How good/bad is a MOT core for making average step down transformers ?

• How good/bad is a MOT core for making average step down transformers ?
  Posted by lordvader88 on 25 Jun, 2019 22:12
• Say I want to take out both windings and make my own 120V to say 40V transformer, at some useful current level like  1A.
  
  Is there anything special about these cores overall that makes them good/bad for re-use ? I can probably find some info on the ones I have, and I want step-down transformers
• #1 Reply
  Posted by Zero999 on 25 Jun, 2019 22:20
• It's not something I've done. The magnetic shunt which increases the leakage inductance will need to be removed, if you want the transformer to have a low impedance and good regulation.
  
  Just one question: why? A microwave oven transformer is well overkill for a 40VA transformer. I recommend just buying one. I know that's not much fun and is probably not the answer you wanted, but it's certainly much easier.
• #2 Reply
  Posted by Ian.M on 25 Jun, 2019 22:31
• You are very much underestimating the VA a typical MOT core can handle, by about an order of magnitude.
  
  They are usually welded cores which makes them slightly lossier than an interleaved E/I core, and a total PITA to rewind.   Cutting the weld is possible, but then its difficult to get the core clamped back togethere adequately to get a small enough gap.   Apart from that, once you've knocked out the magnetic shunts and stripped the windings, they are not significantly different from any other laminated core of the same center limb CSA. 
  
  OTOH if you are keeping the original primary, you'll almost certainly find it runs on the ragged edge of saturation because it just doesn't have enough turns.  Due to the limited access to wind on a welded core, it may be preferable to add an auxiliary winding to put in series with the existing primary to get enough turns, rather than ripping it out and rewinding from scratch.
  
  
• #3 Reply
  Posted by lordvader88 on 25 Jun, 2019 23:34
• Ok I have 2 or 3, and 1 I cut apart before and have it sitting here with 2 intact primary winding's in it. I was going to make a 1:1 isolation transformer, but never had it welded back together, then bought an IT.
  
  I have lots of stranded PVC insulated 18AWG. Thats on the thick side. Lots of 24, 26, +30AWG, in my house. What would be a useful transformer I could make from that for hobby use, or homemade lab test equipent ?
  
  
  1 thing I could try and make is the transformer for a vacuum tube oscilloscope I own whose T. is toast. It had a split +/-325VAC, and up to +550VAC. I have the old winding somewhere, I could find it's rough turns count and wire gauge
  
  I have the core for that too, but it's in 100 pieces and bent up a bit (dummy)
  
  http://rhetoricity.com/misc/tubes/oscope/RCA-Oscilliscope-WO33A.pdf
  
  That can't use much power, ot's only 4 tubes. So with the wire on hand I might try and back of the envelope what I could do with the 18ga for the primary.
  
  There's not much room for PVC insulation. So someone once told me that I could add about 30% more turns to the primary to make it act nicer. If I can make that work, with added 18AWG, then fine. The original scope trans. winding's take little space compared to the MOT primary.
• #4 Reply
  Posted by lordvader88 on 26 Jun, 2019 00:40
• on 2nd thought the 18AWG is only rated for 100V. I have no idea the safety of my insulated solid wire. I have some from 120V induction motors, like bathroom fans, thats the highest I could say for sure.
• #5 Reply
  Posted by gbaddeley on 26 Jun, 2019 02:35
• Given the above advice, it is a bad idea to use a MOT for this purpose. PVC wire should not be used. It will melt, smoke and catch fire if overloaded.