Coil short circuit tester

The motor short-circuit tester introduced to readers here can not only be used to accurately test short-circuit faults of more than a single turn of motor and generator windings. It can also be used to check whether the cast aluminum bars of the squirrel-cage rotor have broken bars. And when rewinding the motor winding, it can be tested while offline. This can avoid the failure to detect the short circuit in time due to improper operation and the wasting of all previous efforts.

1. Introduction to working principle

The circuit of this motor short-circuit tester is as shown in the attached picture, and the principle is not complicated. In the figure, transistor Q1, transformers T1 and C1 form 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 detection transformer T2 winding L1. When the detection transformer core contacts the motor core under test, the main magnetic path of the detection transformer passes through the test. The motor core forms a path (since the core of the detection transformer is a horseshoe "U" shape, when the core of the detection transformer does not contact the 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 wire slot of the motor to be tested (when the core of the detection transformer contacts the core of the coil embedded in the wire slot of the motor 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 wire slot It can be regarded as detecting that the other winding L3 of the transformer is not short-circuited (that is, it is equivalent to K3 being in an open circuit state). Then the audio signal induced voltage will be generated at both ends of the winding L2, and will be sent from the plug Z2 to the transistor Q3 for voltage amplification. The amplified signal voltage is sent to the push-pull power amplifier circuit composed of transistors Q4 and Q5 through the transformer T3 for power amplification. The amplified signal voltage is then coupled to the speaker Y by the transformer T4 to produce sound or is indicated by the changing position of the ammeter's pointer ( Refers to when switch K2 is not closed). If the coil winding embedded in the core slot of the motor under test has experienced an inter-turn short circuit (that is, equivalent to K3 being in a closed state), the inductance of the detection transformer will decrease significantly. No audio signal induced voltage or audio signal will be generated at both ends of winding L2. The resulting audio signal induces a very low voltage. Then the speaker will stop sounding or the volume will be greatly reduced (or the changing amplitude of the ammeter pointer position will be significantly reduced when the switch K3 is not closed). When detecting whether the cast aluminum bar of the motor's squirrel-cage rotor has a broken bar fault, the detection transformer core is also contacted with the rotor core of the motor to be tested. At this time, the motor rotates to the iron core and forms an integral body with the detection transformer core. The rotor core The embedded squirrel cage aluminum strip will also be seen as winding L3 in the test transformer. If the squirrel cage aluminum bar embedded in the iron core is not broken, it is equivalent to a short circuit of L3 (that is, K3 is in a closed state). Obviously, the corresponding situation is that the speaker will stop sounding or the volume will be greatly reduced. On the contrary, if the squirrel cage aluminum bar embedded in the iron core has broken, it is equivalent to an open circuit in L3. The sound volume of the speaker is larger (compared to when the squirrel cage aluminum bar embedded in the iron core is not broken).

2. Production and debugging instructions

R6 and C3 in the circuit form a positive feedback circuit. Its function is to feed back the weak audio signal voltage induced in the winding L2 of the detection transformer to the base of Q2 for re-amplification. Appropriate selection of 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 completed, the core of the detection transformer will be contacted with the core of some other transformer to connect the main magnetic circuit. Then adjust RP1 to start the oscillator (the normal operating current of Q1 is 0.4~0.6mA). If the components are normal, the installation is correct, and the operating point of the transistor is set appropriately. An audio call should be heard from the speaker. Changing the c1 capacity or the number of turns of the primary winding L1 of the oscillation transformer T1 can change the oscillation frequency (ie, change the tone of the sound emitted by the speaker). RP2 in the figure is the variable resistor for adjusting the working current of amplifier tube Q2. Changing its resistance value can adjust the detection sensitivity of the tester. After the circuit is completed, adjust RP2 so that the core of the detection transformer contacts the core of the coil embedded in the motor trunking under test (the coil can be artificially short-circuited), and the speaker just stops sounding or the volume decreases significantly. In the figure, RP3 is a variable resistor for adjusting the working current of push tube Q3. Just change its resistance so that the working current of Q3 collector is 2mA~3mA (actually it also has the function of adjusting the detection sensitivity of the tester). RP4 in the figure is the static operating point current adjustment variable resistor of the power amplifier level. Just change its resistance so that the static operating current is 2mA ~ 4mA (when K2 is disconnected, a milliamp meter can be connected in parallel to both ends of K2 for monitoring). After the working current at all levels of the tester is confirmed to be normal, RP1 ~ RP4 can be replaced with fixed resistors to prevent the variable resistors from deteriorating (value) over time and affecting the work of the tester. The oscillation transformer T1 can use a 10K mid-circle "I"-shaped magnetic core as the skeleton. L1 is wound with 60 turns of φ=0.07mm high-strength enameled wire, and L2 is wound with 6 turns of φ=0.07mm high-strength enameled wire. Pay attention to the correct winding direction of the two windings. on the contrary. Detection transformer T2 uses a 100VA transformer with a "U" shaped core (if it is used to detect small motors, the T2 core can be determined according to the actual situation.), the cross-sectional area of ​​the core is 2x3cm2. L1 is wound with φ=0.21mm high-strength enameled wire for 100 turns, L2 Use φ=0.21mm high-strength enameled wire to wind 200 turns. The push transformer T3 can be used with a small input transformer for transistor radios produced in the 1980s. The primary and secondary turns ratio is 2:1. If you want to make your own, you can use an "E" type iron core with a core cross-sectional area of ​​5x5mm. , the primary L1 is wound with 100 turns of φ=0.1mm high-strength enameled wire, the secondary L2 is wound with 50 turns of φ=0.07mm high-strength enameled wire, and the tap is connected to R9 at 25 turns. The output transformer T4 is also a small output transformer for transistor radios produced in the 1980s. The primary to secondary turns ratio is 8:1. If you want to make your own, use the same core as the above input transformer. The primary L1 is made of φ=0.1mm high strength The enameled wire is wound with 200 turns, and the tap is connected to K3 at 100 turns. The secondary L2 is wound with 25 turns of φ=0.1mm high-strength enameled wire. The speaker can be φ=65mn/0.25W/8Ω, a permanent magnet speaker. Transistors Q1~Q5 all use germanium transistors used in transistor radios produced in the 1980s. Q1 is 3AG11, which requires β>100. Q2~Q5 uses 3Ax31A/B, which requires β>50. If you don't have such a tube on hand, you can also use commonly used silicon tubes. Q1 uses S9015, which requires β>100. Q2 and Q3 use S9012, which requires β>80. Q4 and Q5 use S8550, which requires β>50. The remaining components are shown in the accompanying drawings. Switches K1 and K2 both use small twist switches, and the plugs and sockets use φ=2.5mm plugs and sockets commonly used in small earphones. The available ammeter is 100mA/91L16 small square plastic case ammeter. If the ammeter display is not needed, it does not need to be used. Since the tester is not used frequently, in order to reduce the size of the tester. Therefore, the power supply uses four 5# alkaline batteries. The entire tester circuit components can be installed on an 85x40mm2 printed board. The circuit board is then installed into a 90x45x30mm3 plastic box. Sockets J1, J2, switches K1, K2 and speaker Y are all installed on the upper cover. Use a φ=2.5mm drill bit to drill several evenly distributed small holes on the upper cover where the speaker is located. The area of ​​the small holes should be equal to the area of ​​the speaker cone. quite. The detector is connected to the main circuit board through plugs Z1 and Z2 and sockets J1 and J2. It is best to use two-core flexible wires with metal shielding for plug z1 and Z2 leads, and the length should be greater than 1.5 meters. When in use, insert plugs z1 and z2 into sockets J1 and J2, and turn the power switch to the "on" position to start testing. After the test is completed, pull out the catchers Z1 and Z2 and turn the power switch to the "off" position.