Principle, Characteristics and Application Design of Coil Turn to Turn Short Circuit Tester

In view of the above situation, we have developed a simple and practical coil turn to turn short circuit tester. This tester has the following characteristics:

1. High measurement accuracy

Experiments show that for coils with more than 30 turns of iron core, the tester can detect this fault as long as there is a short circuit between any two turns.

2. It can give sound and light alarm at the same time.

3. Simple, practical and low production cost.

This design detects whether the coil has inter turn short circuit by sensing the oscillator. As shown in Figure 1, when the measured coil has no inter turn short circuit, sense the oscillator to vibrate and have a sine wave output, and then couple and output the sine signal to the normal indication circuit through the coupling circuit. If there is a short circuit between two or more turns in the coil, the short circuit coil will form a closed loop and generate high damping in the magnetic circuit to stop the oscillator, The alarm circuit will give sound and light alarm immediately.

1. Oscillator

As shown in Figure 2, this circuit detects the quality of the measured coil by sensing the start and stop of the oscillator. When the coil is not connected, the oscillator is a venturi bridge oscillation circuit composed of operational amplifier A and rw1, RW2, C3, C2, R3 and R4. By adjusting the coaxial potentiometer rw1 = RW2 = R, the current oscillation frequency f = 1 / 2 RC, about 5.5KHz; When the coil to be tested is connected and there is no fault (at the same time, the capacitor C1 is involved), this circuit becomes a fusion circuit of LC oscillator and Venturi bridge oscillator, in which the LC oscillation circuit is the main one, and the current oscillation frequency is determined by the inductance and capacitor C2 of the coil to be tested. Because the value of capacitor C1 is large and the L inductance is small, the oscillation frequency is deduced as (sinusoidal signal is output at out point). When there is a short circuit between turns in the measured coil, it can be seen from the oscillation circuit and magnetic circuit theory that the coil inductance will drop, the Q value will decrease, and the coil will work in the state of low impedance and high damping, forcing the sensing oscillation circuit to stop vibration (there is no sine wave output at the out point). The operational amplifier C in the circuit constitutes a voltage follower to improve the load capacity of the oscillator. Op amp B, R3, R4, R5, RW3, C4 and D1 form a proportional amplification and rectification filter circuit to make Junction FET Q work in the variable resistance region, so as to realize sine wave amplitude stabilization of oscillator output.

2. Coupling indicating circuit

As shown in Fig. 3, the capacitor C5 and the input resistance of the subsequent amplification circuit form a resistance capacitance coupling circuit. The characteristic of the resistance capacitance coupling circuit is that the static working points at all levels are independent of each other, and the front and rear circuits do not affect each other. In this circuit, if there is no fault in the measured coil, in1 (connected to the out point of Fig. 2) has a sinusoidal signal input, and the capacitor C5 couples this signal to the subsequent rectifier and amplifier circuit to turn on the triode Q1 and Q2, drive the green LED L1 to light up and indicate the normal operation of the coil; If there is a short circuit between the turns of the measured coil, the oscillator stops vibrating, in1 has no signal input, Q1 and Q2 are cut off, so that L1 is extinguished.

3. Alarm circuit

As shown in Fig. 4, the chip 555 and R14, R15, rw4 and C8 constitute a square wave generator. When there is no inter turn short circuit in the measured coil, in2 (connected to the out point in Figure 2) has a sinusoidal signal input. After the rectification and filtering of D4 and C7, Q3 is saturated and turned on, and Q4 is cut off. Therefore, the 12V power supply cannot supply 555, 555 does not work, that is, there is no alarm signal output; If there is an inter turn short circuit in the measured coil, in2 has no sinusoidal signal input, so that Q3 is cut off and Q4 is turned on, so that the 12V power supply supplies 555 through Q4. 555 works, and the continuous square wave signal is output from the 3 pins of 555 to drive the red LED L2 to emit light for light alarm, and drive the speaker for sound alarm.

The working principle of the tester is summarized from long-time work, and its feasibility has been verified through a large number of experiments. It is suitable for all kinds of iron core coils. As long as there is an inter turn short circuit in the coil, the tester can detect this fault. The circuit is simple and the production cost is low. If this tester is widely used, it will bring great convenience to industrial production and equipment maintenance.