Transformer Turns Ratio
Test Sets (TTR) are portable instruments that accurately measure the
Transformer Turns Ratio of power, distribution, current, and other instrument
transformers. The Transformer Turns Ratio is the ratio of the number of turns
in the high-voltage winding to that in the low-voltage winding in a
Transformer. Transformer Turns Ratio Test Sets can also measure polarity, phase
shift, and excitation current of the transformers. Correctly using this test can identify shorted or open windings,
incorrect winding connections and other faults inside transformers.
The TTR Test Set operates
on the principle that the voltage ratio of an unloaded transformer is almost
exactly equal to the true turns ratio. The TTR Test Set applies voltage to the
high voltage winding of a non-energized transformer and accurately measures the
resulting voltage from the low voltage winding. High impedance input
and output voltage monitoring circuits optimize the transformer turns ratio
testing for the highest accuracy.
The major source of error
is the primary impedance drop due to the magnetizing current in the transformer
which can be kept to a minimum by energizing the transformer at a lower voltage.
Although a lower excitation voltage is preferred, voltages over 100VAC can
be used to test a wide variety of transformers in a number of industrial
TTR Test Set Functional Test
Between yearly calibrations of the TTR Test Set, a
functional test can be performed in the field to ensure that the Test Set is
giving the proper results.
This simple check consists of connecting the H and X test
leads from the TTR Test Set and running a typical test. The displayed turns
ratio reading should be exactly 1:1 as it is only measuring the ratio of the
single lead that is connected.
When performing the functional test, the H and X test
connection will cause an impedance mismatch between the input and output
circuits and the indicated turns ratio reading may not be exactly 1:1. A
function test reading may even exceed the manufacturer’s specified accuracy for
the Test Set.
The function test results should not be interpreted as a
calibration check. The function test merely shows that the Test Set is functioning
correctly. If the Test Set does not indicate a ratio of approximately 1:1, a
calibration should be performed on the unit.
TTR Test Calibration
It is recommended that TTR
Test Sets be calibrated at least once every year. The TTR Test Sets require
calibration due to inaccuracies caused by electric component drift, loose
connections, and mechanical wear. Regular calibration will ensure that these Test
Sets are functioning properly and calibrated over the entire measurement range.
of a TTR Test Set is performed by comparison with a Turns Ratio Calibration Standard. A Turns
Ratio Calibration Standard contains a set of rotary switches that offer a range
of selectable turns ratios and a multiplier dial to provide higher turns ratio
For the highest accuracy, most Turns Ratio Calibration Standards use a toroidal transformer core to ensure a very low excitation voltage in order to achieve a high turns ratio accuracy. The accuracy of the standard will allow the verification of the TTR Test Set to a greater than ±0.1% accuracy for most ranges.