What is the Ring of Fire?

When the DC and pulse traction motors are operated under certain harsh conditions, a strong circular arc may form between the positive and negative brushes, accompanied by flashes and loud sounds. This phenomenon is called ring fire (commonly known as "blasting" ). Cause of motor ring fire: One or more coils of the rotor are broken. (No more than 2 sets, because it can't turn because it is broken, and there will be no ring fire)

Ring fire is a vicious accident. In the operation of DC motors, due to the cumulative breakdown between commutator segments, the commutator is seriously deteriorated, or the motor is frequently started and continuously overloaded, all the arcs between the positive and negative brush holders will fly over and surround the commutator surface. A strong arc appears, accompanied by intense arc light and loud noise. This phenomenon is called ring fire. In the short time that the ring fire occurred, the high temperature and huge energy of the arc not only seriously damaged the commutator and the brush device, but also caused different degrees of damage to other windings. At the same time, the shaft current was generated due to the ground fire of the ring fire. , Journals and bushes will also appear pitted and etched. In addition, the loud noise and arc of the surrounding fire will also cause damage to the maintenance personnel near the aircraft.

1.Inter-chip breakdown

(1) When the voltage between the commutator segments is high and an overvoltage is generated during the operation of the motor, a ring fire occurs due to the cumulative breakdown between the commutator segments;

(2) Due to poor maintenance of the commutator and inadequate cleaning, carbon powder and copper powder are accumulated in the mica trench of the commutator, or copper burrs of commutator blades are prolonged due to copper extruding in the bridge, which causes short-circuit of adjacent commutator segments. The conductive materials all constitute the conductive bridge in the mica trench. When such a conductive bridge enters the main pole region with the armature rotation, the potential gradient on the surface of the commutator increases, and the current passing through it gradually increases. When the electromotive force generated by the current exceeds the electromotive force that maintains the positioning of the conductive powder, the conductive bridges formed by these conductive powders will be dislocated, and the bridges will produce flashovers between sheets. If the effective voltage at the breaking point of the conductive bridge exceeds the minimum arc voltage, an arc occurs. When the width of the conductive bridge fracture area increases to

For protection by air circuit breakers or quick-break air circuit breakers, usually only measures can be taken from the aspects of design and operation and maintenance to prevent ring fires. The main measures are:

(1) Use compensation winding. DC motor installation

Inspection after a ring fire accident

(1) Check the insulation resistance. After a ring fire accident, first burn the damaged commutator and purge windings, and then use a megohmmeter to measure whether the insulation resistance of each winding meets the regulations to determine whether the insulation needs to be treated.

(2) Measure the inter-chip resistance to check whether there is open soldering and inter-chip short circuit between the raised chip and the armature.

(3) Check whether the surface of the commutator is oxidized, whether spot-shaped depressions appear due to arc burns, whether the commutator end is locally damaged, and whether there are deeper ablation marks.

(4) Check the damage of brushes and brush holders, and the number of burned brushes and brush holders.

(5) Inspect the brush holder and conductive ring for defects, and determine the parts that need to be repaired or replaced.

(6) Check whether the main pole winding, armature winding, commutating pole winding and compensation winding are damaged.

(7) Since a strong shaft current passes through the bearing after a ring fire, it should be checked whether severe pitting occurs on the contact surface of the journal and the pad, and whether the pasteurized alloy of the pad is molten.

(8) Check whether the fixing screws of the bearing and the stator are loose due to the huge short-circuit torque caused by the ring fire.

Treatment after a ring fire accident

(1) The commutator with black surface can be cleaned and polished with sandpaper or commutator grinding stone; the commutator with slight burns and shallow burn marks on the surface can be polished once with commutator grinding stone; The commutator should be turned externally; the commutator with severe ablation that cannot be repaired should be replaced. After grinding or turning the commutator, it should be chamfered and engraved again, and the inter-chip resistance must be measured after processing.

(2) Replace the burnt brushes and brush holders. The brush brand should be the same and the brush pressure should be adjusted uniformly.

(3) Repair or replace damaged windings and other components.

(4) If the insulation on the surface of the winding and conductive parts is only fumigated by copper and carbon vapor, and the insulation resistance decreases, you can use cotton cloth dipped in gasoline and carbon tetrachloride to wipe off the copper and carbon particles condensed on the insulation, and then measure the insulation resistance ; If the insulation resistance still does not improve after scrubbing, the entire motor should be cleaned and dried. If the surface insulation layer is scorched by an electric arc, the scorched insulation layer should be peeled off and re-insulated. The insulation resistance after the treatment should be higher than the allowable standard.

(5) If there is only a slight burning point on the bearing bush that is burned by the shaft current, the bearing bush can continue to be used after scraping. If the burning point is dense and the burn marks are deep, the spare bearing bush should be replaced and the bearing bush recast. When dealing with bearing pad failure, check whether the insulation of the bearing housing is normal.

(6) Check and tighten the fixing screws of the stator and the bearing seat. ^[2]