Transformer winding direction

One of the issues with people who know everything is that they often don't bother to understand the question before pontificating on the CORRECT answer. So you get a snarky response on their way to solve the problem that they inferred based on their experience. They have no trouble calling you names if you disagree with their interpretation. Since they know everything, anything you say is WRONG. And it's no use trying to support your position, cuz they ain't listenin'.

Communication is a two-way process. The objective of the mentor is to use terminology that the newbie can understand. Sometimes trying to state the exactly perfectly technically correct description obfuscates the key issue. The nitpickers jump on that to tell you that you're wrong...and stupid. It's not about education. It's not about being right. It's about telling the world that the other guy is WRONG!! Throw in a few of your favorite pet names for good measure.

I like to have technical discussions. That requires listening on both sides and supporting the argument with logic. Calling me stupid doesn't help anybody.

Although he is rude, Phil is usually right, and this time is no exception.

I suggest you get a hall-effect current transducer connected to a DSO so that you can measure inrush current, and try out switching the transformer on at both the peak of the mains voltage, and also at the zero-crossing, and look at the current waveforms. When you run out of working triacs you could also google it.

The key to understanding this is to realise that the magnetic flux is proportional to the time-integral of the applied voltage, and that in continuous operation the flux is normally close to zero when the voltage is close to maximum, and the flux is close to maximum when the voltage is close to zero. Have a look at the first link here:

Chris

I had a large input to the writing of that article - Rod is a friend and colleague, we talk and email regularly.

The most surprising thing is how the in-rush surge of an unloaded transformer consists of brief pulses all with the same polarity - it's DC current.

Also, if you power a 240V tranny from 120V, surges are eliminated.

.... Phil

been there done that AEMC MR461 current probe. TEK TDS540 scope. I designed my first production forward converter ~40 years ago. so

with the secondary heavily resistively loaded, and look at the current waveforms. When you run out of

Are we talking about inductors or transformers with load resistors that cause a steady state primary current 2X their design rating?

Thanks for the links. I like to learn new stuff. I'm still trying to get my head around why the graphs in the first link are reversed in time, but if I stand on my head, it looks like the drive signal is optimized to maximize inrush current. I don't have any argument with that. You can certainly manage the drive so the core saturates.

My attempts were to arrange the drive signal to MITIGATE inrush current.

The key point is in the wikipedia link: "Worst case inrush happens when the primary winding is connected at an instant around the zero-crossing of the primary voltage, (which for a pure inductance would be the current maximum in the AC cycle) and if the polarity of the voltage half cycle has the same polarity as the remnance in the iron core has. (The magnetic remanence was left high from a preceding half cycle)." end quote

If you always turn off the current at the current zero crossing with a positive voltage slope, then always turn on the next weld pulse at zero voltage on the positive voltage slope, doesn't that leave you in a remanence position to avoid saturation at the next turn on? If not, why not?

The SSR is gonna turn off near zero current. About all I can control is the slope of the voltage sinewave when I give the command. To turn it on it's far easier to sense the zero crossing of the line voltage than the peak. Isn't a major portion of the primary current in phase with the primary voltage due to the resistive secondary load? Isn't it the leakage inductance that causes the phase shift?

Under the control conditions described above where we control both ends of the waveform to manage remanence and have a very low value resistive load, how much would I gain by waiting for the peak line voltage at turn on?

I'd go look, but it's stored behind a bunch of junk in the garage.

As I recall, I didn't make many measurements without load. But with the secondary (almost) shorted in the weld mode, I don't remember any horrible input current transients. I do know that synchronization with the line made a major improvement in the repeatability of the welds.

I'm up for some education.

My thinking was that, if not for saturation, the SCR would be less stressed if I turned it on at zero voltage when the primary current was zero. And from the unpowered state, the voltage and current can't be anything but zero. And that, if I could arrange the resting place on the B-H curve from the previous pulse such that the first half-cycle wouldn't saturate the core, that's the best I could do easily. Measurements didn't show any horrible first cycle inrush. Welds got more repeatable.

Let me say the same thing in different words. If the load is linear resistive, the transformer current and voltage will be approximately in phase. If the SCR shuts off at zero current, the voltage will also be near zero volts (plus whatever the leakage inductance allows). Case 1, you start the next pulse in a nanosecond. Isn't the initial current still pretty near zero? Isn't the point on the B-H curve still about the same? Case 2, you start the next pulse next week at the zero crossing of the input voltage headed in the same direction. What's the initial current? What's the initial point on the B-H curve? How is restarting it synchronously significantly different from just leaving it running?

I'm not disputing the articles you posted. I'm not saying anything about the general (worst) case. I'm suggesting that this is how you engineer a spot welder using a MOT.

Where did my thinking go wrong?

Why is that surprising? In the steady state, it is traversing a very nonlinear B-H loop. If you restart it from a place different from where you left it, you drive it "off center". To regain steady state, you have to apply a DC, component.

Just like YOU were surprised that zero switching produces maximum surges in transformers.

It's counter intuitive in both cases.

.... Phil

Read the link.

Most of the transformers I see are under heavy load at switch on, charging hefty filter caps. Only makes the combined switch on surge worse, compared to no load.

.... Phil

If there is unnecessarily large inrush current that is due to core saturation late in the first half-cycle, then adding a resistive load on the secondary won't fix that.

It doesn't much matter whether we are talking about transformers or ungapped inductors, in that the transformer with its secondary unconnected can still draw a lot of inrush current, and adding a load on the secondary won't fix that.

Yes I don't know why they put the scope plot backwards. It adds unnecessary confusion, though at least they do mention it in the text. I can only assume they lacked the ability to flip it easily in their chosen method of document preparation.

I know, and I was just suggesting that turning on when the mains voltage goes through zero is not the best way to do that. Turning on at the zero-crossing of the mains voltage is good if your load is capacitive, e.g. the input of a SMPS.

I think the remanence is clouding the issue. It is a relevant effect but even without it, there are good and bad times to switch on the transformer primary, and it would be better to consider remanence only after the basic situation with a soft-magnetic core is thoroughly understood. You might be able to use the remaining flux in the switched-off transformer to choose the least worst of the two zero crossings to switch it on at, but even then, I think you would do better to switch on at a different time. Why not seriously try it out with a current transducer and DSO, (and a vastly over-sized triac or even better a pair of big SCRs, just in case!). It would be nice to see the plots, and it is one way to end an argument.

The mains frequency (or period) is accurate and stable enough, and microcontrollers or even 555 timers are cheap enough that as Phil mentioned, you can figure out the time of the voltage peak from the zero crossing.

That sounds reasonable, but if the core saturates then that's the least of your worries. The primary current is not necessarily a good way to determine the core flux density, as you can make the primary current be whatever you want by choosing the secondary current.

Time for an experiment. It depends a lot on the transformer design. I have read that toroidal transformers produce more problematic saturation effects than E-I types, and if the core was nominally run at less than half of its saturation flux density then there will be no problem. Due to the more uniform geometry I think they can run toroidal transformers close to saturation in normal operation.

I wonder whether they even bother to switch the transformer on at the right time in a microwave oven. The current that your transformer draws in steady state might be so high (due to the secondary current) that saturation doesn't make it all that much worse, but given the choice (or given a bigger transformer than your circuit breaker likes), you might as well make it optimum if that is just a matter of inserting a small delay.

I have an arc welder that sometimes trips the breaker if I turn it on at the wrong time (with a mechanical switch), and it would be nice if it didn't.

The risk of saturation occurs well after the zero crossing when the voltage is turned on. If the load is not capacitive (not a big SMPS) then the SCR won't mind if you turn it on when there is voltage across it, and later on in the cycle it will be much happier.

And the magnetizing current too.

This is not the same thing as I was discussing. If the transformer is in steady-state operation and if you were able to turn off the primary at a zero-crossing of the mains voltage, that is a time when there is maximum flux in the core. If you instantly switch it back on again, sure this will be pretty much a continuation of steady-state operation.

If instead you turn it off for an integer number of mains cycles that adds up to a few seconds, the core flux will not be the same when you go to switch it back on again.

Perhaps but that doesn't really matter as regards the risk of saturation.

Yes, if you only switch it off for nanoseconds. No, if you wait a few seconds until you have repositioned your parts for the next weld.

The flux in the core is different.

Put a big inductor (maybe a car ignition coil) across a 12Vrms AC supply and make sure you are holding the terminals a nanosecond after you disconnect the supply, at the instant when the AC supply is at zero voltage (and the inductor is carrying maximum current).

Then give it to me and I will hold the terminals a week later.

I think you will notice the difference. The state of the flux in the core matters.

Note that in the case of a transformer, it is possible that some value of secondary current could result in the primary current being zero (or any other chosen value) at the zero-crossing of the mains voltage. That is not relevant to my point, which relates to the flux density in the core, which won't be affected much by the secondary current if a low-impedance supply is driving the primary winding.

It may just mean that you need a larger rating for your fuse or circuit breaker and more expensive triac or SCRs than you could otherwise get away with.

When I tried spot welding, I was never able to get enough current from a MOT-sized transformer with a few turns on the secondary. I could sort of weld things if I applied very light pressure so that the workpieces made poor contact with each other and the resistance was high enough for the (insufficient) current to heat them, but this wasn't really satisfactory because getting the force and contact resistance just right was not reliable.

If I made a transformer big enough to weld thick workpieces with proper contact pressure, it might cause excessive drop in my mains supply, and/or trip breakers.

I think the best option for me is a series-parallel array of Maxwell boostcaps. This would eliminate the requirement for a large mains supply capability. The 3000F ones are rated for 1900 Amps each, so about 5 in parallel would probably supply about enough current for any normal sheet metalwork up to a couple of millimetres thick which seems to require close to 10kA capability. (Aluminium welding requires several times more current so I won't try that.) Most of the references that I have seen tend to suggest that the weld itself requires somewhere in the region of

Chris

So, this is a concern that the SCR's dI/dt rating may be exceeded if peak voltage is present at the turnon time (the specific microsecond of time, because SCRs turn on faster than an AC period). That's a valid concern, with two solutions: (1) use an IGBT instead of SCR (yeah, I know, it's a big deal), or (2) use a transformer with enough stray inductance that the worst-case current risetime is tolerable. One hopes that the transformer is appropriate to this kind of frequent-switching use, and fulfills requirement (2). You really HAVE to hope, the microsecond-scale inductance of the transformer cannot be easily measured (the core isn't fully magnetized that quickly, so 60 Hz measurement tells an inappropriate value). Flux coupling to the secondary will also be poor for that short time.

A 'typical' SCR (TYN640, in stock at DigiKey, needs dI/dt under

Again, this depends on the transformer design and material. Remember Phil's comment that a 240V transformer on 120V excitation wouldn't saturate- because a 240V transformer has an oversize core for 120V excitation. The key concept is that the core remanent field also might be zero, and a first half-cycle starting at zero magnetization is more stressful than that same half-cycle starting at the inverse remanent field (which is what subsequent cycles of AC excitation provides). Starting at peak V is a safe bet if the remanent field is negligible, like when the gadget has been powered down for a while.

Most AC supply transformers operate right on their low frequency limit - b ut if you only apply half voltage then that limit is now at half the origin al frequency - so the switch on condition is tolerated.

IOW, a given saturation condition is proportional to V / f.

I often see this in action when testing US model amplifiers meant for a 120 V at 60Hz supply. A step-down device only adjusts the voltage leaving the f requency at 50Hz. The resulting magnetising current is the SAME as if one h ad applied 144VC to the transformer instead of 120V so Imag is way higher.

Some transformers are OK with this and others run very hot PLUS it has a si gnificant effect on the VA rating of a suitable step down transformer.

... Phil

I think he's confused.

I have a professionally-made spot welder, designed for the days when orthodontists needed to weld stainless-steel brackets for people's teeth. It uses a variac and bridge to charge an electrolytic capacitor, which is dumped via a contacter into the primary of a transformer with a secondary having a half-dozen fat turns (think, flattened water-pipe). The secondary has a fat braid running to the contacts.

The contacts are brought together by pressing a foot pedal, and are spring-loaded with a screw adjuster. When the preset force is reached, a microswitch closes and activates the contacter.

To use it, adjust the variac to set the pulse energy, and adjust the activation force, place your work between the contacts and press the foot pedal. You get repeatable energy and force with both hands free to position the work.

It is fairly easy to remove an MOT secondary since they tend to use an EI core that's not interleaved. Grind away the weld line, remove the I section, pull off the secondary and insert your new one made from a few turns of fat copper. Weld the I-section back on the transformer. Not sure if you need to remove the magnetic shunts... anyone know?

The rest is easy, but you'd use an adjustable HV supply not a variac these days.

Clifford Heath.