30-12-2021, 04:55 PM
Thanks Peter and Trevor
The first attempt at a vacuum chamber didn't work out. I used what I thought was a sturdy bowl but the vacuum pump made short work of it.
I was a bit concerned using the vacuum chamber. I thought that the air trapped between the windings might force the windings towards the edges of the layers. The wire could then drop down a few layers and cause trouble. Because of that I let the vacuum build up slowly. I don't think any of the windings have displaced. At least I cant see any at the edges.
One thing that was of concern was immediately after finishing winding it I measured the winding resistance which was 10k. After it coming out of the vacuum chamber I measured the resistance again and it had dropped to 9.7k. The first reading was taking at the end of a long day in the workshop which was good and warm. The second reading was taken the following morning when the workshop was much cooler. I am aware that the resistance of copper changes with temperature but this seamed too big of a change to be put down to temperature difference. I was afraid that while in the vacuum chamber some of the wires got crushed together and caused them to short. But testing the transformer showed no faults.
Over night I had left the transformer in a cooler room. This morning it measured 9.5k. I then left it in front of a fan heater until the core was warm to the touch. Measuring the resistance of the winding again it had risen to 10.8k. So I looks like it is down to temperature. I am just surprised at how much the resistance changed with tempature.
Today I tackled the HT transformer. The insulation was crumbling on its leadout wires.
Unlike the EHT transformer the laminations were varnished so there is very little rust on it. I had thought that I would have to remove the laminations to work on it. But there was sufficient distance between the windings and laminations to allow the removal of the insulation and cloth tape holding the wires in place. I replaced the wires and also the sleeving covering the heater wires.
All windings test OK but I will give both transformers a long soak test.
Frank
The first attempt at a vacuum chamber didn't work out. I used what I thought was a sturdy bowl but the vacuum pump made short work of it.
I was a bit concerned using the vacuum chamber. I thought that the air trapped between the windings might force the windings towards the edges of the layers. The wire could then drop down a few layers and cause trouble. Because of that I let the vacuum build up slowly. I don't think any of the windings have displaced. At least I cant see any at the edges.
One thing that was of concern was immediately after finishing winding it I measured the winding resistance which was 10k. After it coming out of the vacuum chamber I measured the resistance again and it had dropped to 9.7k. The first reading was taking at the end of a long day in the workshop which was good and warm. The second reading was taken the following morning when the workshop was much cooler. I am aware that the resistance of copper changes with temperature but this seamed too big of a change to be put down to temperature difference. I was afraid that while in the vacuum chamber some of the wires got crushed together and caused them to short. But testing the transformer showed no faults.
Over night I had left the transformer in a cooler room. This morning it measured 9.5k. I then left it in front of a fan heater until the core was warm to the touch. Measuring the resistance of the winding again it had risen to 10.8k. So I looks like it is down to temperature. I am just surprised at how much the resistance changed with tempature.
Today I tackled the HT transformer. The insulation was crumbling on its leadout wires.
Unlike the EHT transformer the laminations were varnished so there is very little rust on it. I had thought that I would have to remove the laminations to work on it. But there was sufficient distance between the windings and laminations to allow the removal of the insulation and cloth tape holding the wires in place. I replaced the wires and also the sleeving covering the heater wires.
All windings test OK but I will give both transformers a long soak test.
Frank
