# Transformer Primary coil inductance. Why different inductance values ?

Discussion in 'General Electronics Chat' started by prashanthb, Jul 9, 2015.

1. ### prashanthb Thread Starter Member

May 17, 2015
34
2
The following is a list of different transformers I have and their primary winding inductances.
All are built for 230Vac and having different ampere capacity and different physical dimensions.
My question is, What is the basis for setting the size of primary coil inductance ?
Why are they varying so much when they all are designed to handle same 230Vac. Is it dependent of current handling capacity of the Tx ?

Thank you.

2. ### pwdixon Member

Oct 11, 2012
488
56
If nothing else the transformer power spec differences will obviously change the physical make-up of the coils and have a knock-on effect on inductance. There must be so many differences in the physical build that picking any specific parameter will no doubt pinning the 'blame' on any particular characteristic will be difficult. Don't forget as well your measurement device could also be amplifiying the differences because there will be a difference between measuring a pure inductor and the inductance of the transformer with all the parasitic components interfering with the measurement.

3. ### Bordodynov Active Member

May 20, 2015
643
188
I choose the minimum required inductance using a valid input load current of the transformer to your liking. For example, this current may be 15 mA (I=230V/(2*pi*F*Lp)). Some suggest this current. To reduce this current it is necessary to increase the inductance by increasing the number of turns or the dimensions of the core. The empirical formula for the number of turns per volt: w=50/Area[cm].
It is possible to more accurately count the required number of turns (inductance) using the parameters permissible induction, geometric and magnetic data core.

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4. ### AnalogKid Distinguished Member

Aug 1, 2013
4,546
1,252
The answer is exactly what you said, physical and electronic differences.

ak

5. ### prashanthb Thread Starter Member

May 17, 2015
34
2
Thank you for that formula.
One technique I saw on a youtube video is connecting a general 40W incandescent bulb in series with the inductor, at regular intervals connect the pair to the mains and see if the bulb glows. With each turn the light gets dimmer. You will have finished the winding task when the bulb doesnt glow anymore.

6. ### prashanthb Thread Starter Member

May 17, 2015
34
2
I did not get you please elaborate. Is the inductance of a coil set depending on the current it will handle ? I think it is the voltage.Hmmm..I think its both...

7. ### AnalogKid Distinguished Member

Aug 1, 2013
4,546
1,252
The shape of a coil (round, rectangular, oval, etc.), the diameter of the wire, the thickness of the insulation, the number of turns, the type of metal in the core - all of these things affect the inductance value. Both the inductance value and the wire diameter (and hence, its resistance) affect how much current will flow through the inductor at the power line frequency. The type of insulation and the way the inductor is wound affect the voltage it can handle. All of these parameters interact to set the right inductance value for any particular application. Transformers are even more complex because of the mutual inductance between the primary and secondary windings.

https://en.wikipedia.org/wiki/Inductance#Coupled_inductors_and_mutual_inductance

As a very basic starting point, begin with the amount of power the transformer must handle. For any particular primary voltage you can calculate the current. With those two parameters you can calculate the impedance, and from that calculate the inductance. But transformers are much more complex than a simple inductor.

ak

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8. ### Lestraveled Well-Known Member

May 19, 2014
1,957
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+1 on what AK said.

I would like to add, as the core saturates the inductance changes. So, in part, the inductance of a power transformer is the result of making other more important design choices. Inductance is not ignored in the transformer design, it is just not that critical of a parameter.

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9. ### crutschow Expert

Mar 14, 2008
13,056
3,245
The primary inductance determines the no-load magnetizing current the transformer draws. The inductance is selected to give a low current, but the actual value can vary depending upon how the transformer is designed and how close it is operating to saturation.
It has little direct bearing on the load current that the transformer can carry, which is determined by the wire and core size.

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10. ### Lestraveled Well-Known Member

May 19, 2014
1,957
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If I were to advise a fanatical audiophile, such as yourself ( ), about selecting the best transformer, I would recommend that you pay more attention to the size of the wire on the secondary (bigger = better) and the weight of the transformer (heavier = better). These two factors will generally yield a transformer whose output will sag less from increasing loads, than a transformer that has thinner wires and is lighter.

11. ### prashanthb Thread Starter Member

May 17, 2015
34
2

Point noted.Thank you.

12. ### prashanthb Thread Starter Member

May 17, 2015
34
2
One more question :
Is it right when I say ...
1) Inductors convert change in current to magnetism, then use up that stored magnetism to give out "more" voltage than what was applied(and not much change in current).
2)Capacitors store as charge when there is extra voltage but during discharge, high current can be drawn than what was initially used to charge it(voltage remains more or less same as charging voltage x 1.41 ???).

Thank you.

Last edited: Jul 11, 2015
13. ### BillB3857 Senior Member

Feb 28, 2009
2,400
348
For an interesting experiment, connect the primary of a transformer to your inductance meter, then apply a variable DC to a secondary winding. See what happens to the inductance. The result is the basis of a magnetic amplifier.

14. ### crutschow Expert

Mar 14, 2008
13,056
3,245
You have the general idea but your specifics are a little off.

1) Magnetism is a general term relating to the subject of magnetic materials. Magnetism is not normally a term used when discussing magnetic energy.
It's not a change in current that stores energy, it's the current value. The energy of an inductor is stored in the magnetic flux generated by the current in the inductor. This energy equals 1/2 LI^2. The generated voltage from this stored magnetic energy can be higher than was applied but the current will then be less since energy is volts times amps.
Transformers are inductive devices that transfer energy from one winding to another. This requires an AC voltage which is always changing for the transfer to occur. Transformers won't work on DC.

2) The capacitor action you are referring to applies only to a filter capacitor that is being charged by an AC sinewave through a diode. In that case the capacitor charges to near the peak of the sinewave, which is approximately 1.41 times its RMS value.
The capacitor then discharges between the sinewave peaks with a voltage drop (ripple) which depends upon the capacitor value, the frequency of the sinewave, and the load current.
And the capacitor current during the peak charging time is much higher than the load current during the discharge time since the average current during the peak charge has to equal the average load current.

15. ### Lestraveled Well-Known Member

May 19, 2014
1,957
1,215
There are a variety of ways to look at, and understand, capacitors and inductors. The rule of ones:

If you have a one farad capacitor, and you apply a constant current of one amp through it, the voltage across the capacitor will change at the rate of one volt per second. In other words, a constant current in, will give you a linearly changing voltage.

If you have a one henry inductor, and you cause the current running through it to change at the rate of one amp per second, then the voltage across it will be one volt. In other words, a linearly changing current in, will give you a constant voltage.

So, when you look at how capacitors and inductors behave in the voltage, current and time world, they are functional mirrors of one another. In terms of the duality of the universe, light and dark, good and bad, right and left, capacitance and inductance. One can not be without the other.

I though you might like a more charismatic description. (Oh yea, and you can use them to store energy too.)

Last edited: Jul 12, 2015
16. ### prashanthb Thread Starter Member

May 17, 2015
34
2
great ! I am happy.
Thank you for the explanations.
I will report back after I do that.