transformers back to back - why won't work?

Yes, you are right about the lower voltage and higher current.

You are also right that this would be due to losses, the current would be significantly lower on the output.

He does mention why this would damage the output transformer "they are not meant to be used as a step up transforner" He does however say it unclearly and was hard to notice.

I hope this answers your question,

 

A transformer core is being constantly magnetized and demagnetized following the sine wave of the input. But the core can only magnetize to a certain level. After that, it stops being an inductor and acts more like a resistor, and way too much current can flow. So if the load is trying to draw more current than the saturation point, the transformer will heat up and possibly be damaged. Also the voltage at the output will drop at that point.

Edited: For a small enough load, the step-down step-up would work quite well. And saying a transformer is not designed for step up is wrong. It does not even know which way it is being used, it only knows the currents and the magnetic fields. People use step down transformers in reverse to make inverters all the time.

Bob

 

Small power transformers such as that are typically specified for input voltage and output voltage/current. While I did not hear the video mention anything about a current rating, a small transformer might be rated for 2 amperes, so the transformer could drive a 36 watt load at 18 volts while the 120 volt side would draw 36 watts (plus some iron & copper power losses). Putting two of these transformers back-to-back means the input side could still draw 36 watts at 120 volts, but the output would be capable of 120 volts at 36 watts minus the additional iron & copper power losses, i.e. heat. So this back-to-back arrangement would be capable of powering a 25 watt light bulb, but a 60 watt bulb would cause more current to flow in the 18 volt secondary coils than they are rated for (2 amperes), and the extra current would overheat both transformers.

 

This is actually a poor man's way to make an isolation transformer. Real isolation transformers, those with reasonable power handling capacity and Faraday shielding between primary and secondary, cost big bucks. If all you want to do is isolate your 120 V AC load from mains power, two control transformers or (back in the day) vacuum tube filament transformers wired "back-to-back" will serve the purpose nicely. Just make sure the transformers are rated for the load power.

 

I would disagree with this statement, the magnetization depends on the voltage, not the AC current drawn.
If however you rectify the output in such a way as to put DC through the output transformer, then saturation could occur.

 

That's interesting, I thought saturation occurred when all the magnetic domains lined up due to excessive magnetic flux. This magnetic flux is related to the current and the turns of the transformers primary. But I guess current is linked to voltage anyway.
Adam

 

I am no expert in this field but used to travel to work with an electrical engineer. I have tried to find out without success how to determine which winding may have a shorted turn and we discussed transformers quite a bit.

The energising AC produces a current which is 90deg out of phase. This produces the magnetization. Energy goes into the magnet twice per cycle and is returned to the supply twice per cycle. There is no net power consumption other than losses (wire and core).

Adding a secondary and loading it utilises a current in phase with the voltage and with a normal mains transformer the voltage on the primary changes little, the inductive current is the same and the core is no nearer to saturation.

Putting DC current through a winding shifts the magnetisation 'to one side' so saturation could occur with one polarity of input. Careful design of single valve sound output transformers is therefore needed. A push/pull design which balances the current can produce much more power with a smaller transformer.

Switching on at the wrong part of the cycle effectively puts a DC signal into the transformer and this can cause saturation until the winding resistance damps it out. Toroidal transformers with low resistance and a core running closer to saturation (only one lamination) are prone to blow fuses on switch on so large ones are fitted with a primary starting resistor which is later by-passed.

 

Both statements are correct. Magnetization actually decreases slightly as power transfer in a real transformer increases, because the primary current in the primary winding resistance produces a voltage drop that subtracts from the voltage creating the current for the magnetization of the core. The effect is small if the primary resistance is small.

The magnetization current in the primary depends only on the primary excitation voltage and the magnetizing inductance at any given design frequency, increasing as the frequency decreases. This is why you can't use a 400 Hz transformer at its rated voltage with a 60 Hz supply. OTOH a 50 Hz or 60 Hz transformer generally works fine with 400 Hz excitation, albeit with less core magnetization and greater hysteresis losses.

Only if excessive voltage is applied to the primary does the core go into saturation at the peaks of the primary excitation waveform. This is not always bad, but it does decrease the magnetizing inductance during the peak excursions and it increases the magnetizing current, hence losses increase. However saturation peaking can improve the "stiffness" of the transformer and therefore improve load regulation at the expense of increased transformer losses.

When operated in the "linear" region of the B-H curve, the magnetizing current is virtually constant from no-load to full load. Whatever current is drawn by the secondary load creates a magnetic flux that opposes the increased magnetic flux created by the increased current in the primary. Thus, IsNs = IpNp (transformer equation) where Is is the secondary current, Ns is the number of secondary winding turns, Ip is the primary current, and Np is the number of primary winding turns. The current in the secondary under load creates a magnetic flux in the secondary that is equal and opposite to the flux created by the primary delivering power to the secondary load. The ampere-turns product of any winding on an ideal transformer is always a constant.

DC in any winding is bad news and can lead to core saturation well before maximum power transfer occurs. Avoid loading a secondary with a half-wave rectifier circuit.

 

Interesting explanation as usual Hop, although I didn't understand that much of it! Can you address directly the question of what limits the output power in this step-down/step-up application?

 

Lots of highly technical explanations but no real answer to the original question.
I see no reason that transformers cannot be connected back to back stepdown/stepup configuration. Its done all the time in power distribution networks.
Alternators generate at approx 10Kv and that is stepped up to transmission voltages (about 275Kv -500Kv) then step down again to local supply voltages.
I am sure you can connect a 120v/20v transformer to a 20v/120v transformer and it would work fine(allowing for some extra losses). Of course the total load on the final 120v needs to be considered when designing/specifiying the transformers.

 

The original question isn't whether it will work or not; it's why the output power is limited.

Clearly it's because the transformers have a power limit. I'm interested to know what factor(s) in the transformer limit the output power. Obviously the core size is ultimately the limiting factor but why?

 

There are at least half a dozen back to back transformers between generation and use, so it is possible.

The power limit is determined by the resistance of the wire and overheating thereby.
A bigger core allows fewer turns of thicker wire so a higher power output. Small transformers often have high resistance and low efficiency so running through four windings will give poor regulation.

 

Is that really the only factor that limits the output current?

 

To be specific, in the question, the INPUT 120v will allow more current since it comes from the mains,I assume, so it is only limited by the mains fusing.
The power OUT from the transformer arrangement as queried is limited by the rating of the transformers. Transformers are rated in VA or KVA so the bigger the size the higher the rating.
example: if both transformer used were rated at 1KVA then that is the maximum that you could safely get at the OUTPUT at 120v. One could probably design transformers for this exercise rated at 10000KVA(bit silly though) and get the 10000KVA OUT at 120v. Its all a matter of design requirements.
One could cause damage to the transformers in this setup(as in any transformer connection) simply by overloading the OUTPUT(ie short curcuit). Happens lots in power distribution systems. Thats why they have complex overload protection devices installed.

 

I think it is mainly due to losses in the core material (Iron Loss) and conductors, both the reflected flux from the secondary and flux from the primary don't fully cancel out due to flux leakage.

You also have to consider eddy currents and temperature increase which effect the permeability of the core.

This is one reason an air gap is used to stabilise the core from large variations in permeability due to manufacturing tolerances and temperature changes. And this air gap is also going to have an effect on the amount of current you draw from the secondary winding.

Your right about the size though, as you require more power you need to scale up the size of the core and conductors to reduce the losses and prevent saturation from excessive primary current.

Adam

 

I spent several hours trying to find the answer to this question: what limits the power transfer capabilities of a real transformer? Bottom line appears to be losses creating unacceptable temperature rise. Couldn't find anything, except a side-bar reference on Wikipedia, that relates winding e.m.f. to excitation frequency, core area, number of turns, and peak value of the magnetic field in the core, assuming a sinusoidal field. If you multiply both sides of this equation by the current in the winding... voila! you have a formula for power! Clearly power transfer depends on core area and length of the core (to obtain the induced magnetic field in the core), but where does magnetizing current play a role? All the stuff I found on magnetizing current says you want to keep it small, around one percent of the load current in the primary. Why? Why would you want or need any magnetizing current? More research is required.

Pragmatically we know that more power transfer in a transformer requires more ferromagnetic material in the core, else electrical utilities would purchase much smaller transformers!

As for wiring transformers back-to-back: this does work. You should try to match the VA capabilities of the two transformers, and limit the power transfer to the transformer with the smallest VA specification.

Sorry to be of so little help. Transformer design was not covered in my one power electrical engineering course, although I did find out how a three phase power source can create a constant amplitude rotating magnetic field. Fascinating, as Mr. Spock would say.