The Voltech Handbook of Transformer Testing

An ideal transformer, with the secondaries open-circuit, presents an infinite impedance to an AC voltage applied to the primary, the transformer acts as though it were an infinite inductor.

In practice the transformer presents a finite inductive impedance to the applied voltage given by: -

Inductive impedance (X_L ) = 2πfL (ohms)

Where L is the inductance of the core (Henries) and f is the frequency of the applied voltage

The primary inductance is therefore a measure of the input impedance of the transformer. From this equation it can be seen that the smaller the inductance, the larger will be the current that will flow when the transformer is energized.

To measure inductance, the tester applies an ac voltage across the selected winding; it then measures the voltage across and the current through the winding using harmonic analysis. The measured voltage is divided by the current to obtain a complex impedance and the inductance is calculated.

The test signal can have a frequency in the range 20 Hz to 3 MHz, and an amplitude from 1 mV to 5 V.

Generally, it is not necessary to measure the inductance at the normal operating conditions of the transformer, which could involve, for example, voltage levels of hundreds of volts.

This is because the B-H curve can normally be assumed to be linear in the operating region, and the inductance measured at a low level represents the inductance that will appear in use.

Also, it can usually be assumed that the inductance value does not vary significantly with frequency.

Therefore, although high frequencies are available with the tester, measurement frequencies above a few hundred kilohertz should be used with caution.

This is because the errors caused by the stray inductance and capacitance of your fixture may become much more significant at these frequencies. Compensation can be used to eliminate these errors.

The following table suggests suitable test conditions for different values of expected primary inductance:

a) Current levels

The upper voltage limits should be chosen to give a maximum current level of about 50mA rms. for the lowest inductance expected. In some cases, this current may cause core saturation, and a lower voltage should be used. The minimum voltage level must be chosen so that the test current does not become so low that it cannot be sensibly measured. The lower voltage limits in the table above always give test currents higher than 3uA rms.

b) Self-Resonant Frequency

At lower frequencies, the capacitance of the windings can normally be ignored because its impedance is much higher than that of the inductance. However, at very high frequencies, this is not so, the capacitance dominates and inductance cannot be measured. The self-resonant frequency of the transformer is the change-over point between these two regions. Normally to get a good measurement of inductance, the test frequency should be less than 20% of the resonant frequency of the transformer. In general high values of inductance will have a high inter-turn capacitance and hence a low resonant frequency. Where there is a choice of test frequencies always use the lower value, to minimise any problems due to self-resonance.

c) Non-linear inductance

Normally inductance measurements should be used for transformers where the B-H characteristics are linear.

However, if inductance measurements are attempted for instance with line frequency transformers where the core material is non-linear even at low signal levels, the measured results can be highly dependent on the applied test signal.

This can be a problem when trying to compare measurements made on commercially available impedance bridges, or component testers, with measurements made using the AT. The test signal in such bridges is usually determined within the instrument, and is often at a fixed frequency and at a voltage level, which is not guaranteed to be constant for all value of inductance.

Usually, if the actual test conditions of the bridge can be determined, and the tester is then programmed to deliver the same test conditions across the inductance the results will then agree. (See also the comments below on differences caused by the choice of equivalent circuit)

d) Equivalent circuit

Inductance is always measured as part of a complex impedance, the result being expressed in terms of either a series or parallel equivalent circuit. Note that, for any given winding, the inductance values for the two circuits are not necessarily the same. This should be born in mind when specifying the test limits.