Motor short circuit measuring instrument circuit

The motor short-circuit measuring instrument introduced here can not only be used to quickly and accurately check short-circuit faults above a single turn of the armature of motors and generators,
but can also be used to check the quality of cast aluminum cage rotors and whether there are broken bars in the conductors. Using this measuring instrument, when rewinding the motor winding, you can
test it while taking it offline to avoid short circuit caused by careless operation
    . . l
    (1) Working principle The circuit of the instrument is shown in Figure 5-26. Transistor VT1. Transformer T and other components form a transformer feedback oscillation circuit to generate
an audio signal of about 2000Hz. VT2 and other components form an audio amplifier. The audio oscillation signal generated by the audio amplifier is amplified by VT2 and then added
to the ld winding. When S2 is closed, the S winding forms a closed loop, and the audio signal voltage is added to the base of VT3 through the mutual coupling of L3, L, and k.
After two-stage audio amplification, it drives the speaker or is indicated by the meter.
    In the circuit, R6 and G form a positive feedback circuit, and its function is to act as L. When in a short circuit, the signal induced on the coil L4 is fed to
the base of vr2 for amplification. Appropriate selection of wind resistance can improve test sensitivity. But it should be noted that due to the weak coupling between L3 and S
, coupled with the positive feedback effect of the wind and G branches, when L5 is open, there may still be a weak signal output, but at this time
S2 of Ls is turned on , the speaker should have obvious changes, otherwise the resistance value of Re should be adjusted appropriately.
    (2) Components should be selected from VT1 to VT6 and 3AX31 should be used. The port of VT1 should preferably be larger than 50. The cross-sectional area of ​​the transformer Tl core is 3.5×
5. 5rrrrr12, and the wire diameter of the primary winding is o. The 08mm enameled wire is wound with 1000 turns, and the secondary winding is wound with 120 turns of the same wire. T2 and T3 use transistor
radio input and output transformers.
    L3, L4 and Ls winding cores are cut from "E" shaped cores with a stack thickness of 35mm. After the two cores are completed, they are coated with insulation layers.
    Ls is wound with 350 turns of <PO. 12mm enameled wire, and L is wound with @. 110 turns of 29mm wrapped wire. Then use a thicker enameled wire
to wrap a turn outside L3 and h as L. Ls also plays the role of bringing the two cores together. The two iron cores are in the shape of a "one" and are close together,
with one or two layers of insulation in the middle. For ease of use, install the core, windings, etc. in a shell made of bent aluminum or copper sheets. In order to install firmly,
first grind off the outer edges of the two cores about 1mm with a grinding wheel, and gently knock down the bottom edge of the shell. , so that it covers the outer edge of the iron core, and then seals it with paraffin wax. Windings
L3 and L; use longer flexible leads to lead from the small holes in the baffles at both ends of the shell and connect them to small plugs respectively. The two leads of the Ls winding are connected to s2
. The S2 is replaced with a flashlight switch and mounted on the housing. The circuit part is installed on a 120×70rnrr12 insulating board and put into a
box of suitable size. There are two jacks on the box, so it is very convenient to carry and use. The power supply uses 4 AA batteries:
    (3) Adjustment and use of the circuit After welding, it can be adjusted after checking that there are no empty or wrong weldings, etc. Close switches sl and S3, adjust
RP1, RP2, and RP3 to make I. l—o. 5mA, J. - 4mA. L-3.5mA, L-k-T. 5mA. Use a high-impedance earphone and
a 20FtF capacitor in series and then connect them in parallel between the current collector of VT2 and the ground. You should be able to hear the audio signal. Otherwise, it may be that VT1 has not started to
vibrate. You should change Ll or L2. The two protruding wiring positions. If there is a signal sound in the earphones, close S2 again, and the speaker should have an audio signal sound
. At this time, adjust the bias resistors of VT3, VT4, and VT5 appropriately to maximize the sound of the speaker. If the sound of the speaker does not change much when sz is turned on or off
, it may be that L3 and L prongs are plugged in incorrectly or the wind resistance value is not appropriately selected and should be adjusted appropriately.
    Before use, first insert the two catchers L3 and L4 into the socket L of the circuit box, connect S1 and S3, and then connect sz. If there is sound from the speaker,
it means the measuring instrument is working properly. When using the meter to indicate, if S3 is disconnected at this time, the current indication will increase to approximately .20mA. :
    When in use, disconnect S2. When checking the motor winding, place the measuring head on the slot of the stator core of the motor winding. If the speaker makes a loud
sound, it means that the winding in the slot has a short circuit (equivalent to S2 being closed); if the speaker is silent, it means that the winding is not functioning properly. short circuit condition. When
    checking the armature winding of the rotating armature DC generator , place the measuring head on a certain notch and disconnect S2. The speaker is silent. This means that
the coil of the speaker has no short circuit fault. If you use small tweezers to short-circuit the commutator corresponding to the coil, there will be a sound from the speaker indicating that the coil has no open circuit fault;
otherwise, it means that the coil has an open circuit fault, or it is caused by the wrong connection between the winding and the commutator.
    When checking the quality of the cage rotor, place the measuring head on the slot where the rotor conductor is to be tested, and disconnect S2.
If the speaker makes a loud sound, it means the quality of the cage rotor is good. If the sound is small or silent, it means the cage is damaged or broken. strip.