14-08-2022, 02:27 PM
(This post was last modified: 14-08-2022, 02:30 PM by John Cornforth.)
Hi Joe
I have been on holiday in deepest Wales where mountains and mobile internet don't mix !
An open circuit finely wound "Voltage/Shunt" coil is clearly not going to help matters, but I am still a little surprised at the behaviour you describe. With the cutout initially open, there should be no current flow through the thickly wound "Current/Series" coil either. With no change in magnetic field from either coil, one wouldn't expect any movement of the cutout armature.
As the two windings are wound one on top of the other (Thin one on the inside), it's not impossible there is a short circuit from one to the other due to chafed enamel insulation. The most thorough test is with all wiring disconnected from the cutout. The voltage coil from +D (Yellow Dynamo Output) to E (Earth) should measure about 36 ohms. The current coil from +D to A should be open circuit with the points open, and near short circuit with the points held closed. Do make sure that your meter probes are sharp and pressed hard enough to any contact to pierce through tarnish. If the cutout passes these tests, the finger of suspicion swings more towards the wiring, not forgetting the short connection from the Earth terminal to the car's body. Another item to check is the ignition warning lamp, which should ideally be in working order and definitely not short-circuited.
If there is a fault and you need to get the cutout rewound, here is the detailed information:
1. Shunt winding: 5000 turns 36SWG (About 8 thou), 140 turns per layer, 36 layers. Start and finish at bottom of bobbin. Wind anticlockwise as viewed from Armature. Start and finish to be connected to flexible tails and identified. (Start = Earth, Finish = “D”)
2. Inter-winding insulation: thin PVC or similar
3. Series winding: 50 turns, 16SWG, 17 turns per layer, 3 layers. Start at bottom leaving 10cm tail to connect to “A”; finish at top and connect to armature contact. Wind anticlockwise as viewed from Armature.
The shunt winding has a resistance of 36 +/- 3 Ohms, which means that at 6V approximately 160mA flows in the winding. Thus it produces 0.16 x 5000 = 800 Ampere-Turns of magnetic attraction. In order to counteract this, the force in the series winding must be equal but opposite. As the winding has 100 times fewer turns, this would occur at 100 times more current, about -16 Amps. The reason the cut-out operates at a lower reverse current, as seen on the ammeter, of somewhere between 5 and 10 Amps is that the spring on the armature acts to assist the unlocking action; hence the “drop out” current is significantly lower than the theoretical 16A.
I have been on holiday in deepest Wales where mountains and mobile internet don't mix !
An open circuit finely wound "Voltage/Shunt" coil is clearly not going to help matters, but I am still a little surprised at the behaviour you describe. With the cutout initially open, there should be no current flow through the thickly wound "Current/Series" coil either. With no change in magnetic field from either coil, one wouldn't expect any movement of the cutout armature.
As the two windings are wound one on top of the other (Thin one on the inside), it's not impossible there is a short circuit from one to the other due to chafed enamel insulation. The most thorough test is with all wiring disconnected from the cutout. The voltage coil from +D (Yellow Dynamo Output) to E (Earth) should measure about 36 ohms. The current coil from +D to A should be open circuit with the points open, and near short circuit with the points held closed. Do make sure that your meter probes are sharp and pressed hard enough to any contact to pierce through tarnish. If the cutout passes these tests, the finger of suspicion swings more towards the wiring, not forgetting the short connection from the Earth terminal to the car's body. Another item to check is the ignition warning lamp, which should ideally be in working order and definitely not short-circuited.
If there is a fault and you need to get the cutout rewound, here is the detailed information:
1. Shunt winding: 5000 turns 36SWG (About 8 thou), 140 turns per layer, 36 layers. Start and finish at bottom of bobbin. Wind anticlockwise as viewed from Armature. Start and finish to be connected to flexible tails and identified. (Start = Earth, Finish = “D”)
2. Inter-winding insulation: thin PVC or similar
3. Series winding: 50 turns, 16SWG, 17 turns per layer, 3 layers. Start at bottom leaving 10cm tail to connect to “A”; finish at top and connect to armature contact. Wind anticlockwise as viewed from Armature.
The shunt winding has a resistance of 36 +/- 3 Ohms, which means that at 6V approximately 160mA flows in the winding. Thus it produces 0.16 x 5000 = 800 Ampere-Turns of magnetic attraction. In order to counteract this, the force in the series winding must be equal but opposite. As the winding has 100 times fewer turns, this would occur at 100 times more current, about -16 Amps. The reason the cut-out operates at a lower reverse current, as seen on the ammeter, of somewhere between 5 and 10 Amps is that the spring on the armature acts to assist the unlocking action; hence the “drop out” current is significantly lower than the theoretical 16A.