Simple and practical motor short circuit tester

The motor short circuit tester introduced here can not only be used to accurately test short circuit faults of more than one turn of motor and generator windings, but also to check whether there is a broken bar fault in the cast aluminum bars of squirrel cage felt rotors. It can also be used to test while winding the motor windings, so as to avoid failure to discover the short circuit in time due to improper operation and thus lose all previous efforts.

1. Introduction to working principle

The circuit of this motor short-circuit tester is shown in the attached figure, and the principle is not complicated.

In the figure, transistor Q1, transformer T1 and CI constitute a transformer feedback oscillation circuit. An audio signal of about 2000Hz is generated, which is then coupled to transistor Q2 through C2 for amplification. The amplified audio signal is output from the collector of O2. After plug Z1 is inserted into 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 a horseshoe "U" shape, when the core of the detection transformer does not contact the core of the motor to be tested, the magnetic circuit of the detection transformer can be regarded as an open circuit). If the coil winding embedded in the wire slot of the motor core 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, and the coil embedded in the wire 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). Then an audio signal induction voltage will be generated at both ends of winding L2, and sent to transistor O3 by plug 22 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 to produce sound or indicated by the continuous change of the pointer position of the ammeter (when switch K2 is not closed). If the coil winding embedded in the iron core wire slot of the motor to be tested has a turn-to-turn short circuit (that is, equivalent to K3 being in a closed state), the inductance of the detection transformer will drop significantly, and no audio signal induction voltage will be generated at both ends of winding L2 or the generated audio signal induction voltage will be very low. Then the speaker will stop making sound or the volume will be greatly reduced (or the amplitude of the continuous change of the ammeter pointer position will be significantly reduced when switch K3 is not closed). When testing whether the cast aluminum bars of the motor squirrel cage rotor have broken bars, the detection transformer core is also contacted with the motor rotor core to be tested. At this time, the motor rotor core and the detection transformer core also form a whole, and the squirrel cage aluminum bars embedded in the rotor core will also be regarded as the winding L3 in the detection transformer. If the squirrel cage aluminum bars embedded in the core are not broken, it is equivalent to L3 short circuit (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 bars embedded in the core are broken, it is equivalent to 1_3 open circuit. Then the speaker will sound louder (compared with the case when the squirrel cage aluminum bars embedded in the core are 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 I2 of the detection transformer to the base of Q2 for further amplification. The value of R6 can be appropriately selected to adjust the detection sensitivity of the tester. In the figure, RPI is the variable resistor for current adjustment of the oscillator I. After the circuit is completed, the core of the detection transformer is contacted with the core of another transformer to connect the main magnetic circuit. Then adjust RPI to start the oscillator (the normal operating current of QI is 0.4~0.6mA). If the components are normal and installed correctly, and the working point of the transistor is set appropriately, the audio sound from the speaker should be heard. Changing the capacity of Cl or the number of turns of the primary winding L1 of the oscillation transformer T1 can change the oscillation frequency (that is, change the tone of the sound emitted by the speaker). RP2 in the circle is the variable resistor for adjusting the working current of the amplifier tube Q2. Changing its resistance value can adjust the detection sensitivity of the tester. After the circuit is completed, adjust RP2. 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 coil can be short-circuited artificially). The speaker just stops sounding or the volume is greatly reduced. In the figure, RP3 is the push tube Q3 [as a current adjustment variable resistor, change its resistance value so that the working current of the collector of Q3 is 2mA~3mA. (In fact, it also has the function of adjusting the detection sensitivity of the tester). In the figure, RP4 is the current adjustment variable resistor of the static working point of the power amplifier stage, and change its 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 of each level of the tester is determined to be normal. RPl~RP4 can be replaced with fixed resistors to prevent the variable resistor from deteriorating (value) over time and affecting the work of the tester. The oscillation transformer Tl can use a 10K medium-circuit "I"-shaped magnetic core as the skeleton. L1 is wound with φ=0.07mm high-strength enameled wire for 60 turns. I2 is wound with φ=0.07mm high-strength enameled wire for 6 turns.

And note that the two windings are wound in opposite directions. The detection transformer T2 uses a 100VA transformer with a "U"-shaped iron core (if it is used to detect small motors, the T2 iron core can be determined according to the actual situation.). The iron core cross-sectional area is 2x3cm2. L1 is wound with 100 turns of medium = 0.21mm high-strength enameled wire. L2 is wound with 200 turns of φ = 0.21mm high-strength enameled wire. The driving transformer T3 can be a small input transformer for transistor radios produced in the 1980s, with a primary-to-secondary turns ratio of 2:1. If you want to make it yourself, you can use an "E"-shaped iron core with a core cross-sectional area of ​​5x5mm2. The primary L1 is wound with 100 turns of φ = 0.1mm high-strength enameled wire, and the secondary L2 is wound with 50 turns of φ = 0.07mm high-strength enameled wire. At the 25th turn, the tap is connected to R9. The output transformer T4 is also a small output transformer for transistor radios produced in the 1980s, with a primary-to-secondary turns ratio of 8:1. If you want to make your own, you can also use the same core as the above input transformer. The primary L1 is wound with φ=0.1mm high-strength enameled wire for 200 turns, and the fire is connected to K3 at 100m. The secondary L2 is wound with medium=0.1mm high-strength enameled wire for 25 turns. The speaker can use φ=65mrn/0.25W/8n permanent magnet speaker. Transistors QI~Q5 are all germanium triodes used in transistor radios produced in the 1980s. Among them, Q1 is 3AG11. β>100 is required. Q2~Q5 use 3Ax31A/B. β>50 is required. If you don't have this kind of tube at hand, you can also use commonly used silicon tubes. Ql uses S9015. β>100 is required. Q2 and Q3 use S9012. p>80 is required, Q4 and Q5 use s8550, β>50 is required. The rest of the components are marked in the attached figure. Switches K1 and K2 are small toggle switches, and plugs and sockets are φ=2.5mm plugs and sockets commonly used in small earphones. The ammeter can use a small square plastic case ammeter with a range of 100mA/91L16. If the ammeter display is not needed, it can be omitted. Since the tester is not often used, in order to reduce the size of the tester, the power supply uses 4 5# alkaline batteries. The entire tester circuit components can be installed on a 85x40mm2 printed board, and then the circuit board is installed in a 90x45x30mm3 plastic box. Sockets J1, J2, switches K1, K2 and speaker Y are all installed on the upper cover. The upper cover where the speaker is located should be drilled with a φ=2.5mm drill bit with thousands of evenly distributed small holes, and the area of ​​the small holes should be equivalent to the area of ​​the speaker paper plate. The tester is connected to the main circuit board through plugs Z1, Z2, sockets 2, and J2. The leads of plugs Z1 and Z2 are preferably double-core soft wires with metal shielding, and the length should be greater than 1.5 meters. When using, insert the plugs Z1 and 22 into the sockets J1 and J2. Set the power switch to the "on" position and you can start the test. After the test is completed, unplug the plugs Z1 and Z2 and set the power switch to the "off" position.