The single-phase induction motor is a motor with a main winding connected directly to the mains supply and an auxiliary winding lagged, Gates A74 usually at 90 ° electrical from the main winding. It is a machine used with no other impedance other than that presented by the motor winding itself. When the motor reaches a predefined rotation, the auxiliary winding disconnects from the mains by a switch that is generally driven by centrifugal force. As the auxiliary winding is dimensioned to act only at the start, its non-shutdown will generate its firing.
The angle of lag between the main winding and auxiliary winding currents is low and therefore these motors have a starting torque equal to or slightly higher than nominal, which limits their use to fractional powers and loads requiring conjugate such as office machines, fans and hoods, small polishers, hermetic compressors, centrifugal pumps. The auxiliary winding is placed in the supply circuit only during the starting period of the motor and creates a phase shift which will produce the necessary torque for initial rotation and acceleration.