Standard motor manufacturers conduct inter-turn tests on their windings, but during these tests, suspected inter-turn faults often arise that are difficult to diagnose directly. When using a dual-winding comparison method, if both windings are normal, the two curves will perfectly overlap, meaning only a decaying sine wave curve will be visible on the inter-turn meter screen. If the two curves do not perfectly overlap, it could indicate an inter-turn short circuit fault in the two tested windings, differences in magnetic circuit parameters, or faults in the instrument or wiring. Ms. Can will today compile and share several typical cases with you.
Analysis of several typical situations
Because turn-to-turn meters of different specifications or from different manufacturers may react differently to the same type of winding fault; furthermore, different wiring methods for three-phase windings will also produce different reactions, making it difficult to provide a universal judgment standard. Testers should continuously summarize their experience through experimentation to develop their own effective judgment standards.
● Both curves are very stable, but there are slight differences.
(1) Compared to the total number of turns, a small number of turns are completely short-circuited;
(2) If this phenomenon exists in all specifications of the tested motor, it is likely due to uneven magnetic circuit, such as uneven slot spacing, uneven air gap between stator and rotor (for the whole product), inconsistent magnetic properties of iron core in all directions, etc. After removing the rotor, test again. If the curve becomes normal, it means that it is not a problem with the stator winding.
(3) For windings with a large number of turns, it may be that one phase winding has slightly more or less turns than the normal value;
(4) For multi-strand coils, when connecting the wires, some strands may not be connected or the contact resistance of the nodes may be large. In this case, the DC resistance of the two windings will also be different.
(5) The resistance or reactance of the three-phase windings may vary slightly due to fluctuations in the conductor material, winding coil, end shaping and other operating processes.
(6) The inter-turn meter composed of two thyristors will have different output voltages after a long period of use due to changes in the parameters of the two thyristors or related circuit components (such as the capacitance and leakage current of the capacitor). This will result in a small difference between the two discharge curves. At this time, the same reaction will be observed for each test (such as the three tests of a three-phase motor). However, it should be noted that the reaction will be larger for motors with larger capacity and may not be obvious for motors with smaller capacity.
(7) The instrument was not properly adjusted, which caused the two curves to not overlap when no voltage was applied.
● Both curves are very stable, but the differences are significant.
(1) If the number of turns of the two windings differs significantly or if two or more sides of one of the windings are completely short-circuited (theoretically far, but the actual spatial distance is zero), the DC resistance of the two windings will also be different.
(2) The two windings have the same number of turns, but some coils in one winding have reversed connection at the beginning and end. In this case, the DC resistance of the two windings will be exactly the same or roughly the same.
● One curve is smooth and normal, while the other curve shows chaotic spikes.
(1) There is a short circuit between turns that appears to be connected but not connected inside the winding where the curve shows messy spikes. Under the action of high voltage, the short circuit point generates electric sparks. If it occurs at the end of the winding, you may see blue sparks and hear a "pop pop" discharge sound.
(2) The instrument wiring is loose or has a poor connection.
● Both curves show messy spikes.
(1) Both sets of windings tested had inter-turn short circuit faults;
(2) When the core is placed using a grounding method, the grounding point is loose and not secure.
