2.10 TR - Turns Ratio and Phasing

In practice the output voltage of an actual transformer will be slightly less than this due to the parasitic elements described in the equivalent circuit.

Due to those elements the ratio measured is usually the 'voltage ratio, not the actual 'turns ratio'.

Phasing

Applying an AC voltage to the primary of the transformer will produce an AC voltage on the secondary.

This secondary voltage may be in-phase with the primary voltage, or it may be in anti-phase depending on the direction of winding and the termination of the windings.

This phasing is represented by the 'dot' associated with each winding.

a) In Phase--------------------------------------------b) Anti-Phase

a) In Phase--------------------------------------------b) Anti-Phase

For most applications it is important to know that the windings are phased correctly as well as knowing the turns or voltage ratio.

Where Used

The AT offer 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. (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 numbers 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

To measure turns ratio, a test source voltage is applied to one winding, the energised winding, and the voltages across two other windings (one of which may also be the energised 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.

It is recommended that you choose the winding with the highest number of turns as the one to be energised. A possible exception to this rule is when you wish to measure the ratio between two windings, which should be accurately matched at 1:1. In this case it may be better to energise a third winding with a lower number of turns, to ensure that any measurement errors apply equally to the two windings under test.

You can specify the signal to be applied to the energised winding to have a frequency in the range 20Hz to 3MHz, and an amplitude from 1mV to 5V.

The recommended test conditions depend on the inductance of the energised winding; they are given in the table below assuming that the energised winding is the one with the highest number of turns:

Inductance of the Energised Winding	Preferred test signal
Inductance of the Energised Winding	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

The Test Conditions for Turns Ratio Measurement

Notes: The signal is applied to the primary winding, or the winding, which has the largest number of turns. However, if by doing this, the expected voltage on the winding with the smallest number of turns falls below 1mV, then the test voltage should be increased.

This may also require an increase in the test frequency so that the current taken by the driven winding does not become too large, but in general this frequency increase should be kept as small as possible to avoid problems caused by stray capacitance at high frequencies.

Where Matching in Groups is Important:

In some transformer designs, the turns ratio between a primary winding and a secondary winding is not as important as the ratio between different primaries or different secondaries.

To make the most accurate measurements in such cases apply the test signal to the primary winding and measure the turns-ratio from primary to one of the secondaries.

Then, leaving the primary energised as above, measure the turns ratio between the secondaries.

Next, energise a secondary winding (possibly at a different voltage and / or frequency depending on its inductance) and measure the turns ratio between the various primaries.

In this way windings which should be matched are treated equally during the test.

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.

Prev Next