# Why does the ohmic value measure differently?

Status
Not open for further replies.

#### bunalmis

##### Full Member level 5
I measure the inductance and ohmic value of the secondary winding of a transformer at 40Hz and 400Hz with an LCR meter.

Why does the ohmic value measure differently?

Z=0.1948 Ohm Theta 85.85 Degree F=40Hz L=777.24uH R=0.0145 Ohm
Z=1.8551 Ohm Theta 83.87 Degrees F=400Hz L=734.21uH R=0.1980 Ohm

[moderator action: removed link to external file server]

Last edited by a moderator:

LCR meter can either measure Rdc or Rac. Rac, the real part of complex impedance is of course frequency dependent, representing e.g. core losses and winding skin effect.

### bunalmis

Points: 2
Hi,

Ohmic resistance means "loss" (usually dissipated as heat).
* the DC resistance if a transformer means loss accirding P= I(RMS)^2 × R
* but there is ladditional loss in the core when the magntic field changes
* there may be other loss, like proximity effects
All loss is combined in the R you see.

Klaus

### bunalmis

Points: 2
Yes you are right. I tested that the ohmic value of a coil with an air core value of 1mH remained almost constant in the range of 40Hz ..10Khz.

I measure the inductance and ohmic value of the secondary winding of a transformer at 40Hz and 400Hz with an LCR meter.

Why does the ohmic value measure differently?

Z=0.1948 Ohm Theta 85.85 Degree F=40Hz L=777.24uH R=0.0145 Ohm
Z=1.8551 Ohm Theta 83.87 Degrees F=400Hz L=734.21uH R=0.1980 Ohm

[moderator action: removed link to external file server]
So we assumed that the R value corresponds to zero frequency and should stay constant, right?

Wrong!

What we are actually measuring is a complex impedance. The Ohmic value is the real part of the complex number. But important point is that the real part is no more frequency independent. It is the lossy part and it is notionally called the ohmic values of the impedance.

It is not equivalent to the DC value of the impedance (Z at f=0). But that depends on the circuit.

Lossy circuits are tough to model using traditional L,C,R with frequency independent values.

For a lossy component (transformer coil), the L value is a function of the frequency (because of iron and copper losses). It is not possible to model this with a number of fixed valued components.

Just take a decent air core inductor and measure the L values at the same frequencies; they should be within the limits of the measurement of the meter.

But your readings are good and acceptable for a HF transformer. I would have asked why the L values have drifted?

Ohmic values measure the electrical potential between two terminals in a circuit. Ohm's law states that this potential is directly proportional to the current flowing through the circuit.
The most common application of Ohm's law is with resistors and capacitors, components used in electronic circuits. When you use a resistor, for example, it changes the amount of current flowing through a Circuit by reducing its ohmic value. When you use a capacitor, it increases its ohmic value by storing energy from an electric field.

Status
Not open for further replies.