Gapped core mains transformer? - UK Vintage Radio Repair and Restoration Discussion Forum

Boater Sam · 24th Aug 2018, 5:53 pm

I had microwave mains transformer lying around.
I thought it could be a good source of laminations for making transformers or even if I reused the primary winding, a really heavy current transformer.
Being 21st century imported junk, the lams had 4 arc welds across them and no bolts.
Careful grinding removed the welds and a good battering released the lams from the compound that it had been impregnated with.
And there was the surprise, "E" & "I" lams stacked together on top of one another like an output transformer, not interleaved like a mains one.
It had a negligible gap between the legs of the "Es" and the "Is"
Managed to get the primary off in one undamaged impregnated piece.
Is this method of construction for a good reason?
Sam.

turretslug · 24th Aug 2018, 6:05 pm

I believe that an operating magnetron looks like a low impedance from the supply point of view, so the supply needs to have a high source impedance to limit the current- arranging the lam-stack appropriately is as near "free" as current limiting gets. Also, ISTR that the cathode gets bombarded by some of the fierce cyclone of electrons intrinsic to operation, reducing the necessary heater power from the transformer once HT current starts being drawn.

Ed_Dinning · 24th Aug 2018, 7:56 pm

Hi Sam, this technique is used on many cheaper transformers these days as it is a quicker process.
If the cores are clamped together correctly and then TIG welded the losses in the core are only slightly increased and production costs are lowered.
It would NEVER do on an audio output transformer.

Ed

Radio Wrangler · 24th Aug 2018, 8:10 pm

The magnetron looks like a negative load impedance to the power supply over part of its operating range, and something is needed to stabilise the current it runs at. Consequently the transformers are designed to have appreciable leakage reactance which makes them approximate a constant current source. This is also used in the transformers of plain vanilla AC welders, though those usually have a sliding flux shunt to allow the current to be adjusted.

Attempts by radio amateurs to use microwave oven transformers for high power linear amplifiers have yielded supplies with abysmally poor regulation.

This trickery is designed into the shape of the lams, which makes them unsuitable for use in regular transformers.

David

joebog1 · 25th Aug 2018, 1:53 am

In a basic answer it's this!!
The transformer cannot saturate.
It's self regulating by using very poor self regulations and increased losses.
They will, however make a very handy and quite usable spot welder!!!
search
Instructables.com
using spot welder as the search criteria.

Joe

Argus25 · 25th Aug 2018, 3:48 am

Another lamination trick I have seen is in neon tube transformers, where it is required that the initial voltages are high to strike the tube and drop under load later. The magnetic circuit for the secondary circuit is partially closed off from the primary's magnetic circuit (in a transformer with two separate bobbins), but an air gap is left. I think this idea creates a circuit that looks more like a transformer with a choke in series with its output, but it's cheaper to make.This also has terrible effects on the regulation if the transformer is used for another application. But I got away with using one for a CRT's EHT by filing the residual air gap (which was about 1mm) open to about 4mm.

Though, of all the lamination tricks I have seen, the one that impresses me the most, is the single phase shaded pole motor !

Herald1360 · 25th Aug 2018, 11:11 am

Yup! "Short circuited iron = fresh air" as the prof at uni said.

Boater Sam · 25th Aug 2018, 12:49 pm

As the "Es" and "Is" will line up with one another antiphase if you like, I may use them to build a special with the pre wound primary, but I will have to drill them all to bolt together. Its a big stack, about 3".
Thanks for the info, learned something again.

julie_m · 26th Aug 2018, 3:06 am

Quote:

Originally Posted by Argus25

Though, of all the lamination tricks I have seen, the one that impresses me the most, is the single phase shaded pole motor !

Yes, that one is fiendish!

Someone on another thread tried using a tapped record changer motor stator, without armature, as a crude autotransformer, but it got warm. Would cutting away the copper shading rings to remove the shorted turn do anything to fix this, or would that make things worse by increasing the risk of saturation?

TrevorG3VLF · 26th Aug 2018, 10:00 am

I would think that a replacement for the armature would be necessary to complete the magnetic circuit. How would you cut many discs for the laminations?

A shaded pole motor is very inefficient and no directional force is needed in a transformer, so cut the copper ring.

kalee20 · 27th Aug 2018, 11:04 pm

Cutting the copper rings will help, but really without the rotor in the middle, things will not be very happy.

In a shaded pole motor, there's a bit of iron around the outside of the hole, to define the hole. But only a couple of millimetres at the narrowest point! This can't carry all the flux (it saturates), which forces the flux to go through the rotor. So no rotor plugging the hole, low inductance and high magnetising current.

You could try getting a slug of soft iron or even mild steel and putting it in the middle. There will be eddy currents, but you may still find the benefits outweigh the losses.

24th Aug 2018, 5:53 pm	#1
Boater Sam Banned Join Date: Dec 2013 Location: Middlewich, Cheshire, UK. & Winter in the Philippines. Posts: 3,897	Gapped core mains transformer? I had microwave mains transformer lying around. I thought it could be a good source of laminations for making transformers or even if I reused the primary winding, a really heavy current transformer. Being 21st century imported junk, the lams had 4 arc welds across them and no bolts. Careful grinding removed the welds and a good battering released the lams from the compound that it had been impregnated with. And there was the surprise, "E" & "I" lams stacked together on top of one another like an output transformer, not interleaved like a mains one. It had a negligible gap between the legs of the "Es" and the "Is" Managed to get the primary off in one undamaged impregnated piece. Is this method of construction for a good reason? Sam.

24th Aug 2018, 6:05 pm	#2
turretslug Dekatron Join Date: Nov 2011 Location: Surrey, UK. Posts: 4,396	Re: Gapped core mains transformer? I believe that an operating magnetron looks like a low impedance from the supply point of view, so the supply needs to have a high source impedance to limit the current- arranging the lam-stack appropriately is as near "free" as current limiting gets. Also, ISTR that the cathode gets bombarded by some of the fierce cyclone of electrons intrinsic to operation, reducing the necessary heater power from the transformer once HT current starts being drawn. Last edited by turretslug; 24th Aug 2018 at 6:13 pm.

24th Aug 2018, 7:56 pm	#3
Ed_Dinning Dekatron Join Date: Jan 2004 Location: Newcastle upon Tyne, Tyne & Wear, UK. Posts: 8,194	Re: Gapped core mains transformer? Hi Sam, this technique is used on many cheaper transformers these days as it is a quicker process. If the cores are clamped together correctly and then TIG welded the losses in the core are only slightly increased and production costs are lowered. It would NEVER do on an audio output transformer. Ed

24th Aug 2018, 8:10 pm	#4
Radio Wrangler Moderator Join Date: Mar 2012 Location: Fife, Scotland, UK. Posts: 22,876	Re: Gapped core mains transformer? The magnetron looks like a negative load impedance to the power supply over part of its operating range, and something is needed to stabilise the current it runs at. Consequently the transformers are designed to have appreciable leakage reactance which makes them approximate a constant current source. This is also used in the transformers of plain vanilla AC welders, though those usually have a sliding flux shunt to allow the current to be adjusted. Attempts by radio amateurs to use microwave oven transformers for high power linear amplifiers have yielded supplies with abysmally poor regulation. This trickery is designed into the shape of the lams, which makes them unsuitable for use in regular transformers. David __________________ Can't afford the volcanic island yet, but the plans for my monorail and the goons' uniforms are done

25th Aug 2018, 1:53 am	#5
joebog1 Banned Join Date: Jun 2015 Location: Mareeba, North Queensland, Australia Posts: 2,704	Re: Gapped core mains transformer? In a basic answer it's this!! The transformer cannot saturate. It's self regulating by using very poor self regulations and increased losses. They will, however make a very handy and quite usable spot welder!!! search Instructables.com using spot welder as the search criteria. Joe

25th Aug 2018, 3:48 am	#6
Argus25 No Longer a Member Join Date: Oct 2016 Location: Maroochydore, Queensland, Australia. Posts: 2,679	Re: Gapped core mains transformer? Another lamination trick I have seen is in neon tube transformers, where it is required that the initial voltages are high to strike the tube and drop under load later. The magnetic circuit for the secondary circuit is partially closed off from the primary's magnetic circuit (in a transformer with two separate bobbins), but an air gap is left. I think this idea creates a circuit that looks more like a transformer with a choke in series with its output, but it's cheaper to make.This also has terrible effects on the regulation if the transformer is used for another application. But I got away with using one for a CRT's EHT by filing the residual air gap (which was about 1mm) open to about 4mm. Though, of all the lamination tricks I have seen, the one that impresses me the most, is the single phase shaded pole motor !

25th Aug 2018, 11:11 am	#7
Herald1360 Dekatron Join Date: Feb 2007 Location: Leominster, Herefordshire, UK. Posts: 16,535	Re: Gapped core mains transformer? Yup! "Short circuited iron = fresh air" as the prof at uni said. __________________ ....__________ ....\|____\|\|__\|__\_____ .=.\| _---\__\|__\|_---_\|. .........O..Chris....O

25th Aug 2018, 12:49 pm	#8
Boater Sam Banned Join Date: Dec 2013 Location: Middlewich, Cheshire, UK. & Winter in the Philippines. Posts: 3,897	Re: Gapped core mains transformer? As the "Es" and "Is" will line up with one another antiphase if you like, I may use them to build a special with the pre wound primary, but I will have to drill them all to bolt together. Its a big stack, about 3". Thanks for the info, learned something again.

26th Aug 2018, 10:00 am	#10
TrevorG3VLF Rest in Peace Join Date: Apr 2011 Location: Matlock, Derbyshire, UK. Posts: 1,378	Re: Gapped core mains transformer? I would think that a replacement for the armature would be necessary to complete the magnetic circuit. How would you cut many discs for the laminations? A shaded pole motor is very inefficient and no directional force is needed in a transformer, so cut the copper ring.

27th Aug 2018, 11:04 pm	#11
kalee20 Dekatron Join Date: Feb 2007 Location: Lynton, N. Devon, UK. Posts: 7,082	Re: Gapped core mains transformer? Cutting the copper rings will help, but really without the rotor in the middle, things will not be very happy. In a shaded pole motor, there's a bit of iron around the outside of the hole, to define the hole. But only a couple of millimetres at the narrowest point! This can't carry all the flux (it saturates), which forces the flux to go through the rotor. So no rotor plugging the hole, low inductance and high magnetising current. You could try getting a slug of soft iron or even mild steel and putting it in the middle. There will be eddy currents, but you may still find the benefits outweigh the losses.