11-02-2012, 03:41 PM
Hi,
With a gently nudge from Joe in another thread suggesting I could perhaps add notes on transformer winding I decided to see what notes and pictures I had on record.
I’ll add another thread later on basic coil winding techniques for novices explaining how I go about it being very much a novice myself. Whilst searching through my notes filed on CD I pulled out the following which might be of interest covering a very tricky rewind job I attempted on a Ferranti inter valve transformer.
Even though I’ve wound quite a few coils I’m completely self taught on this subject and members like Ed Dinning could run rings around me when it comes down to technicalities so I can only write from personal experiences.
Ferranti inter-valve transformer.
Occasionally I’ll have a go at re-winding a transformer when asked for help and I can fit it in. This was one such request and I thought it would be an interesting challenge as I’ve seen many of these Ferranti transformers but never seen one with the metal covers removed. What a huge mistake it was to take this on; it quickly got out of hand turning into a major project mostly through my own ignorance of the subject.
Notes together with sketches were made of the connections and a number of digital images taken. The outer covers were easy to remove revealing what looked like a standard type coil with its laminations. Upon closer inspection these laminations were different to anything I’d encountered before; the joints were most strange; how the heck could I remove these laminations in order to release the coils?
Try removing these laminations.
Laminations can be difficult to remove at the best of times especially if they are tightly driven home or are rusty. These laminations tested my patience to the limit and caused much frustration; one piece laminations were new to me and these proved a nightmare to remove; with much tugging and towing at last one of the laminations was sitting on the bench and could be studied. The idea of using one pieced laminations makes a great deal of sense when everything is being newly assembled at the factory but 50 years later they were a different story entirely. So far the job wasn’t going too well but with hindsight this was going to prove the easy part.
One piece lamination.
Screwdrivers; pliers and anything else I could lay my hands on had been tried in order to extract the laminations. During this time I hadn’t realized just how much damage was being caused to the bobbin/spool. The edges on view appeared to be plastic rings so surely these would be simple to replace? With the bobbin freed it became only too apparent that this job was going to turn on me and bite me very hard indeed. The plastic rings turned out to be the ends of three extremely flimsy perforated plastic cylinders for want of a better description. What a truly inspirational bit of design work on the part of Ferranti these were. Three separate plastic bobbins fitting closely together in closed telescope fashion and each having three separate coils of enameled copper wire.
I've never seen one like this before.
Fortunately only the outer larger bobbin had been damaged but even this was going to be difficult if not impossible to repair. What a dilemma I was now in; I’d destroyed a transformer not belonging to me and quickly needed to dream up a solution to get me out of this mess.
Made ready to accept epoxy adhesive.
After a sleepless night a plan of action had been decided upon; repair the bobbin using epoxy adhesive building up a good thickness then when allowed to fully harden to remove excess using the engineering lathe; the idea seemed feasible enough so a lot of time was spent in making a wooden friction fitting plug and a pair of flanges. Sellotape was used to prevent the epoxy sticking where it wasn’t intended to stick and using Poundland epoxy a good thickness was mixed and laid on; this was a mistake because epoxy tends to flow out and this caused more frustration but eventually enough thickness had been applied and the job was left overnight in a warm place to allow the epoxy to fully harden. A quick description of what followed is one word; FAILURE. I’ve never succeeded in gluing plastic and as soon as the lathe tool made contact the epoxy merely parted company with the broken bobbin and now my patience was being stretched.
Total failure.
After another restless night in bed plan “B” was to attempt to make a brand new bobbin from scratch. I knew I had a big lump of black plastic round bar it being an offcut I had picked up whilst buying Tufnol. A quick measurement confirmed it was plenty big enough but there was only enough to make a single bobbin from so any silly mistake would end up rather expensive.
So far all I had succeeded in doing was to take on this job on as a favour in order to help a fellow enthusiast out and ended up with nothing but trouble and grief; now it was going to be my turn as I wasn’t going to be beaten.
Having never turned plastic before the first decision was which lathe to use as I own both woodturning and metal turning lathes. The Myford engineering lathe was decided upon and the plastic blank was gently nipped in the three jaw. A turning job like this needs careful consideration otherwise it is easy to fall into a trap and lose concentricity. I had given the job a lot of thought and had the sequence worked out. First job would be to bore the hole. As an experiment a large woodworking Forstner bit measuring well undersize of the finished hole dimension was mounted in the tailstock and this very efficiently removed the bulk of the waste whilst taking a lot of care not to run the bit too deep fouling the chuck and causing damage. Very accurate measurements had previously been taken and it was found that by playing around with these measurements increased winding area could be gained allowing slightly thicker winding wire to be used. The Forstner bit was cleaned and put away then a boring bar mounted in the cross slide tool post. Roughing cuts brought the hole near to size then great care was taken bringing the bore to exact size. This blank hole was deeper than the finished length of the bobbin.
Using boring bar.
Now the end without hole was cut away giving an open hole throughout the bobbin length. A dummy mandrel was turned from steel allowing a friction fit of the new bobbin blank. With the blank installed onto the mandrel roughing cuts were taken using a new tool bit to remove most of the waste then it was brought down to exact maximum diameter making it like a plastic cylinder. Now the fun could begin as one slip here would result in failure. A special HSS (high speed steel) cutter was ground allowing it to cut on end and both sides; this was needed as three closed recesses had to be turned. A lot of care and many measurements were taken to ensure not only the recesses were accurate to width but also to depth. This was completed successfully much to my relief and the lathe was cleaned then oiled. A digital vernier caliper was used for all measurements.
The original bobbin had many square holes so more thought was needed how to add these to the new bobbin. Three paper strips fitting perfectly into the recesses were cut then these in turn were accurately marked for hole centers before securing to the bobbin with Sellotape. The holes were pricked through with a sharp pointed tool and the paper removed. The previously turned wooden plug was pushed home hopefully allowing the bobbin to be drilled without it being crushed or deformed.
Holes marked on centres.
This was gripped in the machine vice on the big drill press and a suitable bit chucked; any hole not on centre would stand out so once again a lot of care was called for. This big drill proved too difficult to use as the bit chattered giving a ragged hole which was totally unacceptable. I’ve had half a century of using hand tools so mounted the bobbin in the big engineering vice on the bench; the correct sized spur point wood drill bit was chucked in the Bosch drill and each hole drilled bang on the mark by eye. At last the new bobbin was finally in my hand giving me a lot of personal satisfaction.
New home made bobbin/spool.
The actual winding of the coils was straightforward and without trouble; the winding was carried out on the manual AVO Douglas hand winder. Re-fitting the laminations proved to be a chore as each lamination needed “springing” ensuring the sharp ends didn’t rake the new coils; what a terrible job these laminations were and I hope I never come across them ever again. The transformer was assembled and continuity tests taken then it was posted off to a very happy owner with my compliments requesting he keep quiet about what I had done as I don’t want to end up winding hundreds of transformers.
Winding completed successfully.
Monetary wise I was out of pocket but then I’ve never been in this hobby to make money; the experience gained though was priceless to me and what a wonderful learning curve this job turned out to be; the icing on the cake was to later receive a message of thanks and to inform me the transformer was working. It shows what can be achieved in desperation (Panic) and in refusing to accept defeat. What a great hobby this is.
Kind regards, Col.
With a gently nudge from Joe in another thread suggesting I could perhaps add notes on transformer winding I decided to see what notes and pictures I had on record.
I’ll add another thread later on basic coil winding techniques for novices explaining how I go about it being very much a novice myself. Whilst searching through my notes filed on CD I pulled out the following which might be of interest covering a very tricky rewind job I attempted on a Ferranti inter valve transformer.
Even though I’ve wound quite a few coils I’m completely self taught on this subject and members like Ed Dinning could run rings around me when it comes down to technicalities so I can only write from personal experiences.
Ferranti inter-valve transformer.
Occasionally I’ll have a go at re-winding a transformer when asked for help and I can fit it in. This was one such request and I thought it would be an interesting challenge as I’ve seen many of these Ferranti transformers but never seen one with the metal covers removed. What a huge mistake it was to take this on; it quickly got out of hand turning into a major project mostly through my own ignorance of the subject.
Notes together with sketches were made of the connections and a number of digital images taken. The outer covers were easy to remove revealing what looked like a standard type coil with its laminations. Upon closer inspection these laminations were different to anything I’d encountered before; the joints were most strange; how the heck could I remove these laminations in order to release the coils?
Try removing these laminations.
Laminations can be difficult to remove at the best of times especially if they are tightly driven home or are rusty. These laminations tested my patience to the limit and caused much frustration; one piece laminations were new to me and these proved a nightmare to remove; with much tugging and towing at last one of the laminations was sitting on the bench and could be studied. The idea of using one pieced laminations makes a great deal of sense when everything is being newly assembled at the factory but 50 years later they were a different story entirely. So far the job wasn’t going too well but with hindsight this was going to prove the easy part.
One piece lamination.
Screwdrivers; pliers and anything else I could lay my hands on had been tried in order to extract the laminations. During this time I hadn’t realized just how much damage was being caused to the bobbin/spool. The edges on view appeared to be plastic rings so surely these would be simple to replace? With the bobbin freed it became only too apparent that this job was going to turn on me and bite me very hard indeed. The plastic rings turned out to be the ends of three extremely flimsy perforated plastic cylinders for want of a better description. What a truly inspirational bit of design work on the part of Ferranti these were. Three separate plastic bobbins fitting closely together in closed telescope fashion and each having three separate coils of enameled copper wire.
I've never seen one like this before.
Fortunately only the outer larger bobbin had been damaged but even this was going to be difficult if not impossible to repair. What a dilemma I was now in; I’d destroyed a transformer not belonging to me and quickly needed to dream up a solution to get me out of this mess.
Made ready to accept epoxy adhesive.
After a sleepless night a plan of action had been decided upon; repair the bobbin using epoxy adhesive building up a good thickness then when allowed to fully harden to remove excess using the engineering lathe; the idea seemed feasible enough so a lot of time was spent in making a wooden friction fitting plug and a pair of flanges. Sellotape was used to prevent the epoxy sticking where it wasn’t intended to stick and using Poundland epoxy a good thickness was mixed and laid on; this was a mistake because epoxy tends to flow out and this caused more frustration but eventually enough thickness had been applied and the job was left overnight in a warm place to allow the epoxy to fully harden. A quick description of what followed is one word; FAILURE. I’ve never succeeded in gluing plastic and as soon as the lathe tool made contact the epoxy merely parted company with the broken bobbin and now my patience was being stretched.
Total failure.
After another restless night in bed plan “B” was to attempt to make a brand new bobbin from scratch. I knew I had a big lump of black plastic round bar it being an offcut I had picked up whilst buying Tufnol. A quick measurement confirmed it was plenty big enough but there was only enough to make a single bobbin from so any silly mistake would end up rather expensive.
So far all I had succeeded in doing was to take on this job on as a favour in order to help a fellow enthusiast out and ended up with nothing but trouble and grief; now it was going to be my turn as I wasn’t going to be beaten.
Having never turned plastic before the first decision was which lathe to use as I own both woodturning and metal turning lathes. The Myford engineering lathe was decided upon and the plastic blank was gently nipped in the three jaw. A turning job like this needs careful consideration otherwise it is easy to fall into a trap and lose concentricity. I had given the job a lot of thought and had the sequence worked out. First job would be to bore the hole. As an experiment a large woodworking Forstner bit measuring well undersize of the finished hole dimension was mounted in the tailstock and this very efficiently removed the bulk of the waste whilst taking a lot of care not to run the bit too deep fouling the chuck and causing damage. Very accurate measurements had previously been taken and it was found that by playing around with these measurements increased winding area could be gained allowing slightly thicker winding wire to be used. The Forstner bit was cleaned and put away then a boring bar mounted in the cross slide tool post. Roughing cuts brought the hole near to size then great care was taken bringing the bore to exact size. This blank hole was deeper than the finished length of the bobbin.
Using boring bar.
Now the end without hole was cut away giving an open hole throughout the bobbin length. A dummy mandrel was turned from steel allowing a friction fit of the new bobbin blank. With the blank installed onto the mandrel roughing cuts were taken using a new tool bit to remove most of the waste then it was brought down to exact maximum diameter making it like a plastic cylinder. Now the fun could begin as one slip here would result in failure. A special HSS (high speed steel) cutter was ground allowing it to cut on end and both sides; this was needed as three closed recesses had to be turned. A lot of care and many measurements were taken to ensure not only the recesses were accurate to width but also to depth. This was completed successfully much to my relief and the lathe was cleaned then oiled. A digital vernier caliper was used for all measurements.
The original bobbin had many square holes so more thought was needed how to add these to the new bobbin. Three paper strips fitting perfectly into the recesses were cut then these in turn were accurately marked for hole centers before securing to the bobbin with Sellotape. The holes were pricked through with a sharp pointed tool and the paper removed. The previously turned wooden plug was pushed home hopefully allowing the bobbin to be drilled without it being crushed or deformed.
Holes marked on centres.
This was gripped in the machine vice on the big drill press and a suitable bit chucked; any hole not on centre would stand out so once again a lot of care was called for. This big drill proved too difficult to use as the bit chattered giving a ragged hole which was totally unacceptable. I’ve had half a century of using hand tools so mounted the bobbin in the big engineering vice on the bench; the correct sized spur point wood drill bit was chucked in the Bosch drill and each hole drilled bang on the mark by eye. At last the new bobbin was finally in my hand giving me a lot of personal satisfaction.
New home made bobbin/spool.
The actual winding of the coils was straightforward and without trouble; the winding was carried out on the manual AVO Douglas hand winder. Re-fitting the laminations proved to be a chore as each lamination needed “springing” ensuring the sharp ends didn’t rake the new coils; what a terrible job these laminations were and I hope I never come across them ever again. The transformer was assembled and continuity tests taken then it was posted off to a very happy owner with my compliments requesting he keep quiet about what I had done as I don’t want to end up winding hundreds of transformers.
Winding completed successfully.
Monetary wise I was out of pocket but then I’ve never been in this hobby to make money; the experience gained though was priceless to me and what a wonderful learning curve this job turned out to be; the icing on the cake was to later receive a message of thanks and to inform me the transformer was working. It shows what can be achieved in desperation (Panic) and in refusing to accept defeat. What a great hobby this is.
Kind regards, Col.
Happiness is a wreck of a cabinet to restore.
