What does a power factor of 80% indicate about a motor?

A power factor of 80% indicates that the motor converts 80% of the electrical energy it consumes into useful mechanical energy. The power factor is a measure of how effectively electrical power is being converted into usable work output. In this case, an 80% power factor means that there is some reactive power in the system, which contributes to the overall power consumption but does not do any useful work. Therefore, the 80% power factor communicates that the motor is reasonably efficient in its operation, converting a substantial portion of received power into mechanical output.

The other options do not accurately interpret what a power factor signifies. While the motor may be efficient, stating "highly efficient" could be misleading without further context regarding its efficiency rating. The power factor also does not directly correlate with starting torque; a motor’s starting torque is determined by its design and type, not its power factor. Lastly, the operating speed of the motor is influenced by its design and load rather than the power factor alone. Therefore, option C clearly and correctly captures the essence of what an 80% power factor represents in terms of energy conversion efficiency.