Primary transformer resistance

[winnie]

However i did the math and got 0.2 ohms for resistance

You calculated primary voltage divided by primary current. This gives the apparent input impedance of the loaded transformer.

If this impedance was _only_ the transformer resistance, then your calculation would be correct. But most of the impedance is actually the magnetic field in the transformer core preventing the flow of primary current.

The _DC_ resistance of the transformer is a tiny fraction of the primary impedance. If the DC resistance of the transformer were the value you calculated, then all of the input power would be converted to heat in the resistance; just imagine how much power gets dissipated by 3000A and 0.2 ohm!

-Jonathan

 

[jim dungar]

Depending on its actual construction I would expect a 3630kVA transformer to have a nominal impedance of roughly 5% with an X/R ratio of maybe 8. Notice how the math is fairly more complicated in order to determine the actual resistance.

A little more math.
a 5% impedance transformer (5 %Z) means that it takes 5% of the input voltage to generate 100% of the full load current when the secondary is shorted with a bolted fault. So you can do the math to develop the amount of primary + secondary impedance. The X/R ratio is then used to break the total impedance into its resistive and reactive components. But you still cannot easily break the resistance down into the individual primary and secondary winding amounts.

So in the end we don't calculate transformer winding resistances, from nameplate data, as it it is easier to measure them with the proper instrumentation.

 

[Jpflex]

A little more math.
a 5% impedance transformer (5 %Z) means that it takes 5% of the input voltage to generate 100% of the full load current when the secondary is shorted with a bolted fault. So you can do the math to develop the amount of primary + secondary impedance. The X/R ratio is then used to break the total impedance into its resistive and reactive components. But you still cannot easily break the resistance down into the individual primary and secondary winding amounts.

So in the end we don't calculate transformer winding resistances, from nameplate data, as it it is easier to measure them with the proper instrumentation.

What then is the formula E/I good good for if you must use instrumentation?

This is part of equation to find circuit resistance or impedance in ac.

Feeder sizing and breaker primary and secondary use transformer nameplate values and these calculations to determine their AWG size but not instrumentations

 

[jim dungar]

What then is the formula E/I good good for if you must use instrumentation?

This is part of equation to find circuit resistance or impedance in ac.

Feeder sizing and breaker primary and secondary use transformer nameplate values and these calculations to determine their AWG size but not instrumentations

Ohm's Law (E/I) is the formula for determining resistance in loads. It is not used to determine magnetic circuit impedance for determining conductor sizing.

For transformer conductor selection we use the Power formula of VA = EI, or more specifically current equals transformer size divided by voltage (including the √3 if three phase) which for three phase is I = (kVA X 1000)/(V / 1.732).

 

[Jpflex]

Ohm's Law (E/I) is the formula for determining resistance in loads. It is not used to determine magnetic circuit impedance for determining conductor sizing.

For transformer conductor selection we use the Power formula of VA = EI, or more specifically current equals transformer size divided by voltage (including the √3 if three phase) which for three phase is I = (kVA X 1000)/(V / 1.732).

E/I can be used to find circuit impedance. Although impedance uses frequency, knowing a circuits total amperes and voltage you will still be able to find total circuit current opposition or impedance.

You can also use the alternate method 2x pie x frequency x henry for inductive reactance portion as well

And as you mentioned the VA Value of the transformer can be used. This is what i mentioned earlier by using the transformer nameplate values.

 

[jim dungar]

And as you mentioned the VA Value of the transformer can be used. This is what i mentioned earlier by using the transformer nameplate values.

No.
You cannot use a variation of Ohm's Law or the Power Formula to determine the impedance of a magnetic machine, like a transformer or motor, based simply on its nameplate maximum input VA.

The impedance of a transformer primary side windings is the same regardless if the transformer is idling or at 100% load.

 

[Jpflex]

A little more math.
a 5% impedance transformer (5 %Z) means that it takes 5% of the input voltage to generate 100% of the full load current when the secondary is shorted with a bolted fault. So you can do the math to develop the amount of primary + secondary impedance. The X/R ratio is then used to break the total impedance into its resistive and reactive components. But you still cannot easily break the resistance down into the individual primary and secondary winding amounts.

So in the end we don't calculate transformer winding resistances, from nameplate data, as it it is easier to measure them with the proper instrumentation.

I have a book which explained the impedance value and bolted secondary shorting you explained. However when you mention generating 100% of the full load current, you are speaking of the secondary circuit portion correct?

Also what Current effect would the primary have in this situation since the secondary limits the primary and vise versa due to frequency induction and self induction during normal operation.

 

[Jpflex]

No.
You cannot use a variation of Ohm's Law or the Power Formula to determine the impedance of a magnetic machine, like a transformer or motor, based simply on its nameplate maximum input VA.

The impedance of a transformer primary side windings is the same regardless if the transformer is idling or at 100% load.

According to one of my college text books and when finding a circuits impedance neccesary to correct power factor, this is one method in which the reader is told to use.

I am not saying this will find individual resistance vale and inductive reactace but will atleast provide the total circuit current opposition or impedance based on the circuit current, voltage value, sq rt 3 if three phase etc

 

[jim dungar]

Also what Current effect would the primary have in this situation since the secondary limits the primary and vise versa due to frequency induction and self induction during normal operation.

This is partly why using only the nameplate maximum kVA is not sufficient for determining the transformer input resistance.

....when you mention generating 100% of the full load current, you are speaking of the secondary circuit portion correct?

How would you have secondary current flowing without primary current?

 

[jim dungar]

I am not saying this will find individual resistance vale and inductive reactace....

But that is exactly what I took as your question in post #1.

 

[winnie]

No.
You cannot use a variation of Ohm's Law or the Power Formula to determine the impedance of a magnetic machine, like a transformer or motor, based simply on its nameplate maximum input VA.

The impedance of a transformer primary side windings is the same regardless if the transformer is idling or at 100% load.

Expanding on this: If you apply Ohm's law (or the complex impedance variation on Ohm's law) to the nameplate input maximum VA, you will be calculating the apparent impedance of the _entire_ transformer, including the connected full load. You won't get the component impedance of the primary coil alone.

If you treat the transformer along with its load as a single 'device', you can certainly determine the impedance of that complex device. But that will certainly be different than the impedance of a single portion of that device. A transformer is a composite device that internally requires changing magnetic flux to function. When you apply DC current to the primary winding, you are effectively measuring just the primary coil resistance, not the composite impedance of the entire transformer. You can't expect the two numbers to be the same; you are measuring _different_ pieces of the same composite whole.

 

[jim dungar]

An accepted method for determining the effective reactive impedance of a transformer by using the nameplate kVA, input voltage and %Z is:
X_T = (kV² /MVA) X (%Z X 100).

You could then use the transformer X/R ratio to determine the effective resistance.

 

[Jpflex]

How would you have secondary current flowing without primary current?

i was asking if primary or secondary current was examined or both during secondary shorting but not saying that secondary current would flow without primary feed

 

[jim dungar]

i was asking if primary or secondary current was examined or both during secondary shorting but not saying that secondary current would flow without primary feed

The %IZ definition is amount of primary voltage required to have maximum secondary current flowing into a bolted fault effectively at the transformer secondary terminals.
I don't know why you would measure the primary current, if the definition uses the secondary, but I haven't performed one of these tests since college.

 

[Jpflex]

The %IZ definition is amount of primary voltage required to have maximum secondary current flowing into a bolted fault effectively at the transformer secondary terminals.
I don't know why you would measure the primary current, if the definition uses the secondary, but I haven't performed one of these tests since college.

So this maximum secondary is the point where the current is max such as current no longer increases in proportiom to increased primay voltage applied?

I also did not recall every aspect that is in the process that is why i was asking.

 

[winnie]

So this maximum secondary is the point where the current is max such as current no longer increases in proportiom to increased primay voltage applied?

No, just the maximum rated secondary current.

Due to design optimization this is probably pretty close to the core starting to saturate, but core saturation is not a requirement for the max rated current

 

[jim dungar]

So this maximum secondary is the point where the current is max such as current no longer increases in proportiom to increased primay voltage applied?

As Winnie said, it is just the nameplate value.

With proper cooling some transformers can carry up to 1.5X full load amps while maintaining proper secondary voltages, 15% overload capability is very common.
During short circuit conditions a transformer can supply 100/%Z ( a 5%Z would yield 20X FLA) although voltage regulation is not required much beyond about 6X FLA for a few seconds.

Edited: overload value