Polyphase motors: Polyphase squirrel-cage motors are basically constant-speed machines, but some degree of flexibility in operating characteristics results from modifying the rotor slot design. These variations produce changes in torque, current, and full-load speed. Evolution and standardization have resulted in four fundamental types of motors.
Designs A and B: General-purpose motors with normal starting torques and currents, and low slip. Fractional-horsepower polyphase motors are generally design B. Because of the drooping characteristics of design B, a polyphase motor that produces the same breakdown (maximum) torque as a single-phase motor cannot attain the same speed-torque point for full-load speed as a single-phase motor. Therefore, breakdown torque must be higher (a minimum of 140% of the breakdown torque of a single-phase, general-purpose motor) so that full-load speeds are comparable.
Design C: High starting torque with normal starting current and low slip. This design is normally used where breakaway loads are high at starting, but which normally run at rated full load and are not subject to high overload demands after running speed has been reached.
Design D: High slip, very high starting torque, low starting current, and low full-load speed. Because of the high slip, speed can drop when fluctuating loads are encountered. This design is subdivided into several groups that vary according to slip or the shape of the speed-torque curve.
Design F: Low starting torque, low starting current, and low slip. This design is built to obtain low locked-rotor current. Both locked-rotor and breakdown torque are low. Normally used where starting torque is low and where high overloads are not imposed after running speed is reached.