[Guide] 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 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 drying.

Inter-turn short circuit is one of the common faults of the motor, and the causes of the fault are various. At the appointment of a friend, Ms. participated in a practical case and found a communication with everyone.

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 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 drying.

(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 the turn-to-turn 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 0.1us. At the same time the amplitude rises and falls, it enters the motor winding at a certain speed.

If all of 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 impact voltage experienced by the inter-turn insulation is the amplitude divided by the number of turns of the first coil.

It has randomness in the scattered winding. 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. The field operation practice also proves that the failure rate of turn-to-turn insulation is very high.

(3) The short-circuit turn of the inter-turn fault is generated 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 age and become brittle and fall off. Starting from the damage of the inter-turn insulation, it may eventually lead to interphase or ground insulation. Wear it, and eventually burn out the motor. Therefore, it is necessary to extend the operating life of the motor and improve the excellent dielectric strength and process performance of the turn-to-turn insulation.

Interturn fault diagnosis

Inter-turn fault check can use the inter-turn withstand voltage tester to diagnose the inter-turn fault of the motor winding.

Principle: The basic principle is to detect the impedance symmetry balance of the tested product, and the shock wave comparison method is used to perform a non-damage simulation test of the equivalent overvoltage on the motor with high-voltage pulses.

Using the display of the tester to observe the waveform and comparative analysis, it can intuitively, quickly and accurately determine the winding turn-to-turn voltage and various faults. If there is a defect in the winding inter-turn insulation, it will be broken down under a certain shock wave voltage, and an inter-turn short circuit will be formed. When the insulation between the turns breaks down, discharge sparks and discharge sounds are sometimes accompanied. The test waveform shows discharge burrs and jumps, and the waveforms do not overlap.

Obvious fault points will show signs of sparking and blackening, but sometimes the fault point is very difficult to find, and many winding processing companies use a stethoscope to find it.

Handling of turn-to-turn faults

● For low-voltage loose-embedded windings, during the winding and embedding process, carefully check the quality of the wire and observe whether the color of the insulating layer of the paint film is uniform, whether there are scratches, bubbles, etc. If necessary, use a film or scotch tape to cover it. Sometimes the quality of the turn wire is not high, and improper operation during the winding and embedding process will expose the fault in the winding turn test.

● If the fault point cannot be found in the slot and at the end according to the above method or it is not easy to handle, the faulty wire can be pulled out of the slot, the wire can be replaced or rerouted again, and then inserted into it to avoid the weak point between turns , So that the voltage resistance between winding turns can meet the standard requirements.

The key point of turn-to-turn fault control

(1) The quality of the electromagnetic wire itself. Control the quality of the electromagnetic wires through the necessary detection indicators, especially the key points are room temperature withstand voltage, twisting peeling and paint film continuity. Some manufacturers have higher control standards than the industry standards for special requirements of electromagnetic wires for motors. Practical experience has proved that the quality control in this aspect has obvious effect on the control of motor turn-to-turn faults.

(2) Winding link control. In the winding process, on the one hand, it is necessary to ensure the compliance of the winding mold, and it cannot damage the electromagnetic wire from thousands of factories. On the other hand, the drag of the electromagnetic wire and the direct friction between the electromagnetic wire and the spool edge cause damage to the paint.

(3) In the process of wire embedding, the electromagnetic wire and the notch, the scribe board is not smooth, the pressure plate is strongly pressurized, the cutting tool is too sharp, and the end shaping and other links may cause partial damage to the electromagnetic wire.

(4) The non-conformance of the binding tool during the wiring process and the non-conformity of the spreader during lifting will cause partial damage to the winding.

(5) Undesirable factors such as the bouncing of the core slot of the iron core, uneven bottom of the slot, and iron filings in the slot will cause different degrees of damage to the electromagnetic wire, resulting in turn-to-turn.

(6) The stator is damp, especially the infiltration of corrosive substances, which can cause partial damage to the motor and seriously damage the winding of the entire motor.

(7) In the drying process, the furnace temperature is too high and the insulation performance is impaired.

(8) The unsuitability of the design link. If the design product slot full rate is too high, and the end size is too tight, resulting in poor manufacturability, it is a congenital hidden danger of inter-turn failure. The design should be combined with the actual process to avoid problems caused by the decoupling of design and process.

Sometimes, the turn-to-turn fault, phase-to-phase fault and ground fault are not well distinguished, but the appearance of the turn-to-turn fault is characterized by the fact that the coil at the fault point will have different degrees of burn marks, while the ground and phase fault The performance is partial; but these three faults have a certain positive correlation or simultaneity, which fault is not a good cause for distinction, and the accumulation of experience is very important.