One burnt segment is caused by which interpole strength condition?

• A. Interpole strength too weak
• B. Interpole strength too strong
• C. Interpole strength either too weak or too strong
• D. High mica

Answer: (C)

A burnt commutator segment usually points to poor commutation, which happens when the interpole (commutating pole) field isn’t giving the right amount of counter-flux to force the current in the commutating coil to change cleanly as the brushes move. If the interpole strength is too weak, it can’t adequately oppose the armature reaction, so the current in the coil being commutated doesn’t switch properly. This causes sparking at the brush and excessive heat in the corresponding segment, leading to a burnt segment. If the interpole strength is too strong, it overcompensates, upsetting the current reversal as the current transfers to the next coil and again causes sparking and overheating on the segment. So either under- or over-strength of the interpole field can produce the same symptom. High mica isn’t the cause of a burnt segment in this context.