Winding a Toroid Transformer

[ghadden]

I am starting over, again, this time I hope I have the right parts. I have been all over this site and can’t find anything on wire sizes for wiring a transformer. Check my math, is this the right way to figure this? I have three motors, 2ea 1.65V/ 6.8A 1ea 2.9V/4.6A. I went 25 times the 1.65 to get 41.25V max @ 18.2 A. 2/3 of 18.2=12.13 A. I bought a 700VA Toroid transformer kit. From the chart, for wire size is, max current density 1.5A/mm2 (12.13/1.5) so I need wire area of 8.10 or #8awg wire. I have .57V/N (38V/.57=66turns) but I don’t have enough total copper area for this so I reduced it to 62 turns and got 38.6V no load & 35.3V full load. Is this close enough? I can get #16 magnet wire locally but it comes in 62’ spools and I need 36 per coil. The motor shop will sell me #10 wire for $10 plus $40 min. charge. Is #8 too big? Would I have to run six coils of #16 to get the amps right? Did I read the charts wrong? If not, where can I get #8 magnet wire? Will this be OK after the diodes? I am using a Campbell breakout board so I don’t think I need the 5V tap.

[gmfoster]

Originally Posted by ghadden I am starting over, again, this time I hope I have the right parts. I have been all over this site and can’t find anything on wire sizes for wiring a transformer. Check my math, is this the right way to figure this? I have three motors, 2ea 1.65V/ 6.8A 1ea 2.9V/4.6A. I went 25 times the 1.65 to get 41.25V max @ 18.2 A. 2/3 of 18.2=12.13 A. I bought a 700VA Toroid transformer kit. From the chart, for wire size is, max current density 1.5A/mm2 (12.13/1.5) so I need wire area of 8.10 or #8awg wire. I have .57V/N (38V/.57=66turns) but I don’t have enough total copper area for this so I reduced it to 62 turns and got 38.6V no load & 35.3V full load. Is this close enough? I can get #16 magnet wire locally but it comes in 62’ spools and I need 36 per coil. The motor shop will sell me #10 wire for $10 plus $40 min. charge. Is #8 too big? Would I have to run six coils of #16 to get the amps right? Did I read the charts wrong? If not, where can I get #8 magnet wire? Will this be OK after the diodes? I am using a Campbell breakout board so I don’t think I need the 5V tap.

I'd use it like you have it. The extra voltage would not give you a great deal more speed in my thoughts. With the numbet 8 you are going to have very little drop under load. I think you wil be fine. If I needed the five volts I just find room for 8 to 10 turns of something #22 hookup wire and wind a seperate winding.


Where did you get the kit.

Thanks
Garry

[pastera]

#8 seems very large
If you are winding 36' of wire, the resistance will be about 0.023 ohms. This will give you a 3.3 watt copper loss at 12.13 amps.

How about winding multiple strands of 16 guage? Four strands would be equivalent (close) to #10. This would give a copper loss of about 5.3w

The provider of the kit should be able to give you some help on sizing the wire and the expected losses.

Aaron

[ghadden]

Gary
I got the kit from toroid.com. It was $80 for a 700VA. It does not come with secondary wire. After I ordered this I found partsexpress.com had one that was already made, cheaper.

Aron,
If I use 5 strands(that's all they have in stock here) that would be close to #8 and would work?

Thanks for your help! I didn't want to mess up again.
Guy

[Bloy2004]

...I'm interested in obtaining just the core for a toroid. What material can be used and what should be avoided? I've searched for sellors of just the cores, but haven't had much luck. I fiddled with rewinding an 8" 2KVA and had success, but would like to start from scratch for a next project....er at least the core.

Anyone know of sources?

Thanks,
Bloy(John)

[Al_The_Man]

Originally Posted by ghadden Gary I got the kit from toroid.com. It was $80 for a 700VA. It does not come with secondary wire. After I ordered this I found partsexpress.com had one that was already made, cheaper.

I usually get mine from the guy who makes them and sells on Ebay, he has a 700va on right now for $45.00, even if you have to strip the secondary and re-wind. The primary is already in place and is usually 120~240v wound.
Al.

[pastera]

Originally Posted by ghadden Gary I got the kit from toroid.com. It was $80 for a 700VA. It does not come with secondary wire. After I ordered this I found partsexpress.com had one that was already made, cheaper. Aron, If I use 5 strands(that's all they have in stock here) that would be close to #8 and would work? Thanks for your help! I didn't want to mess up again. Guy

What's nice is Toroid.com gives absolutly no info on the specs for the supplied core until you buy - not very good practice

The idea is to keep the total losses to a level that will not overheat the transformer. Did they gove you any information to calculate temperature rise?

Did they give you any info on core loss? If so, then you can match the copper loss (wire size) to the core loss.

I can't see the need for #8 wire on a core that large. You should be able to get away with a lot less copper than they are suggesting. Remember, you are designing the power supply based on a load of all three motors doing a rapid move - not very likely. While you are machining, the load will be much less (if you are using a chopper type driver) and the transformer will cool down.

Wind with three strands and test, if temps are too high just rewind with more strands, just remember to leave a foot or so extra on the leads while testing.


Take a look at this link: http://focus.ti.com/lit/ml/slup126/slup126.pdf

Aaron

[ghadden]

Aaron,
They didn't give me any info on temp. rise or core loss. I will try the #16, I have some #8 ordered, just in case.
Do I understand correctly? The thing I should watch for is heat? I should be OK to test the motors for movement for now.
Thanks,
Guy

[pastera]

If you are have the correct voltages then heat is the only real issue.

Remember, the transformer has a large thermal mass so you need to test the temperature for at least an hour. The transformer should not get more than hot to touch - 115°F.

Aaron

[ghadden]

I got the #8awg wire in the mail. Had my wife help wind the transformer. Used a 30000uF cap, bridge diode, bleed resistor and a bunch of fuses- got enough power to the drivers. With some help from Robert Campbell, after 3 month, I got motors to turn. It helps to use the right components to start with.

Thanks everyone for your help!

Guy