What is a common cause of motor winding insulation breakdown?

The correct answer highlights that operating a motor at higher than its rated voltage can lead to insulation breakdown. When a motor is subjected to voltage levels above its design specifications, it may overheat due to excess current flow, which can degrade the insulation materials within the motor windings. High voltage can lead to increased stress on the insulation, causing it to deteriorate and ultimately fail, resulting in short circuits or motor failure.

To further clarify, the other choices do not generally lead to insulation breakdown in the same way. Excessive lubrication relates more to physical wear and overheating from added friction rather than electrical insulation issues. Running at lower than rated voltage typically causes underperformance issues rather than insulation breakdown. Frequent start-stop cycles can affect mechanical components and thermal management but are less likely to be direct causes of insulation failure compared to operating at excessively high voltage.