Physics going on in a transformer

Good on you - I wish that some folk writing text books bothered to check this sort of thing.

The core sees a magnetic field that is the sum of that caused by current in primary and secondary -  and as the amp turns on both sides are equal and opposite, to a (good) first approximation  the magnetic field in the core is unaffected by changes in the load currents.

The primary current is set by the secondary load (scaled by turns ratio), plus the 'magnetising current' which flows even when there is no load. That is set by the inductance of the primary (with no load on secondary) and the primary voltage, and is based on so many volts per turn and the volume of material in the core to be magnetised.

M.

Does putting a load on the secondary cause a feedback through the magnetic field resulting in the current increasing in the primary

I think that's got to be the case with transformers - otherwise the national grid would not have to vary generation to suit changes in LV consumption.

https://www.gridwatch.templar.co.uk/


  - Andy.

Hello,

I tried to attach a pdf but too big, apparently. Herewith a link to my cloud storage, though I think you need a google account.

See part 3, monograph 2. 
https://drive.google.com/folderview?id=1TbomXeoBbIe-IVADaOOmyyLH9le-3wAt

Chris

Let me know if you have a problem downloading.

There are two other effects too, which I will add to Mikes's description. First the unloaded transformer inductance produces a back EMF as the mains cycle varies in voltage, which opposes the input voltage so the current is small. I=L dv/dt is the equation. There are some resistive and magnetic losses so there is a small current flow, in phase with dv/dt, that is at about 90 degrees to the voltage changing. Load on the secondary decreases the effective inductance (a shorted turn type effect) so the current in the primary increases in direct relation to the load Power (kVA). The losses increase with load, both resistive and magnetic, but these are small compared to the load. Typical large transformer efficiencies are in the high 90%, depending on the size, the quality of the magnetic material, and the disposition of windings on the core.

110V site transformers remember at which point of the ac wave they were switched off. Which is why they trip circuits breakers occasionally. If its switched off at a peak part of the cycle that's where it picks up from when switched back on the result is a massive surge. So I'm told..

It's not memory Jon, it's the switch on at peak'ish voltage is not current limited by the existing magnetic field so the back EMF, which sets the current is given by V= -L dI/dt. So to give say -350-ish volts back EMF in a very short period gives a large I, which trips the circuit. The combination of the R and L of the primary coil gives something called a time constant, which is the next bit of theory, but you should consult Hughes or a similar textbook.

I'll blow the dust off my Newns. Thanks Dave

The switch-on transient is all about the time in the cycle the volts come on, and the magentization state - or rather the lack of it, which is not a good match to the state it should be at that point in the cycle.

It is made vastly worse if you switch near a voltage zero crossing, which would be ideal for adding a flat capacitive load, it is about the worst you can do for adding a non-magnetised  inductor. Also the majority of transformers are sized in terms of turns per volt  and core size, such that the core is within a factor of two of saturation in normal use. If you switch on at the wrong point, the peak tries to be double, but the core saturates, and in effect, once all the little magnetic domains that swing around to give the magnetic response have been jammed at one end, it is as if the core then vanishes, and the windings may as well be air-cored . The result is a spectacular current peak, and several cycles of the transformer going thud-ring to recover.

The correct solution is to pre-magnetise the core via a moderate resistance, and then to short out the resistor after a few cycles,  or to apply power via a thermistor, whose resistance drops to near zero as it self heats.

Modern electronic soft starts, can put the mains on just after the top dead centre of the voltage waveform, which is the point when the core would normally not be very magnetised, so it starts on the falling voltage.


Further to the above, and concerns about how transformers relay the load from one side to the other, to be rigorous, I will clarify that  the B field in the core is only almost constant, and to first order is only a function of applied voltage,frequency, and core geometry.

Drawing current from the secondary causes the current in the primary to rise to keep the field constant, and the change in amp turns is equal and opposite on primary an secondary  sides, (well almost, losses ignored here and good magnetic coupling assumer), so the B field in the core remains (almost) constant.

Looking in at the primary side, the impedance presented is the effective inductance of the primary, then shunted by the transformed version of the secondary load.

B field in core does vary with load for non constant voltage transformers, such as current transformers .

some text book explanations are not always clear on this.

Mike

I think the suggestion made by Mike that the magnetic field strength is constant is slightly misleading, I am sure not deliberately but it leads from another phenomenen which is usually not mentioned in textbooks. I will attempt to explain it. Imagine you have a toroidial core, with a winding spaced from another on the opposite side of the toroid. Imagine the energy flow between the windings, the magnetic field in the core carries this energy from one side to the other. Loading MUST increase this magnetic field energy transfer, and it does, and causes increased losses. But this may be imagined in the second winding reducing the magnetic field in the same way as the primary winding increases it. However magnetic circuits do not work quite like that, and that is why practical transformers have the secondary winding wound on top of the primary winding, but still with a complete magnetic circuit (the core in a loop). Magnetism in this toroidial confguration has "resistance" and loss, but if the same piece of magnetic material has both the primary gain and secondary loss in the same place it is very efficient. One still needs the core to work, but the whole arrangement and "magnetic circuit" is not quite as often described. There are many things about magnetism (and electricity) which we do not fully understand, whatever you read in "text books", these are usually just restatements of that which is written elsewhere. The important thing is to keep an open mind and think about the problem, it may well not be quite as others describe. but consider things like energy flow above. The idea that a coupled winding reduces the magnetid field is somewhat counter intuitive, but if you think about it inductors of all kinds have slightly strange energy flows, think about "back EMF" in energy terms and it is obvious that the magnetic field energy must go somewhere when the magnetising current stops!


David CEng etc.