How to test whether the motor is short-circuited between turns!

　　The same winding of the motor is made up of many turns (turns) of wire. If the insulation is not good, the coils stacked together will be short-circuited. In this way, it is equivalent to that some coils are directly short-circuited and do not work. After a turn-to-turn short circuit, the winding of the motor is short-circuited due to a part of it, the magnetic field is different from the previous one, asymmetrical, and the remaining coil current is larger than before, the vibration of the motor will increase during operation, the current will increase, and the output will be relatively reduced small.

　　Test motor short circuit

　　One: visual inspection.

　　Two: measure the motor no-load current. But this needs to know the normal no-load current of the motor.

　　Three: Motor temperature rise. Generally, there is a local short circuit and the local temperature will increase significantly.

　　Four: Impedance method: use the bridge to test the impedance comparison of each winding to determine if the motor has been disassembled. Can be tested with low voltage AC.

　　It is more difficult to detect the fault between the turns of the Y-connected high-voltage motor according to the conventional detection technology. First, it is necessary to check the DC resistance of each phase with a dual-arm DC bridge to determine the fault range. Pull out the rotor and connect the three-phase windings in series. -12 volts AC voltage, use a short-circuit detector to detect. When the short-circuit detector is moved, a large suction force occurs, and this part is the fault point; even if the fault point is detected, maintenance is also a difficult task. Most of the time To ensure the quality of maintenance, the manufacturer will rewind the coil. According to the requirements of the equipment, for example, the lightning impulse withstand level of your transformer is 450kV, and your winding has 100 turns, then the turn-to-turn insulation requirement can be 450/100=4.5kV×K1; K is the safety factor, specific needs According to the structure of the equipment, because the potential distribution on the winding is not uniform under the impulse voltage. If the wiring is wrong, the motor response is obvious. The excitation current will be a bit large, and the excitation can be done according to the current. Do not excite according to voltage. It is literally understood to be a copper wire that plays a short-circuit role, such as a three-phase short-circuit copper bar temporarily installed at the stator outlet when the generator performs a short-circuit test.

　　Causes and hazards of interturn faults

　　(1) The contact area between the turns of the motor winding is basically the same as the turn length of the winding. Inter-turn insulation is often the insulation of the electromagnetic wire itself or very thin additional inter-turn insulation, such as thin films or mica pads. The dielectric strength of the turn-to-turn insulation is far inferior to the insulation to ground. In addition, the inter-turn insulation may be damaged during the processes of winding, embedding, drawing, complexing, and baking.

　　(2) The winding insulation of the motor is subjected to power frequency voltage, instantaneous overvoltage, operating overvoltage and lightning overvoltage during operation. These voltages act on both ground insulation and turn-to-turn insulation.

　　The rated inter-turn power frequency voltage is only a few tens of volts, with little damage to the turn-to-turn insulation. The main factors that damage interturn insulation are various overvoltages. Overvoltage is a non-periodic transient voltage, called impulse overvoltage, its peak value can be up to tens of times the rated voltage, and the wavefront time can be as short as 0.1 us. While going up and down in amplitude, it enters the motor winding at a certain speed. If all the wavefront part enters the first turn of the coil within the wavefront time, the inter-turn insulation is subjected to the peak voltage, and if it enters the first and second turns, it is reduced to half. It is generally believed that in high-voltage motors, due to the neat arrangement of the wires, the amplitude of the shock wave is evenly distributed between the turns of the first coil of the winding, and the shock voltage experienced by the inter-turn insulation is the amplitude divided by the number of turns of the first coil. It has randomness in random embedded windings. Starting from the second coil, the effect of distributed capacitance attenuation decreases the amplitude. Therefore, the inter-turn insulation is a weak link in the motor insulation structure. Field operation practice also proves that the failure rate of turn-to-turn insulation is the highest.

　　(3) The short-circuit turn of the inter-turn fault is inside the motor. Under the action of the alternating magnetic field, an induced electromotive force is generated. The short-circuited turn forms a loop. The induced electromotive force generates a large current in this closed loop with a small resistance. This current is as high as several times the rated current, making the temperature of the short-circuited turns higher than other turns. As the time goes on, the insulation material will be aged and scorched and brittle. It will start from the damage of the inter-turn insulation, which may eventually lead to the breakdown of the inter-phase or ground insulation , And eventually burn out the motor.