Some single-phase AC electric motors require a run capacitor to energize the second-phase winding (auxiliary coil) to create a rotating magnetic field while the motor is running. Run capacitors are designed for continuous duty while the motor is powered, which is why electrolytic capacitors are avoided, and low-loss polymer capacitors are used. Run capacitors are rated in a range of 1.5 to 100 µF, with volt classifications of 250, 370 and 440 V.