Simple measurement of the number of turns in a transformer.
RADIO Miesięcznik dla Techników i Amatorów, Rok IV, Styczeń-Luty 1949r., Nr 1/2
(RADIO Monthly magazine for Technicians and Amateurs, Year IV, January-February 1949, No. 1/2)
(Trioda website is not responsible for the content of the article)

  We often have difficulties with determining the number of turns in a transformer. In many cases, unwinding the transformer and recalculating the turns in this way is pointless, especially when we want to use one of the factory windings in an undamaged transformer, and the other, based on the calculation, to be wound up.

Fig. 1.

  Fig. 1 shows a system with which we can easily determine the number of turns in a transformer winding, without the need to unwind it with sufficient accuracy for practice.

  On the core of the transformer, one winding of which we want to examine, we wind one coil of thick (about 1mm) insulated wire. This one turn is connected through an adjustable resistor Rr and through an ammeter [A] for alternating current, with the heating winding of some other transformer Tr1 (eg with a voltage of 4V), which we will use as the current source in our measurement. The winding of the transformer Tr2, on which the measurement is performed, is connected through the switch [W] with a sensitive milliammeter [mA] for alternating current. First, leave the tested winding circuit open (the switch [W] is open).

Connect the transformer Tr1 (supplying) to the network and set any current, eg 2A, on the ammeter [A] using the regulated resistor Rr. Now, when we close the circuit of the tested winding with the [W] switch, a short-circuit current of the value of, for example, 0.002A will flow through the milliammeter [mA]. Because there is such a relationship between the currents and the number of turns in the windings that:

n1/n2 = I2/I1

so since n1 = 1, we get:

 n2 = I1/I2 = I1/0,002 = 1000 turns

where I1 is the current set on the ammeter [A] using the Rr resistor, and I2 is the short-circuit current that will flow after the winding tested with the circuit breaker [W] is short-circuited through the milliammeter.

  The accuracy of the measurement - as you can immediately see - depends primarily on the accuracy of the short-circuit current measurement, ie the accuracy of the milliammeter [mA].

  In the number of turns obtained in this way, correction should be made for winding resistance and iron losses, etc., which affect the accuracy of the method. A more precise number is obtained by adding about 10% of the turns to the result.

Jerzy Gaisler