BIG PT - One other question about transforners

[kagliostro]

Some time ago I recovered the transformer of an arc welder (~1500W) but yesterday as a present I received a very BIG transformer

380v (single phase) primary

55v-0-55v secondary

2500VA !

My intention will be to use it for a DIY Spot Welder, of course the secondary will be unwound and rewound using few turns of a HUGE wire

My line has 220V and I think I can connect without problems the 380V primary to a 220V line, but my knowledge is poor and my curiosity large, what changes havind a primary winding planned for 380V instead of 220V ?

Obviously if the secondary is maintained as is the output voltage will be less (in the order of 31.8v-0-31.8v) but this isn't important to me because I must rewind the secondary with few (may be also only one) turns of a very BIG wire (I'm interested in current, not voltage for a Spot Welder)

So, in which way I must consider the fact that I've a primary winding that is planned for 380V and I'm going to supply it with 220V ???

Which are the differences ??

Many Thanks

Franco

[trobbins]

The peak flux level in the core is related to the applied voltage.


There's nothing wrong with going lower in supply voltage - same situation as operating the transformer via a variac.


Do you have to, and can you, remove the secondary windings?  And can you identify the volts/turn?  Is there an earthed screen?


2.5kVA would reduce, so based simply on 1kVA and say 5V, you may have only a few hundred amp - that's not too much.  If you can find insulated flexible cable like 35-50mm2, you may be able to use bi-triflar etc to get the sq.mm up.

[kagliostro]

Do you have to, and can you, remove the secondary windings?

My intention is to unwound the secondary and install a new secondary using those flexible cables used to connect electrodes on Arc Welder

I'm thinking to use 50mm2 cable (100mm2 if I find it) to wind one or two rounds (doubled or tripled in parallel)

And can you identify the volts/turn?

If I unwound the transformer I can identify the volts/turn but I don't think it is necessary as the DIY Spot Welder projects I've read all talk about a secondary under 2v (so 1 or 2 turns will be OK for me)

Is there an earthed screen?

No, there isn't

The peak flux level in the core is related to the applied voltage.
....
2.5kVA would reduce .....

There is math to apply as to determine the VA disposable if the 380V primary is supplied with 220V ?

My brain is a bit in confusion, I was thinking that VA depends on core measures .... but effectively a relationship with the input voltage and primary wire section and turns ...............

I don't would like to rewound also the primary .....

Thinking about the use of the 220V on the 380V winding I was assuming that the number of turns in excess is not so important because I don't want a particular output voltage, only a big current

---

I'll make a pair of photos of the two transformers and I'll post here, may be other councils come ......

Thanks

Franco

[Moonage]

If, as you say the the power of the PT is 1500VA when connected 380V to primary, the the power when connected to 220V is 868VA since the current through the primary can not be higher then when connected to 380V, or you would exceed maximum current density through windings of the primary.

[PRR]

His old biggie was 1500VA. His new biggie is 2500VA. As you say, with no other adjustments, 220V on a 380V winding suggests a lower VA rating in the *same* service. Perhaps 1440VA. (So not really "larger" than the old 1500VA, unless he can re-wind the primary closer to 220V.)

He does not say what the design service was for the "2500VA". However a home *spot welder* is probably the "easiest" possible service. Very short (second) time on load with long rests as the work is moved to the next spot. A car-factory spot welder might go zzp-zzp-zzp non-stop all day long; a DIY spot welder might not run 2% of the time and maybe never long enough to get hot.

So it is all about voltage sag, in the windings and also in the house. In the US it is unusual to have a house circuit over 240V 50A, the kitchen stove circuit. OK, that is 12,000VA so might be at the limit of a 2500/1440VA transformer even for short duty. If a 32A 220V circuit (clothes dryer? hot water?) is available, that might be a better supply. You can probably get a "spot" of 10,000VA, but any steady load (stuck contacts) above 8,000VA will blow the fuse within seconds.

I was going to say that a 32 Amp switch is fairly big, but I guess you will never switch on load (unless something is terribly wrong). The 1440VA core may take 100VA-200VA at idle, which an ordinary lamp switch will break easily. (I am installing a 100 Amp switch, which may never see 100A, but has to be designed for it.) And the "right way", on the US market, could be a $17 30A switch box sold for air conditioning safety-switch. (There's three kinds: pull-out jumper, pull-out fuses, and a handle-switch which looks like a circuit breaker but isn't.)

[kagliostro]

Thanks Moonage & PRR

At home (here in Italy) is common to have 230V (220V in the past) and the counter (on the most) is set for delivery 3KW and 3.4KW max for a small periode

As PRR say the use is really short time and I hope the counter didn't stop erogation (I hope)

The first transformer I recovered was stolen from a Micro Wave Oven and was a 600VA

on the web seems this transformer can give 800A (I've seen other measures with 1000A)

! No longer available

The second was from an Arc Welder and it was 1500VA (EDIT: now I'm in doubt about this value)

The third is an industrial monophase 380V - 55V-0-55V 2500VA (use unknown)

The most part of the DIY progect of Spot Welder I've seen use the Micro Wave Owen transormer, but I've those other transformers .....

The Arc Welder transformer will be easily converted because the secondary is in a separate gorge and can be rewind easily

but the other transformer is very larger and I'm tempted ....

A good thing of the Arc Welder Transformer is that there is a threaded shaft that move a part of the core as to have more or less current disposable ....

Here some Photos

The small transformer is the Oven transformer, the intermediate is the Arc Welder transformer and the bigger is the 2500VA transformer

Here the 1500VA and the 2500VA


I think that 1500VA is usable only for short time periodes at full roar



Here the comparision between the 2500VA and the 600VA



Here the comparision between the 1500VA and the 600VA



Here the movement that performs the core saturation regoulation



The advantages of the 1500VA are that the secondary can be easily rewound as is in a separate gorge and there is a shaft to regoulate the core saturation as to have less or more current available, the counterpart is that the winding is aluminium not copper

The advantage of the 2500VA is that it has a really BIG core and all windings are genuine copper, the counter part is that may be I must rewound also the primary and I don't like the idea so much ...

The measures of the 2500VA are

180mm x 105mm x 150mm (core 90mm (?) x 105mm)

The measures of the Arc Welder transformer are

175mm x 65mm x 130mm (core 45 x 65mm) there is a regoulation shaft

What would you do ?

Franco

p.s.: Damn ..... I've a doubt about the 1500VA value ..... I didn't remember if the value refers to the Arc Welder transformer or to one other transformer that had my friend ..... the Arc Welder was labeled 145A and I think it was around 45V of secondary .....

[trobbins]

The 2.5kVA PT looks nice - perhaps for battery charging.

Based on 380VAC then full rated primary current is about 7A.  Even though that implies a 1.5kVA output capability, it is worth noting:

- the core is large, so if you have enough incentive to snip out the secondary without damaging the primary and insulation barrier, then you should have more winding window to play with (?).

- the primary winding is now not covered by secondary, so cooling may be a titch better.

- that transformer looks like it is rated for continuous operation, so you could 'push' the primary current rating if you can ensure a short operation period and suitable duty cycle for cooling.

You may be able to make up some solid boxing to act as insulation within the winding windows.

How were you going to sequence the operation?  Was the primary going to be manually switched on/off, with the load connected prior?  What AC mains over-current protection devices do you have available - such as a D-curve CB. 

[kagliostro]

Ciao Tim, Thanks

You may be able to make up some solid boxing to act as insulation within the winding windows.

Can you explain a bit more ?

Was the primary going to be manually switched on/off, with the load connected prior?

Yes, I was thinking to do exactly that, as switch I think I've a Solid State relay (or two) that I can use on the primary

What AC mains over-current protection devices do you have available

In my house the line arrives then there is a counter that automatically disconnect the load if there is a consumption higher of 3.4KW for a short time (the disposable continuous current is of 3KW at 230V - currently I measure 220-222v)





then the line reaches my shop (my atelier) and there there is an electrical panel with outlets and thermal switches, if I remember correctly 32A is the value of the thermal switch, more I've a CFCI (differential switch) but I don't remember the max A of this device, may be 16A

I'll post some photo later

As secondary I was wondering if I can use a piece of copper plate (or alluminium)



bended as a U, to form a single 3/4 turn secondary winding instead to use Arc Welder cable

Franco

EDIT: Here the electrical panel of my shop (my atelier), I remembered badly, there are two CFCI (differential switch) each rated for 25A, one serves the whole panel the other is in series and serves only one outlet

[PRR]

> disconnect the load if there is a consumption higher of 3.4KW for a short time

Ha! Different places.

3KW is a couple of coffee makers. Just 15 Amps at 230V.

Here I could not get a service for a house less than nominal(*) 240V 100A or 24KW. For a mobile home (large fancy trailer with wheels removed), a 60A 14KW service may be available. Larger homes "always" get 200A service. I have heard of 400A and 800A (192KW!!) service to homes with large air-conditioning systems and workshops. True, they may never pull that much current. And the larger fuse-boxes can hold more fuses (breakers) for more circuits so that one short does not darken half the house.

I was "fused" (circuit breaker) for 100A. I added a 50A breaker, work done by licenced electrician and power company, and nobody objected. In theory I can pull 150 Amps until the rubber falls off my hot overhead line. It would be quite safe to pull 100 Amps

(*) The "service" is the wires from street to house. #2 Aluminum is standard for small homes and is rated 95-105 Amps, hence 100A or 125A main breaker. If you are 50 feet 16m from the street that is fine. I am 500 feet back. If I pulled 100 Amps I would have 40 Volts drop in the line, 16.6% sag. (If I were at 50 feet this would be 1.7% sag which is within the 2% recommendation.) Lights would burn very dim. Heater would give 68% of rated heat. I believe the power company was told to wire for a trailer home. They may have offered to bring in high voltage feed and a transformer at the meter, but several thousand dollars more cost. (They fund the first ~~~100 feet, after that the property owner pays.)

The guy in the video puts a lot of trust in a very poor connection: toy vice-grips on the corner of the connector. 850 Amps deserves a lot more area and pressure. Just for 100 Amps long-term I think I need over 1 square inch and 50 pounds pressure.

The meter readings are interesting, and say something about short-term abuse. Resistance of metal is higher when hot. The copper winding stays at 850A for many seconds. This winding is not hot (yet). The iron nail current starts falling in a few seconds, it is smaller and heats up faster. From >660A to <360A at melt.

Note also that when the current is 425A, this is half the short-circuit output of the transformer, so the voltage and resistance of the iron is equal to the voltage and resistance of the winding. Since the winding is physically larger than the nail, you can melt the nail before the copper will melt. I think this is true for any practical welder.

I do not know which of your transformers I would use. Probably start with whichever one looks easier to convert.

[kagliostro]

Also here we have contracts higher than 3KW, but 3KW is the standard and the most part of the contracts are within that target

Also here now we have aluminium lines, but I've no tecnical data about it

---

I've not yet decided which will be the transformer that I'll use, today I give an accurate look to the 2500VA and seems it will be possible to unwound the secondary without dismout the laminations ..... I must investigate more about

---

The larger is the section of the secondary the higher is the current I can take, I was wondering if a big copper or aluminium plate bended as a U can be used instead of using a big flexible cable wound for some single turns connected in parallel

I don't remember well but I think I've seen something like tath (the use of a big plate instead of wire) in a commercial Spot Welder, those that are used by car coachbuilders

Franco

[PRR]

Yes, the conductor can be about any shape. When you have "many" turns, round wire is 78% as good as any other and is most available. For MAXimum conductor in limited space, rectangular gets a little more metal in a space. For 1 or 2 turns, bending a thin wide sheet is easier than a fat round wire.

> a single 3/4 turn secondary winding

It really goes by the number of times you go through the window in the iron. These cores are "one turn is two window passes". If you put a straight wire through the window, that is a half turn. If you U-bend it but only go through one window, that is a half turn. If you U-bend but thread it through both windows, that is a full turn.

[PRR]

> we have aluminium lines, but I've no tecnical data about it

Nearly all "large" electricity travels on Aluminum. Copper is better conductivity for size, but Al is much better per pound and (at present prices) per dollar. Al can also span longer distances (strength to weight ratio).

The problem with Aluminum is that it is hard to make good connections. For "small stuff" like inside wiring of 6 Amp to 50 Amp circuits, Copper is much easier to connect reliably and the cost of small copper is not large in the overall installed cost with labor.

If I changed my 500 foot line to the same size copper, I would have a little less lamp-dim when the pump came on. Not a big difference. A BIG difference in price. For Copper, but also because I would probably need an additional pole to support over that distance. So I have Aluminum. Where the main line connects to the house, those terminals are designed to apply proper pressure on Aluminum conductor. They also need to be cleaned and treated with No-Al-Ox grease, which all electricians know to do.

For the smaller line out to my fairly generous (50 Amps) garage I used Copper. #6 Aluminum exists but is very un-common.

I'm sure the wires along the street are Aluminum, actually ACSR which is one strand of steel with six strands of aluminum.

When you buy a house in the USA, 1960s-1970s, have the wall-wires checked. We were throwing so much brass in an Asian swamp that copper prices soared and Aluminum was used for "small" circuits. This was a Very Bad Idea. Next steps were copper-coated aluminum and specially designed terminal screws. These still fail, with great heat. While most of that aluminum has been repaired/replaced (at great expense), it is still something to be aware of.

[kagliostro]

Thanks PRR for the info about line in the USA (Many Thanks)

---

It really goes by the number of times you go through the window in the iron. These cores are "one turn is two window passes". If you put a straight wire through the window, that is a half turn. If you U-bend it but only go through one window, that is a half turn. If you U-bend but thread it through both windows, that is a full turn.

"one turn is two window passes" .........

I don't understand well what you are trying to explain me, can you try to open my brain to knowledge ? Thanks

talking of a 3/4 of turn (or a U) I was meaning to do something like this (I hope you can understand what I mean)



This will be a full turn ???

Franco

[PRR]

Only the number of passes through windows "counts". For 100+ turn windings, we don't need to know this. We count turns on the bobbin machine. For "1 turn" windings, we really want to count "windows" not turns.

So there is no "3/4 turn". The iron you show are all "two windows per turn", so the few-turn math is like this.

[kagliostro]

AH, OK, now I've understand Many Thanks

So, as I'm interested in current the number of turns I'm able to accomplish is not really important and also a U bended aluminium plate can do the job

now I must hope that the current protection on my counter didn't  stop the erogation when the welder is switched on and try to collect info

in a soft start to be applied

Thanks Again

Franco

[kagliostro]

I have think a way to "soften" the startup

putting a 1200W 220V boiler resistor in series with the primary of the transformer and using 2 Solid State relays

one as ON/OFF switch, the other in parallel to the boiler resitor to perform a soft start with the resitor in series and then the full performance when the SS relay shorts the resistor

Do you think that the resistor will be OK for that use ? (1200W is an easy to be find value) Or do you think that I must use higher resistance ? (a 1200W resistor I think is 40.4R when hot)

Thanks

Franco

[PRR]

> a 1200W 220V boiler resistor

Yes. Maybe.

I assume something like this: Water Heater Element, goes in something like this: water heater. Or maybe a coffee boiler.

They are cheap. They are 1,000+ Watts. But they need water cooling. In air they will burn-up in a few seconds.

BTW, if this is your sink-and-shower washing-up water heater element, here in the US we use 3,500 and 4,500 Watt monsters. Home coffee makers are 1,500W. I do not know how big restaurant coffee-makers go (ah, 18.9 gallons/hour, 16,800 W, $1,200-$2,400).

The US water-heater elements fit a standard pipe-thread. You could thread them into a bushing, a foot of pipe over that to hold a part-Liter of water to allow several seconds of full power.

For that matter, a standard home coffee-maker or room heater will be about 1,500 Watts and of course 220V in your home. So you could just make coffee in the summer, heat the workshop in the winter, though very slowly unless you start welding-up new car-bodies at home.

[kagliostro]

Thanks PRR

Yes, the resistor looks like you think

this is one of the most common shape (straight with a small copper tube for thermostat probe but exists also without)



this is the thermostat



and it fits in an unit like this ( Like is the brand )



and as you say is treaded as to fit a standard pipe thread (1"1/4)


BTW I've sold those items for 24 years 

To invent someting as to keep the resistor water cooled will not be difficult, originally I was thinking to an air heatsink with a fan

more, the ON cycle is very short respect to the OFF cycle in a hobbyist Spot Welder

Also here we have bigger resistors, but the common user has a mere 1200W (not very small if your counter gives only 3KW)

Do you think one resistor is enough or it will be better to use a pair in series ?

Thanks

Franco
 

[shooter]

water cooled

just thinking, what about an electric stove element?  Have no clue on Resistance, but it doesn't need water, can handle high currents.

[PRR]

I would think two in -parallel-. Then the dead-short load is 2,400 Watts, which is less than your 3,000 Watt cut-out (if all other house loads are at a minimum). The maximum available energy to an external load is just 600 Watts (110V across resistors, 110V across transformer), and that is not much for spot welding.

How fast is this cut-out? It shouldn't be "instant". It should allow start-up loads such as large motors. The intent is to not burn-up wires, either the wire to your house or the large wires feeding many houses,. But a few-second overload won't hurt house wires, and a wire serving several houses is unlikely to have 3,000 Watts in all houses at the same moment.

How much trouble is it to re-set? Do you press a button or does the company have to send someone to do that?

[PRR]

> In my house the line arrives then there is a counter that automatically disconnect the load

Italy's electric system is slightly different than most. In the US we have 1 or 2 houses per 13KV:240V transformer. Italy drops to 230V and powers 10 to 100 houses from one transformer. This may not be good power efficiency, but it became MUCH easier to use "smart" meters. The data-path does not work through transformers without an added coupling part. In Italy you have many more meters per coupler so the data cost is lower.

I found a picture of the Enel meter. In a study of how average customers failed to get simple readings in 67% of attempts, and averaged 3 times more clicks than was necessary.

OTOH, the Enel meter tells MUCH more than my meter tells me. I can't get peak demand, I don't (yet) have time-of-day rates, and the meter does not know a billing-month. I get current demand by watching how fast a rotor spins. I see total lifetime delivery on 7 dials which turn different directions. To know my month bill I have to read those dials on the Billing Day and keep records. (Actually this is a "smart" meter and there was a web-site where I could get a daily report.)

Also: allowing an unexpected power cut-off is odd in many places. In the US the company usually has to contact you several times over a period of months before they can turn-off for non-payment. In the UK they are moving from whole-house GFI/RCB leakage breakers to small-area breakers, so the whole house does not go dark unexpectedly.

I see you can request a 6KW maximum but did not find the cost. And if your wire is only safe for 15 Amps then you could not pull 6KW for long.

[PRR]

Meter manual (different country?):
https://www.smartutilities.com.mt/wps/wcm/connect/d47973b5-7a4d-4460-bff2-fb7faae188dc/ENEMALTA%2BSMART%2BMETER%2BUSER%2BMANUAL%2Beng.pdf?MOD=AJPERES&attachment=true

ADDITIONAL MESSAGES IF YOU EXCEED THE MAXIMUM POWER ALLOWED
If you exceed the maximum power allowed the Smart Meter will perform checks after 2, 32 and 62 minutes and will display the following messages:
Reduce Load
Power exceeded by (pct) xx
The first warning that you are exceeding the standard power (9.2kW) load by the percentage (pct) displayed.
Risk Disconnect
Power exceeded by xx pct
You have continued exceeding the standard power by the percentage (pct) displayed and you are now risking disconnection in the next 30 mins if you do not lower the power you are using.
Disconnection
Your supply has been disconnected. Lower the power and switch on the circuit breaker.

This suggests you can exceed the limit for a short time. However it also says you can re-set yourself, while info I found for Italy said there was a re-connection charge.

[kagliostro]

@ Shooter the reason for my choice about the resistor is that a boiler resistor is compact and mechanically robust easier to be managed than a stove element and easier to be found

@ PRR Oh, you really collected a lot of info about the italian sistem ...

The disconnection can be restored simply acting on the switch present on the counter and didn't happen immediately, there is a time (I don't know how long, someone says minutes) that a 3.3KW  load is allowed (now I want investigate on this details)

You say to put 2 x 1200W resistors in parallel, please can you explain why in parallel instead of in series ?

The resistance will be halved from 40.4Ohm to 20.2Ohm and this will increase the load (or not ?)

Thanks

Franco

p.s.: An interesting TIP for those hobbyist spot welders that want to spot weld aluminium chassis (the gy is US but he say the trick comes from an old italian guy)

http://www.youtube.com/watch?v=gcgC3V3mkcw


BTW I like Lazze's videos

[PRR]

I think you will want 10KW for a few seconds.

You do want to know the disconnection system: fast or slow? Maybe slow for slight overload and faster for heavy overload?

Yes, start with one ~~40 Ohm resistor in series. But I think that will not be enough-- only a few hundred Watts in the spot. You need a LOT of power to get the metal white-hot quickly (before the heat is absorbed by surrounding metal). Unless you only do very thin metal, this will be several thousand Watts in the spot, and more than that to cover transformer and arm losses.

When you need momentary HIGH power and have steady small power, capacitor storage is interesting. But I think the capacitor will have to be the size of a trash can.

Another storage spot-welder, but only for round rod. You spin the rod with a motor, usually with a flywheel to store more energy. Turn off the motor and slam the spinning rod into another piece of metal. Friction will tear off all oxide and melt the metal. Pressure will merge the rod and other metal together. If you get the size of rod and flywheel just right it makes a very good weld with very little energy, and average power much lower than weld power.

[shooter]

spin the rod with a motor

I see broken windows and impaled humans in somebodies future

[kagliostro]

OK PRR I understand

I'll try to have informations about disconnection time/method

The motor method is a bit more difficult to be used, may be an application with the drill press, but I think it requires a lot of HP

The easiest method I think will be this (I've a Wire Welder)

! No longer available

Franco

[PRR]

> I've a Wire Welder

So why do you need a spot welder?

Spot-welder used to be cheaper, but wire-welders have become popular low-price toys because they do things a spot-welder can't touch. Spot-welder may be less set-up, but not if you have to invent and perfect your own spot-welder for a few personal projects. That Eastwood clamp and holder is neat, but you don't really need it. Or you can DIY something like it.

[PRR]

> The motor method ... I think it requires a lot of HP

No, that is the sweet thing. You could weld rail-road locomotive axles with a 1/4HP motor, if it does not mind a heavy torque for a long time (a "torque motor", not a fan motor). Turn it on, it slo-o-o-o-wly builds up speed. By lunch time you have a 1-ton locomotive wheel spinning 2,900 RPM, LOT of stored energy, but only 0.25KW of electric demand (for several hours). Now turn off the motor and shove the axle in the wheel hole, hard. Either the hub goes red-hot and melts to the axle, or something snags and throws a ton of iron through your roof.

[Willabe]

Now turn off the motor and shove the axle in the wheel hole, hard. Either the hub goes red-hot and melts to the axle, or something snags and throws a ton of iron through your roof.

Oo, oo, I wanna try that.         

[shooter]

SEe reply #24

[kagliostro]

So why do you need a spot welder?

......Good question ......... I think principally because I would like to build/have one

Yes I can use the wire welder but a spot welder will result in a cleaner job

sometime can happen that wire welding I've some iron splatter around

and if I can accept that on a small carpentry work I don't on an amp chassis were I dislike to use the grinder on top and sides

(I'm not a perfect trained welder)

More I would like to try to spot weld aluminium chassis using the trick presented on Lazze's video (my wire welder isn't a professional one and didn't allow to weld aluminium) and this is the reason for which I didn't stop on the Microwave Oven Transformer and I've think to use something larger (to weld aluminium requires big currents and I don't know which will be the minimum request also using the sheet metal trick)

---

SEe reply #24

I readed the post previously, but I didn't realized your avatar is a Vlad Sheep 



Or I'm wrong and on the contrary your avatar shows a Werewolf Sheep 


 

Franco

[shooter]

 

your avatar

It came from a RockClimbing Advertisement for shoes, rope, I forgot what.  I liked it so I painted It, the painting is 24" X 24", oil on canvas.  It hung in my Office at work, because PPl would always come down saying; "Dave can you fix......."
 

[kagliostro]

Cool story

Franco