Q3Electrical Machines and Drives

A sudden increase in load current causes a corresponding sudden increase in armature reaction, which distorts and shifts the air-gap flux distribution, crowding flux lines near the trailing pole tips. This causes the voltage induced between adjacent commutator segments (the inter-segment voltage) to rise sharply in that region, occasionally exceeding the breakdown strength of the thin air/mica gap between segments, causing an arc. This arc can bridge across the commutator surface (ring fire), spreading rapidly into a sustained flashover across the entire commutator, short-circuiting it.