Which phenomenon is primarily caused by dirt accumulation on a single-phase motor?

The phenomenon primarily caused by dirt accumulation on a single-phase motor is decreased airflow and overheating. When dirt and dust build up on or around the motor, it obstructs the airflow necessary for cooling the motor during operation. Effective cooling is crucial for maintaining optimal performance and preventing overheating.

As the motor runs, the heat generated from the electrical currents and mechanical friction needs to be dissipated. If the airflow is restricted due to debris, the motor cannot cool efficiently. This can lead to excessive heat buildup, which can damage the motor's insulation, reduce its lifespan, and lead to premature failure.

The other options are not relevant in this context. Increased torque output, improved energy conversion, and enhanced rotational speed typically relate to the motor's design, load, and efficiency rather than being affected by dirt accumulation. In contrast, decreased airflow directly results from dirt buildup, making it the correct answer to the question.