Short circuit indicator. Encyclopedia of radio electronics and electrical engineering

Encyclopedia of radio electronics and electrical engineering / Measuring technology

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During the manufacture of inductors, mechanical damage to the insulation of the winding wire sometimes occurs. This usually results in the formation of shorted turns.

The presence of an interturn short circuit can be determined using the proposed device (see Fig. 1).

Fig. 1

The main part of the device is a series resonant circuit consisting of coils L1-L2 and capacitor C1. The circuit is set to a frequency of 100 Hz and is powered by the alternating voltage component of a full-wave rectifier (D4, D5). Coils L1-L2 are located in the lower part of a two-rod open magnetic circuit with different cross-sectional areas of the rods - one 12x14 mm, the other 25x35 mm. The upper part of both cores of the magnetic core serves for putting on the tested coils L_x (with different hole diameters).

In order for the sensitivity of the circuit to be the same when exposed to short-circuited turns on any of the rods, a coil with a smaller number of turns (L1) is located on a rod of a larger cross-section.

The operation of the device is based on reducing the quality factor of the resonant circuit when introducing losses into it, created by short-circuited turns of the coils being tested.

When losses are introduced into the circuit, its quality factor decreases and, accordingly, the voltage U drops_K. Table 1 shows the effect of short-circuited turns on the quality factor of the circuit (at U_п=30 V).

Table 1

The device works like this. The voltage that is removed from the coils L1-L2 is rectified by the selenium column D2 and supplied to arm R3 of the comparison circuit. It is proportional to the network voltage and depends on the quality factor of the L1L2C1 circuit. The reference voltage removed from winding II is applied to the other arm (R1-R2) of the comparison circuit

As a result, the top sector L1 is narrowing. Zener diode D3 limits the voltage on the control grid L1 with a large unbalance of the comparison circuit.

Coil L1 has 3000, and L2 - 4600 turns of PE 0,31 wire. To reduce their own capacitance, L1 and L2 are wound in bulk on frames with four partitions (it is advisable to make several taps to tune the circuit to resonance). The magnetic core (Fig. 2) is assembled from transformer steel plates.

Fig. 2

Transformer Tp1 is made on a Ш25x35 core. Its winding data is given in table. 2.

Table 2

All fixed resistors used in the device (except R6) are wire resistors, grade PEV-10) type BC. Potentiometer R1 - VK-a-0,5-A. Capacitors C1 (made up of two 0,1 μF), C2-C4 - MBGC-2, designed for a voltage of 600 V.

If the diameter of the winding wire is more than 4 mm, as well as if there is an electrostatic screen in the form of a metal tape between the windings of the transformers being tested, the test is impossible. In this case, the transformers (or coils) will be rejected due to false readings from the device. It occurs as a result of significant distortion of the magnetic field of the sensor, which leads to detuning of the resonant circuit.

Setting up the device comes down to setting the L1L2C1 circuit to resonance, which is done by selecting the capacitance of capacitor C1. The electronic light indicator is adjusted by setting the variable resistor R1 to the largest shadow sector in the absence of short-circuited turns.

Author: I.Vasiliev

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