Circuit Principle, Manufacturing and Debugging Instructions of Short Circuit Tester

Here to introduce the reader to the motor short-circuit tester. It can not only be used to accurately test the short-circuit fault of more than one turn of motor and generator winding. It can also be used to check whether the cast aluminum strip of squirrel cage rotor is broken. When rewinding the motor winding, it can be tested while offline. Avoid the failure to find out in time after the short circuit caused by improper operation.1 Introduction to working principle

The circuit of the motor short-circuit tester is shown in the attached figure, and the principle is not complex. In the figure, transistor Q1, transformer T1 and C1 constitute a transformer feedback oscillation circuit. An audio signal of about 2000Hz is generated and then coupled to transistor Q2 via C2 for amplification. The amplified audio signal is output from the Q2 collector. After the plug Z1 is inserted into the socket J1, the audio current will flow through the winding L1 of the detection transformer T2. When the core of the detection transformer contacts the core of the motor to be tested, the main magnetic circuit of the detection transformer forms a path through the core of the motor to be tested (because the core of the detection transformer is horseshoe "U") When the iron core of the detection transformer does not contact the iron core of the motor to be tested, the main magnetic circuit of the detection transformer can be regarded as an open circuit). If the coil winding embedded in the core slot of the motor to be tested (when the core of the detection transformer contacts the core of the embedded coil of the motor slot to be tested, the core of the motor to be tested and the core of the detection transformer will form a whole. The coil embedded in the slot can be regarded as another winding L3 of the detection transformer) is not short circuited (that is, K3 is in an open circuit state). An audio signal induced voltage will be generated at both ends of winding L2 and sent to transistor Q3 by plug Z2 for voltage amplification. The amplified signal voltage is sent to the push-pull power amplifier circuit composed of transistors Q4 and Q5 through transformer T3 for power amplification. The amplified signal voltage is then coupled to speaker y by transformer T4 or indicated by the constant change of pointer position of ammeter (when switch K2 is not closed). If the coil winding embedded in the iron core slot of the motor to be tested has an inter turn short circuit (i.e. K3 is closed), the inductance of the detection transformer will be greatly reduced. No audio signal induced voltage will be generated at both ends of winding L2 or the generated audio signal induced voltage is very low. Therefore, the speaker will stop making sound or the volume will be greatly reduced (or the changing amplitude of ammeter pointer position will be significantly reduced when the switch K3 is not closed). When detecting whether the cast aluminum bar of the motor squirrel cage rotor is broken, the iron core of the detection transformer is also in contact with the rotor iron core of the motor to be tested. At this time, the rotation of the motor to the iron core also forms a whole with the detection transformer iron core. The squirrel cage aluminum bar embedded in the rotor iron core will also be regarded as winding L3 in the detection transformer. If the squirrel cage aluminum bar embedded in the iron core is not broken, it is equivalent to L3 short circuit (i.e. K3 is closed). Obviously, the corresponding situation is that the speaker will stop making sound or the volume will be greatly reduced. Conversely, if the squirrel cage aluminum strip embedded in the iron core is broken, it is equivalent to L3 open circuit. The sound volume of the loudspeaker is large (compared with that when the squirrel cage aluminum strip embedded in the iron core is not broken).

2 Production and commissioning instructionsR6 and C3 in the circuit form a positive feedback circuit. Its function is to feed back the weak audio signal voltage induced in winding L2 in the detection transformer to Q2 base for re amplification. Properly selecting the value of R6 can adjust the detection sensitivity of the tester. RP1 in the figure is the variable resistor for adjusting the operating current of the oscillator. After the circuit is made, the detection transformer core will contact with some other transformer core to connect the main magnetic circuit.

Then adjust the RP1, so that the oscillator starts (Q1 normal working current is 0.4 0.6mA). If the components are normal and installed correctly, the working point of the transistor is set properly. The audio call from the loudspeaker shall be audible. Changing the C1 capacity or the number of turns of the primary winding L1 of the oscillation transformer T1 can change the oscillation frequency (i.e. change the tone of the sound emitted by the speaker). In the figure, RP2 is the working current of amplification tube Q2, adjusting the variable resistance, and changing its resistance value can adjust the detection sensitivity of the tester. After the circuit is made. Adjust RP2. When the iron core of the detection transformer contacts the iron core of the motor slot embedded in the coil to be tested (the coil can be artificially short circuited), the speaker just stops making sound or the volume decreases greatly. RP3 in the figure is to push the Q3 working current of the tube, adjust the variable resistance, and change its resistance value to make the Q3 collector working current 2mA 3mA (in fact, it can also adjust the detection sensitivity of the tester). RP4 in the figure is the power amplifier level static operating point current regulating variable resistance.

Change the resistance value so that the static working current is 2mA 4mA (when K2 is disconnected, the milliammeter can be connected in parallel at both ends of K2 for monitoring). After the working current at all levels of the tester is determined to be normal, RP1 RP4 can be replaced with fixed resistance to prevent the deterioration (value) of variable resistance over time from affecting the operation of the tester. The oscillating transformer T1 can use 10K medium cycle "I" type magnetic core as the skeleton and L1 as the core = 60 turns of 0.07mm high-strength enamelled wire for L2 = 0.07mm high-strength enamelled wire shall be wound for 6 turns, and it shall be noted that the winding directions of the two windings are exactly opposite. The detection transformer T2 uses 100VA, and the transformer uses "U" shaped iron core (if it is used to detect small motors, T2 iron core can be determined according to the actual situation). The cross-sectional area of the iron core is 2x3cm2.l1 = 100 turns of 0.21mm high-strength enamelled wire for L2 = 200 turns of 0.21mm high-strength enamelled wire. The push transformer T3 can be a small input transformer for transistor radio produced in the 1980s. The primary turn ratio is 2:1. If you want to make it by yourself. "E" type iron core can be used, with a cross-sectional area of 5x5mm., For primary L1 = 100 turns of 0.1mm high-strength enamelled wire for secondary L2 = 0.07mm high-strength enamelled wire is wound for 50 turns and tapped at 25 turns to connect R9. The output transformer T4 is also a small output transformer for transistor radios produced in the 1980s. The number of turns ratio of the primary stage is 8:1. If self-made, the same iron core of the above input transformer is also used, and the primary L1 is used = 0.1mm high-strength enamelled wire is wound for 200 turns, tapped at 100 turns to K3, and used for secondary L2 = 0.1mm high-strength enamelled wire is wound for 25 turns, and the loudspeaker can be used = 65Mn / 0.25w/8 , permanent magnet loudspeaker. Transistors Q1 Q5 are germanium triodes for transistor radios produced in the 1980s, of which Q1 is 3ag11 》 100, 3ax31a / B for Q2 Q5, requirements 》 50 If you don't have such tubes at hand, you can also use commonly used silicon tubes.

Q1 adopts the requirements of s9015 》 100, Q2 and Q3 adopt s9012, requirements 》 80, s8550 for Q4 and Q5, requirements 》 50. See the attached drawings for other components. Switches K1 and K2 are small twist switches, and plugs and sockets are commonly used by small ear machines = 2.5mm plug and socket. The available range of ammeter is 100mA / 91l16 small square plastic case ammeter. If ammeter display is not required, it can also not be used. Since the tester is not often used, in order to reduce the volume of the tester. Therefore, the power supply adopts 4 5# alkaline batteries.

The whole tester circuit element can be installed on a 85x40mm2 printed board. Then put the circuit board into a 90x45x30mm3 plastic box. Sockets J1, J2, switches K1, K2 and speaker y are installed on the upper cover plate, and the upper cover plate at the position of the speaker shall be applied = Drill several evenly distributed small holes with a 2.5mm drill bit. The area of the small hole shall be equal to the area of the loudspeaker paper basin. The tester is connected to the main circuit board through plugs Z1 and Z2 and sockets J1 and J2. The lead wires of plugs Z1 and Z2 shall preferably be double core flexible wires with metal shielding, and the length shall be greater than 1.5m. When in use, insert the plugs Z1 and Z2 into the sockets J1 and J2, and turn the power switch to the "on" position for detection.

After the test. Pull out the catcher Z1 and Z2. Turn the power switch to the "off" position.