This type of electric motor has a primary winding, which is the main and an auxiliary (for starting), the two lagged in the space 90 degrees electric. The auxiliary winding creates a phase shift which creates the essential torque for initial rotation and acceleration. When the motor reaches a predetermined rotation, the auxiliary winding is disconnected from the network by means of a key which is generally driven by a centrifugal force, leeson motors which is the centrifugal switch or breaker. In special situations, also by relay of chain, manual key or other special instruments, so that the auxiliary winding is dimensioned to act only at the start, if it remains on, its burning may occur.
The angle of lag that can be obtained between the main winding and auxiliary winding currents is negligible and, for that reason, these motors have a starting torque similar to or slightly above the nominal, which limits their application to fractional powers and loads requiring reduced or balanced starting torque eg small polishers, office machines, fans and exhausters, hermetic compressors, centrifugal pumps, etc.