Indicator of short-circuit turns in coils with ferromagnetic magnetic circuits. Encyclopedia of radio electronics and electrical engineering

Encyclopedia of radio electronics and electrical engineering / Measuring technology

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The proposed indicator was developed to check for the presence of short-circuited (short-circuit) turns of the windings of various electrical devices - transformers, DC and AC machines, magnetic amplifiers, etc. To reduce material costs, their magnetic circuits are often made of soft magnetic materials with relatively large specific losses. For this reason, it is often impossible to obtain reliable information about the presence of short-circuit turns in the traditional way - by disrupting the oscillations of a low-power generator [1-3], which is possible not only due to the presence of short-circuit turns, but also due to hysteresis and eddy current losses. in the magnetic circuit.

The principle of operation of the proposed device is based on registering the response of the shock excitation circuit formed by the built-in capacitor and the coil under test to a voltage pulse: if there are no short-circuited turns, then when a charged capacitor is connected to it, damped oscillations occur in the circuit, and if there are such turns, aperiodic ones.

Rice. 1 (click to enlarge)

The indicator scheme is shown in fig. 1. It contains a capacitor C2, which, together with the tested coil L_x forms a circuit of shock excitation; a switch on the assembly of field-effect transistors VT1, the operation of which is controlled by the button SB1; RS-trigger on the elements of the DD1 microcircuit, which serves to suppress the bounce of the button contacts, the pulse shaper on the VT2 field-effect transistor and the binary counter on the DD2 microcircuit. LED HL1 indicates the state of the counter "two or more".

The device works as follows. After turning on the power, the output of the RS flip-flop (pin 4 of the DD1.2 element) is set to the log level. Oh, so the transistor VT1.1 is open, and VT1.2 is closed. Through an open transistor VT1.1, capacitor C2 is charged to the voltage of the power source. Since it is greater than the threshold voltage of the transistor VT2, the latter opens by connecting the input of the CP counter DD2.1 with a common wire. The counter triggers are set to an arbitrary state when the power is turned on.

To test the inductor L_xconnected to terminals X1 and X2, press and hold the SB1 button in this state. In this case, the RS-flip-flop changes its state - at the output (pin 4) of the DD1.2 element, a log level appears. 1. At the moment of switching the RS-flip-flop, a short pulse appears at the output of the element DD1.3 (pin 11), resetting the counters DD2.1 and DD2.2. The transistor VT 1.1 closes at a high level at the gate, disconnecting the charged capacitor C2 from the power source, and VT1.2 opens, connecting the coil under test in parallel. In the absence of short-circuited turns in the circuit L_xC2 damped harmonic oscillations occur with a frequency depending on the capacitance and inductance of its elements. When the capacitor C2 is recharged, the transistor VT2 periodically opens, generating pulses that are fed to the input of the counter DD2.1. As soon as the voltage amplitude in the circuit becomes less than the threshold voltage of the transistor VT2, the flow of pulses to the counter input stops and at least one of the counter outputs is set to log 1, so the HL1 LED lights up, indicating that the coil under test is working. When the button is released, the device returns to its original state. The counter is again reset by a reset pulse from the output of the element DD1.3.

If there are short-circuited turns in the coil, only one pulse enters the counter input, and since output 1 (pin 3) of the DD2.1 counter is not connected to the OR element on the VD1-VD5 diodes, the HL1 LED does not respond to it. Circuit R3VD1-VD4 protects the gate of the transistor VT2 from static electricity.

There are no special requirements for most parts of the probe: resistors and capacitors can be of any type, diodes - any low-power silicon, LED HL1 - any, preferably with increased brightness. The main requirement for the transistor VT2 is a low threshold voltage. For transistors of the KP504 series, it does not go beyond 0,6 ... 1,2 V, so you can use a transistor with any letter index. You can use the KP505G transistor (it has a threshold voltage of 0,4 ... 0,8 V).

Fig. 2

The device is assembled on a fragment of a universal breadboard with dimensions of 50x30 mm. To facilitate the installation of the transistor assembly VT1 (it is available in the SO-8 package with a lead pitch of 1,27 mm), a riser board was made. To do this, a fragment was cut out of a prototyping board for microcircuits with planar leads (Fig. 2), designed for mounting four leads with a pitch of 1,27 mm. A cut is made in the foil of the wide printed conductor on the opposite side of the fragment to create a gap between the leads 5, 6 and 7, 8 of the assembly. Adapter board leads - pieces of tinned copper wire with a diameter of 0,7 mm are soldered to the resulting pads for pins 5-8 and soldered into round pads that terminate the printed conductors for pins 1-4. By bending the pins of the riser board at the right angle, it can be mounted either parallel to the main board or perpendicular to it. Unused inputs of the DD1 chip (pins 8, 9) should be connected either to the positive power line or to a common wire.

The assembled device, together with a battery, made up of four AAA size cells connected in series, is placed in a case, which can be conveniently used as a plastic soap dish. The position of the board in the case is fixed with pieces of foam rubber, and the halves of the case are fastened one to the other with miniature self-tapping screws. The device does not require adjustment.

As the test showed, the indicator confidently determines the presence of short-circuit turns in transformers with a power of several watts (transformer from a network adapter) to several kilowatts (welding transformer), moreover, when connected to both the primary and secondary windings (the short-circuit loop was created artificially, by closing a piece of the mounting wire passed through the window of the magnetic circuit). In devices with a branched magnetic circuit (three-phase transformers, magnetic amplifiers, etc.), it is necessary to check the windings on each rod. In AC machines, due to the different spatial orientation of the windings, the test should also be carried out by winding.

In most cases, electric motors with a squirrel-cage rotor can be checked without disassembly - apparently, the air gap between the rotor and the stator creates sufficient magnetic resistance, which weakens the effect of short-circuited turns of the rotor (the need for disassembly arose only in those cases when the device showed the presence of short-circuit turns in all windings). Engines of very different design and power were tested - from low-power single-phase (EDG of various modifications, KD-3,5) to three-phase imported with a power of 3,5 kW (from a woodworking machine). Collector motors must be checked at different armature positions.

Literature

1. Krivonos A. Determination of short-circuited turns in the windings of transformers and chokes. - Radio, 1968, No. 4, p. 56.
2. Dmitriev V. A device for determining interturn short circuits. - Radio, 1969, No. 2, p. 26.
3. Pozdnikov I. Probe for testing inductors. - Radio, 1990, No. 7, p. 68, 69.

Author: K. Moroz

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