pm

I hope that it continues to work for you but don't go for any long night trips just yet. As Don knows, through his attempts to help, I did all the things that you did. I even re-soldered the armature connections and the dynamo motored very well. But no real charge when connected to a drill (running the drill backwards) or on the 'bike. Electro Diesel place tested the armature with the said growler and said it was u/s.

Johnny B

Biscuit · Post by **Biscuit** » Mon Jun 18, 2007 3:39 pm

Sounds like weak residual magnetism in the field core.