Percent slip of an induction motor refers to which of the following?

The percent slip of an induction motor refers to the difference between the synchronous speed, which is the theoretical speed of the magnetic field in the stator, and the actual rotor speed. It is calculated as the difference between these two speeds divided by the synchronous speed, often expressed as a percentage.

When referring to the correct choice, understanding that percent slip is a measure of how much slower the rotor is running compared to the synchronous speed clarifies its significance in motor performance. The rotor always runs at a speed less than the synchronous speed because of the principle of electromagnetic induction. This slip is necessary for torque production; without it, no current would be induced in the rotor, and thus no torque would be generated.

In terms of the other options, the average speed of the rotor does not provide insight into how it compares to synchronous speed, nor does the maximum speed of the motor define the relationship between rotor and synchronous speeds. The ratio of synchronous speed to rotor speed does describe an aspect of their relationship, but it does not convey the direct difference which, when expressed as a ratio to synchronous speed, gives the percent slip. Thus, the correct response succinctly captures the operational effect of slip in the context of induction motor functionality.