The inductance match test calculates the ratio between two inductances on two windings. An equivalent series inductance measurement is performed on each winding by measuring the complex impedance.

This test is suitable for switched mode power supply transformers, and audio & telecommunication transformers. It checks matching between windings.

Measurement Conditions

When calculating inductance match the tester performs two inductance measurements.

Firstly the unit applies an ac voltage across the first 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.

This is then repeated for the second winding.

The inductance match is the ratio of first to second winding inductance.

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

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 average inductance: -

Average Inductance (Geometric Mean)	Preferred test signal
Average Inductance (Geometric Mean)	Frequency	Voltage
100nH → 1uH 1uH → 10uH 10uH → 100uH 100uH → 1mH 1mH → 10mH 10mH → 100mH 100mH → 1H 1H → 10H 10H → 100H 100H → 1KH 1kH → 10KH	300KHz 100KHz 30KHz 10KHz 1KHz 100Hz 100Hz 50Hz 50Hz 50Hz 20Hz	10mV 30mV 50mV 100mV 100mV 100mV 300mV 1V 5V 5V 5V

Test Conditions for Inductance Match Measurement

Wherever possible, this table should be used for all inductance tests. The inductance range should be chosen based on minimum value of inductance expected.

When choosing the test conditions, the following potential problems should be considered: -

a) Current levels

The upper voltage limits should be chosen to give a maximum current level of about 100mArms. 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 dependant 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 Series testers. 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 two circuits are not necessarily the same; this should be born in mind when specifying the test limits.

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