# Need a little help (transformer)

• Ruby_338
In summary: The primary occupies about an inch and a half. The magnetizing inductance may not be large enough, so it may be drawing a high current and causing a larger voltage drop across the source impedance. Assuming your primary coil is 1 inch long (2.5 cm) , I get a magnetizing reactance of about 8 ohm, which is not that big. If the coil length is about 3 inches, the magnetizing reactance reduces to about 2.7 ohm, which is definitely very small and is the reason for the reduced voltage across the primary.In summary, it seems that the magnetic reactance of the primary coil is not large enough and is the reason for the reduced voltage across the primary.
Ruby_338
I am a student of class 12. I am making a transformer for a project. I took an iron rod about 3 inches long and cross sectional area about 0.5 square centimetres as the core. The primary coil has 100 turns and the secondary has 25. I used an ac source of 16 volt. I connected the primary coil to the ac source and measured the voltage across both the primary and secondary coil. I found that voltage across the primary coil is about 2 volt and there is no voltage across secondary coil. I used a multimeter for taking readings.
I'm pretty sure I didn't scratch the insulation anywhere in the wires except at the ends, the secondary is insulated from the primary and the primary from the iron core by polyester paper, the coils are wound pretty tight, the ac source and multimeter are fine (I checked), the core gets magnetised, the connections were made well, the resistance of the wires are not that high(primary is about 6 ohms and secondary, about 3)...i can't see where the problem lies. Please help.

You say the primary voltage is 2 , this must also be the source voltage ...check your voltmeter is set to measure AC Volts ... make sure the source is AC volts ...connect voltmeter across source ..it should read 16 ...then connect to the primary, the voltage should drop , but not too much...try this first.

Ruby_338
Ruby_338 said:
I am a student of class 12. I am making a transformer for a project. I took an iron rod about 3 inches long and cross sectional area about 0.5 square centimetres as the core. The primary coil has 100 turns and the secondary has 25. I used an ac source of 16 volt. I connected the primary coil to the ac source and measured the voltage across both the primary and secondary coil. I found that voltage across the primary coil is about 2 volt and there is no voltage across secondary coil. I used a multimeter for taking readings.
I'm pretty sure I didn't scratch the insulation anywhere in the wires except at the ends, the secondary is insulated from the primary and the primary from the iron core by polyester paper, the coils are wound pretty tight, the ac source and multimeter are fine (I checked), the core gets magnetised, the connections were made well, the resistance of the wires are not that high(primary is about 6 ohms and secondary, about 3)...i can't see where the problem lies. Please help.
Try oz93666's suggestion.

What is the length occupied by the primary coil on the rod? The magnetizing inductance may not be large enough, so it may be drawing a high current and causing a larger voltage drop across the source impedance. Assuming your primary coil is 1 inch long (2.5 cm) , I get a magnetizing reactance of about 8 ohm, which is not that big. If the coil length is about 3 inches, the magnetizing reactance reduces to about 2.7 ohm, which is definitely very small and is the reason for the reduced voltage across the primary.

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Ruby_338
I'd connect the source to the secondary and measure V across the terminals of both sides. I may end up with a clue.

Ruby_338
cnh1995 said:
Try oz93666's suggestion.

What is the length occupied by the primary coil on the rod? The magnetizing inductance may not be large enough, so it may be drawing a high magnetizing current and causing a larger voltage drop across the source impedance. Assuming your primary coil is 1 inch long (2.5 cm) , I get a magnetizing reactance of about 8 ohm, which is not that big. If the coil length is about 3 inches, the magnetizing reactance reduces to about 2.7 ohm, which is definitely very small and is the reason for the reduced voltage across the primary.
The primary occupies about an inch and a half

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oz93666 said:
make sure the source is AC volts ...

I did. The source voltage is 16

GoodPost said:
I'd connect the source to the secondary and measure V across the terminals of both sides. I may end up with a clue.
Did that too. In that case, voltage across secondary is 0.5 volt and there is no voltage across primary

What's magnetic reactance?

Ruby_338 said:
What's magnetic **magnetizing** reactance?
The reactance responsible for establishing magnetic flux in the core.
In this case, it is the self inductance of the primary coil. You have no of turns=100, area of the coil=0.5 sq cm, length of the coil=1.5 inch, and relative permittivity of iron=1000. Inductance L=μ0μrN2A/l. The inductive reactance then becomes XL=2πfL, where f is the supply frequency (50Hz I assume). With an inch and a half long coil, magnetizing reactance is around 5.3 ohm. You should check the internal impedance of the source. At least check the internal resistance using a multimeter.

Ruby_338

cnh1995
Hi Ruby,

If the supply is on the 100 turn coil, the power output on the 25 turn coil should resemble .25 of the supply.
If the supply is on the 25 turn coil, the power output on the 100 turn coil should resemble 4 x supply.
It is worth noting that the coil connected to the supply is the primary, the other (output) coil is the secondary.

The supply voltage should travel through the first coil generating a magnetic flux field, which must be transferred to the second coil through the core, which will in turn induce a reciprocal time shifted voltage charge in the 2nd coil.

I am not convinced that using a single rod material is the best choice for this step-up transformer you have built, as it may be awkward to have 2 magnetic fields existing in the same section of the material. This alone could explain much of the voltage loss in that; the time shifted voltage from the second coil would be cancelling out a large portion of the voltage of the first coil, because the 2 magnetic fields are in the same space, where they would be better placed in 2 separate but over-lapping spaces.

Having each coil wrapped around it's own iron core material may be a better option, or Imagine a square of iron, where one coil is wrapped on the east side and the other coil wrapped on the west side, of the iron square.

e.g. http://www.electronics-tutorials.ws/transformer/transformer-construction.html

Ruby_338
Wee-Lamm said:
Hi Ruby,

If the supply is on the 100 turn coil, the power output on the 25 turn coil should resemble .25 of the supply.
If the supply is on the 25 turn coil, the power output on the 100 turn coil should resemble 4 x supply.
It is worth noting that the coil connected to the supply is the primary, the other (output) coil is the secondary.

The supply voltage should travel through the first coil generating a magnetic flux field, which must be transferred to the second coil through the core, which will in turn induce a reciprocal time shifted voltage charge in the 2nd coil.

I am not convinced that using a single rod material is the best choice for this step-up transformer you have built, as it may be awkward to have 2 magnetic fields existing in the same section of the material. This alone could explain much of the voltage loss in that; the time shifted voltage from the second coil would be cancelling out a large portion of the voltage of the first coil, because the 2 magnetic fields are in the same space, where they would be better placed in 2 separate but over-lapping spaces.

Having each coil wrapped around it's own iron core material may be a better option, or Imagine a square of iron, where one coil is wrapped on the east side and the other coil wrapped on the west side, of the iron square.

e.g. http://www.electronics-tutorials.ws/transformer/transformer-construction.html
Our project requires that we do it on the same core. Thanks for the suggestion anyway

It seems clear from what you said the supply transformer is not up to the job ...you have an open circuit voltage of 16 , but when connected to primary it drops to 2 and when connected to secondary drops to 0.5 .. this can only mean it has a very high internal resistance/reactance ...insanely high ...
Give details of supply transformer (it may be fried)/try a different one.

Ruby_338 said:
Our project requires that we do it on the same core. Thanks for the suggestion anyway
Well, Wee-Lemm made a fine point and it turns out that my caculations for the magnetizing inductance are valid for a square core where most of the flux is contained in the core , and not a rod. For the rod, the magnetizing reactance is even smaller because of the air-path. If you are using a rod, the flux completes its path through the air and that wouldn't be effective.

oz93666 said:
It seems clear from what you said the supply transformer is not up to the job ...you have an open circuit voltage of 16 , but when connected to primary it drops to 2 and when connected to secondary drops to 0.5 .. this can only mean it has a very high internal resistance/reactance ...insanely high ...
Give details of supply transformer (it may be fried)/try a different one.
I'll try that

100 turns doesn't sound nearly enough. Try 400 turns for the primary.

Can you attach a photo of what you have constructed?

https://www.physicsforums.com/attachments/110502.gif

cnh's point about a square core, more specifically one with a closed all iron path for magnetic flux, is the key to your dilemma.

You understand no doubt Kirchoff's Current Law, to effect '...current must get back to where it started from...'
Magnetic circuits are similar to electrical ones, magnetic flux too must get back to where it started from .

Observe flux that gets pushed out the top of the coil comes back to its bottom. I only showed two lines, in theory some of the lines go clear out past Alpha Centauri.
Now it is very difficult to push flux through air.
Current is what pushes flux,
and your source is incapable of delivering enough current to push enough flux through the air surrounding your core to make the voltage that you want in the coils.

Try surrounding your transformer core with a path of iron and see if voltage goes up.
(should be no air gaps - I'm just not very good with graphics)

Be aware the C-Clamp won't work at frequency above a few hundred hz so try line frequency.

Look up statement "There are no magnetic monopoles" ...
and call this experiment your 'introduction to ferromagnetism' .

By inserting paper between core end and clamp you will be able to show how tremendous is the effect of air gap.

old jim

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He would get a better transformer if he reduced the current on the coil by winding a longer length of wire. And/or fixing the current from the supply transformer. I get that.

Would he also get a better transformer if he wound the primary on the clamped iron rod in Jim's picture and the secondary around the back of the C-clamp? That looks more like the transformers I see.

That didnt work either. Besides, the project is to make a rod transformer. Maybe i should just make a new one :)​

Ruby_338 said:
That didnt work either.

What didn't work? the C-Clamp?
I didn't expect it to work well, only to come nearer working.

What is frequency of your voltage source?
What voltages did you measure on primary and secondary? Did they go up when gave flux an all iron path ?

Mother Nature makes us work hard for our lessons. She rewards persistence.

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There was still no voltage across the secondary

jim hardy said:
What is frequency of your voltage source?
.
The source frequency is 50 hz

Ruby_338 said:
There was still no voltage across the secondary
Jim Hardy asked 4 questions, you give a proper answer to only one.

Looking at your photo I'd say that any flux in the primary does link with the secondary and you should therefore get the correct transformer action.
What it seems to me is in question, is how much of your measured primary voltage is inductive and how much resistive? Since you have not many turns and a poor magnetic circuit, the inductance may be quite low compared to the resistance.
Guessing at copper wire of about 0.25mm diameter (based on the 100t primary occupying 1/3 of the length of the 75mm rod) and 100t around a 5mm diameter bar being about 1500mm of wire, I get a resistance of about 0.5Ω.
For an air cored coil, the inductance is only about 5μH, and the iron bar I think would only about double this, giving a reactive impedance of 3mΩ at 50Hz, over 100 times less than the resistance.
So if you are measuring 2V across the primary, 99% of that is resistive and only 0.02V is inductive. Step that down 100:25 and you are only looking at 5mV induced in the secondary. But I'm surprised you detect nothing, unless you're using an analogue multimeter.

E&OE: It's late, I've guessed most of the variables and my method is very rough and ready. But I'm getting an inductive reactance much smaller than the resistance. Try checking it yourself and see what you come up with. Or measure the resistance of your primary and the current through it, then calculate the values.

jim hardy and sophiecentaur
cnh1995 said:
At least check the internal resistance using a multimeter.
You need to add a known resistive Load and measure the voltage dropped when it's connected to the supply.
Ruby_338 said:
Our project requires that we do it on the same core. Thanks for the suggestion anyway
This is a common difficulty for students. They are given a basically flawed project and they get helpful advice from PF which they can't follow. because of the limitations of equipment they have been given. You really should approach your tutor and repeat the ideas that you have gathered here - particularly the numbers you obtained from PF. The replies you have been given are telling you that you are getting results that you could be expecting with such equipment.
Talk to your fellow students (I assume there are others doing this experiment too) They are likely to be as confused as you are.
Your teachers should be in a position to be helping you here - you are certainly not just being lazy or clueless if you ask them.
Edit: You should be able to expect that the person who gave you the project can actually do the project themselves. At your level of EE education, you should be able to rely on your equipment (transformer) to behave pretty well ideally.

jim hardy and cnh1995
Rod transformers don't work at all well, here's why

It's really difficult to push magnetic flux through air.
So not much of your flux makes the long round trip between the coils and you get not much coupling.

To demonstrate that point is the only reason i can think of for this exercise.

Winding your primary and secondary right atop each other will give you better coupling. I'd hold two strands of wire and wind them at same time, that's called "bifilar winding" ..

Still, with half your magnetic circuit being air it's going to be a feeble transformer. That's why they'd gone to closed loop cores by 1880's.

We had another transformer thread where the student plotted volts on both secondary and primary versus primary amps. That graph would be a starting point for understanding this apparatus. Do it twice, first as you've built it then again with windings bifilar .
old jim

cnh1995
jim hardy said:
Rod transformers don't work at all well, here's why
That sure is a low permeability core, Jim. Are you sure that one isn't just cardboard?

jim hardy
NascentOxygen said:
That's a low permeability core, Jim. Are you sure that one isn't just cardboard?

With bifilar on a rod, cardboard core gives roughly half as much flux per amp as iron. That's counterintuitive because iron's relative permeability is so high.

Could it be that's the point of the experiment ?

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I wasn't counting the flux lines, it's their distribution that doesn't look right. Insertion of the iron core should bring a concentration of field lines with some emerging from the ends of the rod.

The pattern in your image looks appropriate for an air core.

NascentOxygen said:
I wasn't counting the flux lines, it's their distribution that doesn't look right. Insertion of the iron core should bring a concentration of field lines with some emerging from the ends of the rod.

The pattern in your image looks appropriate for an air core.
You're quite right

sorry , N.O. i thought you were teasing me about the primitive drawing

My thinking is iron core roughly doubles the flux density in its vicinity
because around any loop that traverses it, roughly half the path is low reluctance iron and the other half remains high reluctance air

so yes of course replacing my cardboard core with an iron one should add a few lines to my sketch, like this blue one,
and roughly double the number of purple ones.

I've done that experiment on a 12 foot long coil. We too were surprised flux only doubles.

Thanks for your sharp eye !

PS i removed the snickering kitty

NascentOxygen
jim hardy said:
With bifilar on a rod, cardboard core gives roughly half as much flux per amp as iron. That's counterintuitive because iron's relative permeability is so high.

Could it be that's the point of the experiment ?
I don't think there would be any point in presenting that to students at the apparent level of the OP.
I think it's time to sit back and wait for some feedback - hopefully with information from the teacher, even.

Re. #30 and #31 of Jim and NO : my understanding was that a solenoid had a uniform field inside and nearly all the flux passed through the ends, as shown on the hyperphysics solenoid page or

This would seem to suggest that if the secondary were wound on top of the centre portion of the primary (as shown in Ruby's photo), then all the flux (caused by primary current) would link both coils irrespective of the core or bifilar winding. (OTOH, perhaps not all of the flux due to any secondary current would link the primary, as the secondary is much shorter. But I (we?) haven't got that far yet.)

Re. #32 Jim, I agree with the nearly doubling of flux for any decent permeability core. The length of the air path is nearly halved and the air contributes nearly all the reluctance.

Meanwhile I've found an 8mm bolt and a couple of metres of copper wire and made my own transformer. It was about 1.7Ω primary, but I didn't have time to measure the reactance, because, my step-daughter decided to have her son a few days early and I've been whisked away to do something useful instead of playing with my toys. At least I've got my laptop, so I can keep up with the chat.

Merlin3189 said:
Re. #30 and #31 of Jim and NO : my understanding was that a solenoid had a uniform field inside and nearly all the flux passed through the ends, as shown on the hyperphysics solenoid page or

agreed. nearly all but there is leakage flux , integrate h dot dl around any single turn ?

https://sharepoint.umich.edu/lsa/ph...et Library/Two solenoids, B-field_5H15.40.JPG
Merlin3189 said:
This would seem to suggest that if the secondary were wound on top of the centre portion of the primary (as shown in Ruby's photo),
I didnt read his photo that way

i take yellow tape as underneath primary and green tape as underneath two halves of secondary.
But i didnt think about hooking secondaries in series with turns all going same direction...

Might that be another trouble..?

cnh1995

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