AC motors are designed to operate at +/- ____ Percent of their rated potential difference, and +/- _____ percent of their rated frequency?
To approach the question the way it is asked would be to say few. Only synchronous A/C motors operate at a fixed speed based on the number of poles and the frequency of the power supply. A synchronous speed can be in increments of 3600 RPM or 2500 RPM depending on the frequency. Induction A/C motors will operate at the synchronous speed when there is no applied load and will begin to slow down slightly as load is applied. That slip is usually around 2- 3% when the nominal design horsepower is produced. . For example, an 1800 RPM synchronous motor will operate at 1800 RPM when fully loaded. A common induction motor will operate between 1800 and 1750 RPM. Most motors are manufactured to specific horsepower ratings at their design speeds and so cataloged. When a device needs a motor and it's design horsepower is known, the motor selected will be the next larger size. For instance a direct drive pump may be rated at 17.5 horsepower non-overloading (maximum) at 1750 RPM. There are no standard motors made to that specific horsepower so a 20 horsepower motor will be selected and will run efficiently at that partial load. Depending on the motor cooling system and the motor wiring insulation design, some will have transient service factors of 10-15% above the full load rating, but prolonged operation in an overloaded condition will shorten the motor life. In the larger sizes, motors can be custom built to closely match a non-standard horsepower within the normal speed range.
