To measure turns ratio, a test source voltage is applied to one winding, the energized winding, and the voltages across two other windings (one of which may also be the energized winding) are measured using harmonic analysis. The turns ratio is measured by dividing one measured voltage by the other, and making a compensation for the effects of winding resistance.

On selecting ‘TR Turns Ratio’ from the available tests window, a dialogue box will be displayed.

To program the test :

In the fields provided, insert the voltage and frequency you require for the test (see chapter 7 - ‘Test Conditions’).
Note also that the Measure button may be used here as outlined in section 3.9, with the combinations of test parameter specification shown in the following table:

Voltage Specified Auto Auto

Frequency Specified Specified Auto
Select (mouse click the button) the integration time you require.
'Medium' is the default setting. 'Long' will give more stable readings (for tighter limits) at the expense of an increased test time, and 'Short' will test at the maximum speed, but may give a slightly noisier reading.
Select the high and low terminals of the coil to be energized. (Click on the arrow to the right of the fill-in box and you will be provided with a choice of the available terminals. Click the required terminal.)
Normally, choose the energized winding to be the one with the highest number of turns. An exception to this rule can be made in the case when there are three separate windings, and you wish to check that two windings with the same number of turns are accurately 1:1; in this case it is preferable to energize the third winding, even if it has fewer turns.
Similarly, select the high and low terminals of the primary and secondary windings.
In the case of a two winding transformer, choose both the primary winding and the energized winding to be the one with the highest number of turns.
Select (mouse click the button) Ratio (%), or Turns (#) limits.
Enter the required limits:
Ratio (%)
Enter the nominal Primary to Secondary ratio, together with the limits as positive and negative percentage values.
Turns(#)
In turns mode you may set upper and lower limits on the primary and secondary windings. The transformer tester will then set limits on the turns ratio which are equal to the sum of these two errors, e.g.:

primary 100 turns ± 2 turns

secondary 100 turns ± 2 turns

the tester will set the following limits on the ratio:
upper limit = 104/100 lower limit = 96/100.
If you wish to set limits on only one winding, you must enter zero (0) as the limit for the other winding.
Note that the Measure button may be used here as outlined in section 3.9.
If required, the polarity of the test can be checked. Click on the expected polarity of the transformer turns, otherwise disable the test by selecting ‘No Test’.
Select OK. The test specification will be displayed in the program window on the screen.

Choosing the test limits

The limits may be entered as :

1. A nominal value together a percentage tolerance.(%)
Enter the nominal Primary to Secondary ratio, together with the limits as positive and negative percentage values.

2. An absolute number of turns(#)
When limits are set as +/- an absolute number of turns, the tester converts these limits to the % type in the following way:

(You can see the exact result of this conversion by clicking on % limits in the Editor dialogue once the absolute, # , limits have bee entered.)

For example, take a typical Switched Mode Power Supply transformer with windings in the ratio of 200:10.

Typically, a prior measurement of inductance will have confirmed the correct number of primary turns have been fitted with the correct cores. Setting absolute limits of 0 on the primary and 1 on the secondary will result in limits of;

Nominal Ratio: 200/10 =20
Ratio + 1 sec turn =18.18
Ratio –1 sec turn = 22.22

Limits calculated by tester = (0/200 + 1/10) x 100%= +/- 10%
The equivalent ratio limits used by tester are:
Upper limit = 22. Lower Limit = 18

Which equates to the actual ratios with a one turn error on the secondary.

In general, the voltages obtained during any voltage turns ratio test will not equate exactly to the theoretical calculation of Np / Ns, because, for example, of the relative positions of the windings with respect to each other and the core.

General Rule

When setting turns ratio limits in absolute terms, use a limit of ½ turn less than the number of turns you want to detect. In this case set a limit of 0 (pri) and ½ (sec). This will result in equivalent limits of 19 and 21. Ideal for detecting an errors of 1 turn or more.

What happens if I set both absolute limits for both primary and secondary windings?

If you want to detect up to a 10 turn error on the primary and 1 turn on the secondary in the above example, set 91/2 (pri) and 1/2 (sec).The tester uses the equivalent ratio limits: 21.95 and 18.05. Setting both limits WIDENS the limit to allow for an error of 10 Turn on the primary SIMULTANEOUSLY with 1 turn on the secondary.

i.e. to allow 190:11 (17.27) and 210:9 (23.33).

Notice that these limits will NOT detect just a 1 turn error on the secondary because you have now chosen limits to allow for errors on both primary and secondary, resulting in a broader tolerance on the absolute ratio.

How so I set limits that will detect a 1 turn error in either the primary or secondary winding?

To do this you must set limits that will detect the smallest possible error. This will be due to the 1 turn error being on the winding with the largest number of turns.

In the above example then, set absolute limits of ½ turn (pri) and 0 turn (sec). (Remember to use the n-1/2 rule).

The equivalent limits used by the tester will be:

20.05 and 19.95

This will certainly detect a 1 turn error in the primary, but because of the physical errors already described these limits may be too severe for a normal good part. This is why a combination of inductance and turns ratio tests are normally used to confirm the correct assembly of transformers.

The polarity test may be selected as (+)ve, (-)ve or no test. To measure the exact interwinding phase angle use the PHAS test.

Use the PC Editor’s ‘measure’ button (together with a sample part) if you are unsure of the nominal value.

Prev Next

Voltage	Specified	Auto	Auto
Frequency	Specified	Specified	Auto