What is the insulation generally used between commutator bars on a DC motor?

Mica is the material commonly used as insulation between the commutator bars on a DC motor due to its excellent electrical and thermal properties. Mica can withstand high temperatures and has a high dielectric strength, making it an ideal choice for preventing electrical shorts between the bars while also enduring the heat generated during the motor's operation. Its natural abilities to resist fragmentation and maintain structural integrity under mechanical stress also contribute to its effectiveness in this application.

The other materials listed – rubber, plastic, and fiber – do have their own uses in electrical applications, but they do not offer the same level of thermal stability and electrical insulation that mica does. Rubber, while providing some insulation, does not hold up well at high temperatures and can become conductive over time. Plastic can be used in various electrical components but may not endure as well under the conditions present in a DC motor's commutator environment. Fiber materials can provide some benefits as insulating materials, but they do not match the performance of mica in this specific context.