2.11 TRL - Turns Ratio by Inductance

The AT offers two basic alternative ways to confirm that the transformer has been assembled properly, with the appropriate number of primary and secondary turns.

Turns ratio is the preferred test for signal, pulse and switched mode power transformers, where the normal operating conditions require only small excursions of the B-H curve, never extending beyond the linear regions. Where the magnetic coupling between the primary and secondary is poor, it is preferable to measure the turns ratio by inductance.
This test measures the inductance of both the primary and secondary and calculates the turns ratio from these measured values.

(For line frequency transformers, designed to operate over the full extent of the B-H curve, including the non-linear regions, the preferred method is to use an open - circuit voltage test to check for the correct number of turns on each winding.)

Clearly a turns ratio test cannot tell you the actual number of turns on a winding, only the ratio between one winding and the next.
You should therefore have at least one inductance test in your program to give confidence that the absolute number of turns is correct as well as the ratio.

Measurement Conditions

The inductance of a winding can often depend upon the flux density in the core/windings.
Since during measurement, the flux density will depend upon the signal energising the winding, it is important that both windings are energized at the same level.
This will ensure that both inductances are measured along the same region of the B/H curve of the core, to give a true ratio.

Setting the Test Parameters

The simplest method of setting the test parameters is to use the "Measure" button, to do this you have to program the test from a computer which is connected to the Auxiliary port of the tester.
There are two other methods of inputting the test parameters, one is to set the primary voltage and frequency and let the editor set the secondary voltage, and the second is to set both voltages manually.

Using the Measure Button to Set Test Parameters

To do this you must be programming the test from a computer that is connected to the testers auxiliary port.
Select the integration period you require, enter the primary and secondary terminals, then click on the measure button.
The editor will then enter the test signal and show the measured turns ratio. Set percentage limits on this ratio and click "OK" (you may select a polarity test before clicking "OK").

Using the Auto Button to Set Secondary Voltage

To do this you must know the primary inductance.
Select the test voltage and frequency for the primary from the table below and enter them in the TRL dialogue box.
Enter the turns ratio, then press the "Auto" button next to the secondary voltage parameter, the tester will automatically select the appropriate test voltage for the secondary winding when the program is running.

Setting the Primary and Secondary Parameters Manually

To do this you will need to know the inductance of the primary and secondary windings.

The optimum test conditions are chosen for an inductance value that is between the primary and secondary (L_m).

Look up the recommended test signal for this inductance.

Enter the recommended frequency for this inductance as the test frequency.

The primary and secondary voltages can be calculated from the following:

Where:
L_m = Intermediate inductance
L_p = Primary inductance
L_s = Secondary inductance
V_p = Primary voltage
V_s = Secondary voltage
V_m = Intermediate voltage
N_p = Primary turns
N_s = Secondary turns

If you calculate V_s or V_p to be greater than 5V, you should set 5V as your test signal. If you calculate V_s or V_p to be less than 1mV, you should set 1mV as your test signal.

Intermediate Inductance (L_m)	Preferred test signal
Intermediate Inductance (L_m)	Frequency (V_m)	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

The Test Conditions for Turns Ratio by Inductance Measurement Specifying the Test Limits

When specifying turns ratio tests, it is preferable to avoid limits which are unnecessarily tight, and which may therefore lead to measurement difficulties.

For example, if two equal secondary windings should have 10 turns each, the ratio should be 1:1.
One turn in error would produce a ratio error of 10% or - 10% (i.e. 11:10 or 10:11), and therefore limits of +5% and -5% would be suitable to detect the error.

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