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
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 inductance of a winding can often depend upon the flux density in the
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
The optimum test conditions are chosen for an inductance value that is
between the primary and secondary (Lm).
Look up the recommended test signal for this inductance.
Enter the recommended frequency for this inductance as the test
The primary and secondary voltages can be calculated from the following:
Lm = Intermediate inductance
Lp = Primary inductance
Ls = Secondary inductance
Vp = Primary voltage
Vs = Secondary voltage
Vm = Intermediate voltage
Np = Primary turns
Ns = Secondary turns
If you calculate Vs or Vp to be greater than 5V,
you should set 5V as your test signal. If you calculate Vs or Vp
to be less than 1mV, you should set 1mV as your test signal.
|Intermediate Inductance (Lm)
||Preferred test signal
| 100nH → 1uH
1uH → 10uH
10uH → 100uH
100uH → 1mH
1mH → 10mH
10mH → 100mH
100mH → 1H
1H → 10H
10H → 100H
100H → 1KH
1KH → 10KH
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
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