First attempt at transformer experiment failed

[11bgrunt]

I wanted to demonstrate a simple transformer showing turns ratio with step up and step down. I found a little experiment on the web and came close to following the directions but those materials were not on hand so I continued with the experiment anyway. The original instructions ask for several hundred turns of 28 gauge mag wire and apply low voltage AC.
I used insulated 14cu. solid, 150 turns to 100 turns and connected to 120 VAC. The source breaker tripped. Moved source to the other winding with the same result. Tomorrow I plan to connect to a variac and start out at a much lower voltage. If you agree that a transformer is nothing more than conductors wrapped around a steel core, what are the mistakes I made?

The experiment was titled; Build a Transformer Chapter 4 - AC Circuits

 

[GoldDigger]

Biography:
Old Broke Down Lineman.


...ask for several hundred turns of 28 gauge mag wire and apply low voltage AC.

I used insulated 14cu. solid, 150 turns to 100 turns and connected to 120 VAC.

The impedance (resistance to current flow) of a transformer winding with no load on the other winding will depend on the number of turns and the quality of the magnetic core they are wrapped around.
Either your core is not adequate or you do not have enough turns or both AND the applied voltage was too high.
The result is that too much current flows at line voltage.

If you do not understand this and you are not willing or able to follow published instructions you should not be doing this experiment!

 

[petersonra]

I think you need to see if you can get the magnetic fields more closely aligned. try winding the secondary over top of the primary, or vice versa.

 

[GoldDigger]

I think you need to see if you can get the magnetic fields more closely aligned. try winding the secondary over top of the primary, or vice versa.

Not the problem since one winding with no load on the other popped the breaker.
Once he goes to a low enough test voltage the slightly loose coupling between windings will affect efficiency, but not the rough experimental results.

 

[don_resqcapt19]

You can make a very simple one with two 500' spools of #12. Assuming a non-metallic spool with both ends of the conductor brought out. Connect one spool to the supply, the second to the load, stack them on top of each other and insert a length of ferrous metal through the two spools....not very efficient, but it does work.

 

[junkhound]

what are the mistakes I made

You have too small an iron cross section and not enough turns

V=N*A*integral(B*dv/dt), or simplistically, = NBAw, where N= # of turns, B= max flux in Teslas, about 1.8 for iron, A = area of magnetic core in meters square; w=2*pi*f or abuot 377 for 60 Hz.

You look like your highest voltage at 60 Hz would be 150*1.8*377*(1.5*.125/(39.4^2) =about 12 Vac, or scale up to about 1500 turns for 120 Vac. Expect your 1/8" thick steel to get hot from eddy currents. The bolted flux path has an air gap so the losses will be high from that also (high leakage reactance) For 150 turns, you could stack 10 bars thick and interleave them at the corners. Or, 300 turns, 5 bars, etc........

 

[templdl]

The iron core may work with DC devices but with AC it will work very poorly as the AC current one would expect would induce an alternating magneting flux in your core. It must be noted that the mag flux alternates poles at the same frequency as the AC current in the coil only the flux lags the coils polarity because of the hysteresis in the core. This hysteresis induces a counter EMF back into the coil as inductive reactance measured by impedance, a measure of resistance to AC current though a coil. Then the question becomes is there enough impedance to resist current flow in the coil as Nd enough amp turn in the coil. You current is extremely high.
Then, the core as shown I would expect would get hotter than han a pistol as the AC in the coil induces an alternation mag field in the core which actually induces an alterating current to flow around in the core, the core being iron and a poor conductor, the resistance to current ft low results in heating.
You will then realize as to why the cores of Tranformers, AC relays, and solenoids are made of laminations that are insulated from one another we which breaks up the current flow in tty he iron by not allowing current to flow between laminations to much smaller amounts thus greatly reducing heating.
Make any sense?

 

[Tony S]

I wanted to demonstrate a simple transformer showing turns ratio with step up and step down. I found a little experiment on the web and came close to following the directions but those materials were not on hand so I continued with the experiment anyway. The original instructions ask for several hundred turns of 28 gauge mag wire and apply low voltage AC.
I used insulated 14cu. solid, 150 turns to 100 turns and connected to 120 VAC. The source breaker tripped. Moved source to the other winding with the same result. Tomorrow I plan to connect to a variac and start out at a much lower voltage. If you agree that a transformer is nothing more than conductors wrapped around a steel core, what are the mistakes I made?

The experiment was titled; Build a Transformer Chapter 4 - AC Circuits

Just to cheer you up, we built a transformer at engineering college.

It caught fire

 

[steve66]

As Golddigger mentioned, more turns and less voltage to start with.

Note that 28 ga. magnet wire would allow you to get a lot more turns in the same volume than your #14 plastic coated wire.

 

[ggunn]

Not the problem since one winding with no load on the other popped the breaker.
Once he goes to a low enough test voltage the slightly loose coupling between windings will affect efficiency, but not the rough experimental results.

Could another problem be the fact that his core is four separate bars bolted together rather than a single contiguous piece of metal? Also, what are the ferromagnetic characteristics of those bars?

 

[Sahib]

Tomorrow I plan to connect to a variac and start out at a much lower voltage.

Why not use a power electronics device instead of variac for voltage change.(Remember you are already making a transformer for voltage change) It would also be energy efficient and more fun. You may search web for details.

 

[GoldDigger]

Why not use a power electronics device instead of variac for voltage change.(Remember you are already making a transformer for voltage change) It would also be energy efficient and more fun. You may search web for details.

A "power electronics device", as in triac or SCR based? That would introduce a lot of harmonics which would overly complicate the "simple" transformer experiment.

 

[Ingenieur]

Is there a load on the secondary?

 

[raider1]

You can make a very simple one with two 500' spools of #12. Assuming a non-metallic spool with both ends of the conductor brought out. Connect one spool to the supply, the second to the load, stack them on top of each other and insert a length of ferrous metal through the two spools....not very efficient, but it does work.

This is what I have done for my electrical apprenticeship class when we discuss transformers. One spool is connected to a light bulb and the other to an extension cord. Not overly efficient but the light bulb does light up dimly.

Chris

 

[templdl]

Other than the length of wire and the size of the wire that the primary coil is made of we hat limits the current which flows in that coil? In a previous post I tried to explain the importance of the core and it's effect on current flowing in the coil wrapped around it. That iron core as s hi own is enemic at best in its ability to induce a counter EMF in the primary coil. It is difficult to assemble a model transformer in the field when you are not able to use s laminated core.

 

[11bgrunt]

what are the mistakes I made

You have too small an iron cross section and not enough turns

V=N*A*integral(B*dv/dt), or simplistically, = NBAw, where N= # of turns, B= max flux in Teslas, about 1.8 for iron, A = area of magnetic core in meters square; w=2*pi*f or abuot 377 for 60 Hz.

You look like your highest voltage at 60 Hz would be 150*1.8*377*(1.5*.125/(39.4^2) =about 12 Vac, or scale up to about 1500 turns for 120 Vac. Expect your 1/8" thick steel to get hot from eddy currents. The bolted flux path has an air gap so the losses will be high from that also (high leakage reactance) For 150 turns, you could stack 10 bars thick and interleave them at the corners. Or, 300 turns, 5 bars, etc........

Thank you all for your comments. I agree we have a poor core and not enough windings. I think there is a reason it is called an experiment. The objective was to show the coupling, transformation and turns ratio. This transformer is so bad, the coupling from one coil to the other is obvious and a transformation of sorts takes place but this is not efficient enough to show turns ratio.
When 100 turns was primary; 16 amps, 10 volt primary fell to 4 volt secondary.
When 150 turns was primary; 16 amps, 17 volt primary fell to 3 volt secondary.
A 40 watt bulb with 25 ohm cold filament had no effect on the source or load when connected to the load side. Maybe I will get a chance to do more testing with smaller loads.
For sure the primary coil and bar can probably get hot enough in this model to destroy itself.

 

[GoldDigger]

To be useful a transformer must have a much higher inductive reactance than the DC resistance of the winding. And equally important the magnetic field from the coil must not be high enough to drive the iron core to saturation.
Saturation will limit the maximum voltage that can be applied to the coil.
Once you deal with that, you can look at increasing the number of turns to increase the inductance of the coil and you should see better tracking between primary and secondary.
Finally, you should start by looking at the voltage ratios at very low load rather than the short circuit current ratios.
The primary current consists of both the magnetizing current and the transformed secondary current. To see the effect of turn ratio you need to separate the two components. The easiest way to do this is to look at the unloaded voltages instead.

 

[templdl]

To be useful a transformer must have a much higher inductive reactance than the DC resistance of the winding. And equally important the magnetic field from the coil must not be high enough to drive the iron core to saturation.
Saturation will limit the maximum voltage that can be applied to the coil.
Once you deal with that, you can look at increasing the number of turns to increase the inductance of the coil and you should see better tracking between primary and secondary.
Finally, you should start by looking at the voltage ratios at very low load rather than the short circuit current ratios.
The primary current consists of both the magnetizing current and the transformed secondary current. To see the effect of turn ratio you need to separate the two components. The easiest way to do this is to look at the unloaded voltages instead.

This is the direction that I was going with my posts, the core. Just because you have an iron core doesn't mean that you with transforme coupling. I attempted to explain the inportance of hysteresis in the core and the iinfluence it has on generating counter EMF in the coil as well as impedance. This isn't happening in his model. Besides that his core should be getting hotter that a pistol in very short order.

 

[11bgrunt]

You can make a very simple one with two 500' spools of #12. Assuming a non-metallic spool with both ends of the conductor brought out. Connect one spool to the supply, the second to the load, stack them on top of each other and insert a length of ferrous metal through the two spools....not very efficient, but it does work.

This is perfect for what I want. You can't teach or learn all there is to know about transformers in 4 hours, but this is a place to start. and That solid bar will get hot enough to melt the plastic centers.
Thanks,

 

[kwired]

This is perfect for what I want. You can't teach or learn all there is to know about transformers in 4 hours, but this is a place to start. and That solid bar will get hot enough to melt the plastic centers.
Thanks,

What was your primary to secondary voltage ratio. Theoretically should have been 1:1, if both reels of wire were the same thing, but just curious how close that may have come out.