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Power Transformer Tests

A power transformer is tested to discover whether the transformer will perform as required, or whether it will give reliable service life. Some tests perform both functions.

(a)   D-C Resistance. This test is usually made on transformers at the factory as a check on the correctness of wire size in each winding. Variations are caused by wire tolerances, and by difference in winding tension between two lots of coils or between two coil machine operators. About 10 per cent variation can be expected in the d-c resistance of most coils, but this value increases to 20 per cent rather suddenly in sizes smaller than No. 40. The test is made by means of a resistance bridge or specially calibrated meter.

(b)   Turns Ratio. Once the correct number of turns in each winding is established, correct output voltage can be assured for a coil of given design by measuring the turns. A simple way of doing this is by use of the turns ratio bridge in Fig. 74.

Fig. 74. Turns-ratio bridge.

If the turns are correct, the null indicated by the meter occurs at a ratio of resistances


If there is an error in the number of turns of one winding, the null occurs at the wrong value R1/R2. A source of 1,000 cycles is preferable to one of 60 cycles for this test. The smaller current drawn by the transformer reduces IR and IX errors. Harmonics in the source obscure the null, and so the source should be filtered. The null is often made sharper by switching a small variable resistor in series with R1 or R2 to offset any lack of proportion in resistances of windings N1 or N2.

An accuracy of 0.1 per cent can usually be attained with four-decade resistances. Polarity of winding is also checked by this test, because the bridge will not balance if one winding is reversed.

(c) Open-Circuit Inductance (OCL). There are several ways of measuring inductance. If the Q (or ratio of coil reactance to a-c resistance) is high, the check may be made by measuring the current drawn by an appropriate winding connected across a source of known voltage and frequency. This method is limited to those cases where the amount of current drawn can be measured. A more general method makes use of an inductance bridge, of which one form is shown in Fig. 75.

Fig. 75. Modified Hay bridge for measuring inductance.

If direct current normally flows in the winding, it can be applied through a large choke as shown. Inductance is then measured under the conditions of use. Source voltage should be adjustable for the same reason and should be filtered to produce a sharp null. Rc is provided to compensate for coil a-c resistance. Without it an accurate measurement is rarely attained. Enough data are provided by the test to calculate a-c resistance as well as inductance.

When Q is low, as it is in coils with high resistance, better accuracy is obtained with the Maxwell bridge, which is like the Hay bridge except that Xc and Rc are paralleled. Then the equations for bridge balance become


The Maxwell bridge has the further advantage that the null is independent of the source frequency.

(d) Temperature Rise. Tests to determine whether a transformer overheats are made by measuring the winding resistances before and after a heat run, during which the transformer is loaded up to its rating. Where several secondaries are involved, each should deliver rated voltage and current. Power is applied long enough to allow the transformer temperature to become stable; this is indicated by thermometer readings of core or case temperature taken every half hour until successive readings are the same. Ambient temperature at a nearby location should also be measured throughout the test. The average increase in winding resistance furnishes an indication of the average winding temperature. Figure 76 furnishes a convenient means for finding this temperature.

Fig. 76. copper resistance versus temperature in terms of resistance at 25°C.

(e) Regulation. It is possible to measure voltage regulation by connecting a voltmeter across the output winding and reading the voltage with load off and on. This method is not accurate because the regulation is usually the difference between two relatively large quantities. Better accuracy can be obtained by multiplying the rated winding currents by the measured winding resistances and using equation 13. If the winding reactance drop is small this equation works well for resistive loads. To measure winding reactance drop, a short-circuit test is used. With the secondary short-circuited, sufficient voltage is applied to the primary to cause rated primary current to flow. The quotient E/I is the vector sum of winding resistances and reactances. Reactance is found from


where R includes the resistance of both windings and the meter.

Sometimes it is more convenient to measure the leakage inductance with secondary short-circuited on a bridge and multiply by 2πf.

(f) Output Voltage. Although the method described under (e) above is accurate for two-winding transformers, it is not applicable to multi-secondary transformers unless they are tested first with newly calibrated meters to see that all windings deliver proper voltage at full load. Once this is established, values of winding resistance and reactance thereafter can be checked to control the voltage. The interdependence of secondary voltages when there is a common primary winding makes such an initial test desirable. This is particularly true in combined filament and plate transformers, for which the best test is the actual rectifier circuit.

(g) Losses. Often it is possible to reduce the number of time-consuming heat runs by measuring losses. The copper loss is readily calculated by multiplying the measured values of winding resistance (corrected for operating temperature) by the squares of the respective rated currents. Core loss is measured with open secondary by means of a low-reading wattmeter at rated voltage in the primary circuit. If these losses correspond to the allowable temperature rise, the transformer is safely rated.

(h) Insulation. There is no test to which a transformer is subjected which has such a shaky theoretical basis as the insulation test. Yet it is the one test it must pass to be any good. Large quantities of transformers can be built with little or no insulation trouble, but the empirical nature of standard test voltages does not assure insulation adequacy. It has been found over a period of years that, if insulation withstands the standard rule of twice normal voltage plus 1,000 volts rms at 60 cycles for 1 minute, reasonable insulation life is usually obtained. It is possible for a transformer to be extremely under-insulated and still pass this test (see Dielectric Strength); conversely, there are conditions under which the rule would be a handicap. Therefore it can only be considered as a rough guide.

The manner of making insulation tests depends upon the transformer. Low-voltage windings categorically can be tested by short-circuiting the terminals and applying the test voltage from each winding to core or case with other windings grounded. Filament transformers with secondaries insulated for high voltage may be tested in similar manner. But a high-voltage plate transformer with grounded center tap requires unnecessary insulation if it is tested by this method. Instead, a nominal voltage of, say, 1,500 volts is applied between the whole winding and ground; after that the center tap is grounded and a voltage is applied across the primary of such value as to test the end terminals at twice normal plus 1,000 volts. Similar test values can be calculated for windings operating at d-c voltages other than zero. Such a test is called an induced voltage test. It is performed at higher than normal frequency to avoid saturation. An advantage of induced voltage testing is that it tests the layer insulation.

If insulation tests are repeated one or more times they may destroy the insulation, because insulation breakdown values decrease with time. Successive applications of test voltage are usually made at either decreased voltage or decreased time. In view of their dubious value, repeated insulation tests are best omitted.

Corona tests are not open to this objection. A voltage 5 per cent higher than normal is applied to the winding, and the leads are run through blocking capacitors to the input of a sensitive radio receiver as in Fig. 38.(1) RETMA standard noise values for this test are based primarily on radio reception, but they do indicate whether standard insulation practice is followed. See Table X.

Table X. Corona Voltage

RMS Working Voltage
Corona Level
Up to 8.61,000
8.61 to 152,500

Transformers which are subjected to voltage surges may be given impulse tests to determine whether the insulation will withstand the surges. Power line surges are the most difficult to insulate for. The electric power industry has standardized on certain impulse voltage magnitudes and wave shapes for this testing.(2) The ratio of impulse voltage magnitude to 60-cycle, 1-minute insulation test voltage is called the impulse ratio. This ratio is much greater for oil-insulated transformers than for dry-type transformers, and is discussed further in Chapter 4.

(1) See RETMA Standard TR-102-B, "Power Transformers for Radio Transmitters.
(2) See ASA Standard C57.22-1948, paragraph 22.116.

Last Update: 2011-02-17