Extracting star point on a 3-phase motor?

Has anyone here had real experience of converting star-wound motors to star/delta?
I'm swapping my CVA lathe to inverter drive. I thought I had the job
cracked, I had a spare motor of the same power and nominal frame size as the original, but dual-voltage (and with duff bearings). I sorted out the bearings, swapped the pulley, bolted it to the (bl**dy heavy!) sub-plate, then found it wouldn't go into the space available with any room for adjustment or for wiring it up :-(( The frame is about an inch bigger across the fins, and the terminal box is about another inch bigger, than the original. There might, just about, be room if I reverse the frame so that the terminal box is on the other side, but it's highly doubtful. Plan B is to convert the original motor and put it back. I've had the armature out, & I'm pretty clear that I can identify the star point and break it out. If I do this, any recommendations about type of wire to use, what to use to replace the lacing I'll have to cut away, and is there a 'domestic' substitute for the varnish used to tie it all together? Last time I had one 'done' at the local rewind place they charged me 50 quid (without a bill), the three extra wires were just left as 'tails', though they did to a thorough job of the lacing & varnishing.
Cheers Tim
Dutton Dry-Dock Traditional & Modern canal craft repairs Vintage diesel engine service
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload

to
size
the
varnishing.
Tim,
I've done it twice. First time was not strictly a star point as it was a dual winding single phase motor that I wanted to make reversable so wanted both ends of both windings. Was dead easy as the join was on the surface so to say. I just separated the join, soldered on ptfe insulated wire as it was what I had to hand, sleeved with high temperature silicone sleeving, tied it down with strong cotton thread to the coils, and gave it all two coats of polyurathane varnish. Second time was the suds pump on a Colchester Student. This one was a bit like brain surgery as it was a smaller motor and for some reason the star point was tucked inside a coil between windings which had then been varnished. After very careful separating of the windings with a wooden tooth pick I mangaged to extracate the star point and make off the new wires. Again after insulating and tying off I soaked it in polyurathane, but this time virtually potted it as I was concious that it spent its life a few inches above the water level. Problem with this motor was finding somewhere for the extra terminals ! Both conversions were entirely satisfactory - The single phase motor drives the roller shutter on my garage and has done for the last 15 years. The Student I had for perhaps ten years after the conversion and never had problems.
AWEM
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload
Hi Tim,
I've done this twice too (or once on a dual-wound two-speed machine in fact). Firstly, you need to make sure that you have a machine which is star connected for 415 V. If your machine is delta connected for 415 V, you can't convert it to 240 V operation, but I'm sure that you already knew this. Usually the star point is fairly visible once you've removed the end bells and rotor. It will normally be inside some glass fibre sleeving, tied and varnished to the windings at one end of the stator. Get the motor off the machine and into a comfortable, well lit location. When you've got plenty of time and are feeling patient, carefully cut the cords which tie the windings together and identify which length of sleeving has three wires leaving it (two from one end of the sleeve and one from the other). This will be the star point. Carefully slit the sleeving with a sharp knife and remove it to reveal the soldered joint. You may or may not be able to neatly desolder the joint. If you can't, cut the lacquered wire as close to the joint as you can. Try not to nick the windings with your knife or nippers, although if you do it isn't a complete disaster because you can paint over the nicks with insulating varnish. Now take some emery paper and draw it along the ends of the wires to remove the lacquer. Be very careful not to bend the wire, or it may break. I broke a wire doing this and only just had enough to solder onto. If it breaks near where it enters the windings you'll have to get a rewind, so it's worth taking your time here. Now tin some lengths of stranded wire to use as your lead-outs. I used 105 C equipment wire from Farnell, but I understand that the temperature rating should officially be higher. It worked fine for me, though. I'd attach one lead-out to each of the three wires at the star point, and take all six wires out of the motor. I don't think it's worth trying to create a delta connection internally when you can just attach two wires to each of the three terminals in the terminal box. I fitted heat-shrink sleeving over my soldered joints, tied everything together with thick cotton and painted it with a bit of glass fibre resin. You can get proper insulating varnish from Farnell if you want. My motor probably isn't quite standards-compliant but it has been working fine for six years now. The whole job took me about 3 to 4 hours as I remember.
Good luck!
Best wishes,
Chris
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload
On 20 Oct 2006 14:15:47 -0700, "Christopher Tidy"

Thanks to both Christopher & Andrew for helpful replies. I don't have any wire which I would be happy about using, the two likely sources around here aren't open on a Saturday morning, so it'll have to wait for Monday. It's one of those annotying jobs, I had a couple of hours available on Friday afternoon, thought it was plenty of time to get the motor fitted, I wanted to test it with the new drive before committing to all the rest of the rewiring. There's a bit of pressure to get the job done, I've got a bit of space to get round the machine at the moment but not for long. The 'new' 50-year old big lathe is getting closer, it's only half a mile away now <G>
Cheers Tim
Dutton Dry-Dock Traditional & Modern canal craft repairs Vintage diesel engine service
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload
Tim,
I've just done my first one. It was hard to spot the sleeve with three wires, but eventually found it after a couple that looked like three wires turned out not to be..
I scrounged some high temperature sleeving from work. The sparks guy said it was woven fibreglass so I should wear gloves - didn't see why. Its about 5mm ID which is slightly larger than ideal but did the job. I used araldite to fasten back in place anything I had disturbed.
I have plenty of the sleeving left over, so can send it to you if you want.
Steve
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload
wrote:

Steve
Many thanks for the offer. I'll see what I can pick up locally on Monday, I might get back to you if my luck is out ;-)
Cheers Tim
Dutton Dry-Dock Traditional & Modern canal craft repairs Vintage diesel engine service
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload
Tim Leech wrote:

I bought fibreglass sleeving from B and Q not so long ago. It is needed in some of the lo-volt halogen fittings.
just a though, asB and Q are open on sundays.
Dave
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload
On 21 Oct 2006 12:48:52 -0700, " snipped-for-privacy@bem.fki-et.com"

Ta, I didn't try it as the nearest big B&Q is quite a trek. Stored for future refernce, though.
I can report (see my note in the 'selecting a relay' thread) that the motor now runs as sweet as a nut from the inverter, when just sitting on the floor with no load. Any suggestions as to how I can determine whether it really is wired for 240V operation now, apart from seeing how well it stands up to a load? The inverter isn't one which can display current.
Cheers Tim
Dutton Dry-Dock Traditional & Modern canal craft repairs Vintage diesel engine service
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload

Polytechforum.com is a website by engineers for engineers. It is not affiliated with any of manufacturers or vendors discussed here. All logos and trade names are the property of their respective owners.