In another thread on the restoration of an Ekco AC77 I mentioned that I would need to rewind the field coil of the mains energised speaker. The coil has approximately 2km of 38 SWG wire, so some form of powered winder is called for. In researching this topic and looking at videos on youtube, one of the considerations isn't simply adding turns to the former, but ensuring that the feed wire coil turns smoothly, but isn't allowed to 'free-wheel' because if the winder is stopped or slowed down, there is a risk that the feed coil will continue to rotate, unravelling wire. Thus, some means of gently tensioning the feed wire is called for, but must not risk snapping the wire.
Another consideration is that of at least some attempt at trying to lay the turns as neatly as possible, though without a precise mechanism, when using such fine gauge wire, the turns aren't going to lay side by side, nor do they need to. The risk of course is that if turns are in contact with turns on lower layers, the electrical potential between them may be high enough to rupture the insulation. However, I'm advised by Gary that modern enamelled copper wire is good for 1,000V between turns, and in this coil of 2,000 Ohms, only 110 Volts are dropped across the whole coil. That said, I'm including a wire guide consisting of two pulleys which I can guide by hand to at least try to make a neat job of the winding. (The finished wire guide will be like the one in the youtube video below).
I've added some pcs which I hope make things clearer. It will be evident right away that I've borrowed several ideas from Gary's coil winder on the basis that imitation is the sincerest form of flattery!
I've shown details of the components of the adjustable spring-loaded tensioning arrangement to apply slight tension to either side of the feed bobbin. First I turned a Tufnol rod to fit the hole through the feed coil and drilled out the rod to fit onto the 1/4" brass rod, so that the bobbin will rotate freely with no jerkiness. To fit onto the brass rod at each end of the feed bobbin I turned a couple of brass end plates about 2" diameter, onto which I soldered two brass 'cups' to hold a 7mm diameter weak compression spring. Then I added two 'cupped' brass bushes with 2BA grub screws, which can be slid along the brass rod at the other end of each spring, with the position of those bushes set so as to allow just enough tension so that when the foot pedal stops the motor, the feed coil will stop rotating rather than 'free-wheeling' and unravelling more wire. That's the theory, but as we know, the difference between theory and practice in practice, is often greater than the difference between theory and practice in theory! I'll soon find out. I've attached a blank spool on the winding bobbin to show how it's held in place with two tapered cones to allow the turns to be wound onto that bobbin.
I've turned a plastic pulley for the drive shaft and have made that pulley about eight times larger than the pulley on the motor, so it slows the motor speed down still further than does the foot controller - it can go down to about 50 RPM, or faster than it will ever need to. I've made rudimentary 'bearings' at each end of the drive shaft from little squares of bright mild steel. Quite adequate for the limited use to which the winder will be put. The plastics I used was what I had to hand, which is why it's several colours. The large pulley is turned from a type of plastic made from re-cycled yoghurt pots, and the little flecks are bit of foil left on the scrap cartons. (The motor and controller are an e-bay find).
At the RH end of the drive shaft, there's a little cover which can be moved to allow the shaft to be withdrawn to remove the completed coil. At the LH end of the drive shaft there is a cam which operates the micro-switch to trigger the ‘=’ key of the calculator to count the turns. I had a job to find a calculator which has an actual PCB for the keypad - most seem to have a plastic membrane to which contacts can't be added. The model I used is the 'KC-138AP' from Wilko. It's dual power so has an internal battery, whereas many only use solar. It set me back £1.45!! Really, in this application a turns counter isn't essential - it's the total resistance of the coil that matters and that will be more or less evident when the bobbin is full.
I've added a pic which shows what a state the field coil pole piece was in and a pic of it cleaned up and sprayed. I've also shown the state of the coil.
I hope the pics help clarify what I'm up to.
Progress on all of my projects is erratic and spasmodic, as we spend so much time away from home, so I only get an hour or two each week for hobbies. But then hobbies are about the relaxing, enjoyable use of leisure time, which for me, is a scarce commodity! I do hope to make steady progress over the winter months and will update the thread as and when.
Here is another source if my inspiration and ideas, though the wire gauge is coarser than that which I'm using. (Note the rudimentary, though effective, spring tensioning device!):
http://www.youtube.com/watch?v=yfppXiNRnrc
Another consideration is that of at least some attempt at trying to lay the turns as neatly as possible, though without a precise mechanism, when using such fine gauge wire, the turns aren't going to lay side by side, nor do they need to. The risk of course is that if turns are in contact with turns on lower layers, the electrical potential between them may be high enough to rupture the insulation. However, I'm advised by Gary that modern enamelled copper wire is good for 1,000V between turns, and in this coil of 2,000 Ohms, only 110 Volts are dropped across the whole coil. That said, I'm including a wire guide consisting of two pulleys which I can guide by hand to at least try to make a neat job of the winding. (The finished wire guide will be like the one in the youtube video below).
I've added some pcs which I hope make things clearer. It will be evident right away that I've borrowed several ideas from Gary's coil winder on the basis that imitation is the sincerest form of flattery!
I've shown details of the components of the adjustable spring-loaded tensioning arrangement to apply slight tension to either side of the feed bobbin. First I turned a Tufnol rod to fit the hole through the feed coil and drilled out the rod to fit onto the 1/4" brass rod, so that the bobbin will rotate freely with no jerkiness. To fit onto the brass rod at each end of the feed bobbin I turned a couple of brass end plates about 2" diameter, onto which I soldered two brass 'cups' to hold a 7mm diameter weak compression spring. Then I added two 'cupped' brass bushes with 2BA grub screws, which can be slid along the brass rod at the other end of each spring, with the position of those bushes set so as to allow just enough tension so that when the foot pedal stops the motor, the feed coil will stop rotating rather than 'free-wheeling' and unravelling more wire. That's the theory, but as we know, the difference between theory and practice in practice, is often greater than the difference between theory and practice in theory! I'll soon find out. I've attached a blank spool on the winding bobbin to show how it's held in place with two tapered cones to allow the turns to be wound onto that bobbin.
I've turned a plastic pulley for the drive shaft and have made that pulley about eight times larger than the pulley on the motor, so it slows the motor speed down still further than does the foot controller - it can go down to about 50 RPM, or faster than it will ever need to. I've made rudimentary 'bearings' at each end of the drive shaft from little squares of bright mild steel. Quite adequate for the limited use to which the winder will be put. The plastics I used was what I had to hand, which is why it's several colours. The large pulley is turned from a type of plastic made from re-cycled yoghurt pots, and the little flecks are bit of foil left on the scrap cartons. (The motor and controller are an e-bay find).
At the RH end of the drive shaft, there's a little cover which can be moved to allow the shaft to be withdrawn to remove the completed coil. At the LH end of the drive shaft there is a cam which operates the micro-switch to trigger the ‘=’ key of the calculator to count the turns. I had a job to find a calculator which has an actual PCB for the keypad - most seem to have a plastic membrane to which contacts can't be added. The model I used is the 'KC-138AP' from Wilko. It's dual power so has an internal battery, whereas many only use solar. It set me back £1.45!! Really, in this application a turns counter isn't essential - it's the total resistance of the coil that matters and that will be more or less evident when the bobbin is full.
I've added a pic which shows what a state the field coil pole piece was in and a pic of it cleaned up and sprayed. I've also shown the state of the coil.
I hope the pics help clarify what I'm up to.
Progress on all of my projects is erratic and spasmodic, as we spend so much time away from home, so I only get an hour or two each week for hobbies. But then hobbies are about the relaxing, enjoyable use of leisure time, which for me, is a scarce commodity! I do hope to make steady progress over the winter months and will update the thread as and when.
Here is another source if my inspiration and ideas, though the wire gauge is coarser than that which I'm using. (Note the rudimentary, though effective, spring tensioning device!):
http://www.youtube.com/watch?v=yfppXiNRnrc
Regards, David.
BVWS Member.
G-QRP Club Member 1339.
'I'm in my own little world, but I'm happy, and they know me here'
BVWS Member.
G-QRP Club Member 1339.
'I'm in my own little world, but I'm happy, and they know me here'
