Single Phase AC Motors

Operation A 3 phase motor self starts due to a rotating magnetic field in the windings If a single phase was applied to one winding, it would simply pulse and produce no torque If we could simply ‘push’ the rotor, it would begin to spin for the same reasons in a 3 phase motor

Operation To make the spin happen, we add a start winding to the motor A start winding usually has the same number of turns as the run winding but a smaller diameter wire This creates an inductive reactance causing the current to lag

Split Phase Motor A split phase motor has 2 windings: Run winding Start winding

Split Phase Motor The two windings cause the current to lag depending on the inductive reactance of each This mimics the effect of a changing magnetic field Once started, the switch cuts out to stop burning out

Split Phase Motor Can be reversed by changing connections to any one winding Has a high starting current Used in simple domestic machines

Capacitor Start Motor Similar in effect to the split phase motor Capacitor causes current in start winding to lead voltage Start and Run windings are approximately 90° out of phase

Capacitor Start Motor High torque Used in compressors, refrigerators and when high torque starting is needed

Capacitor Start – Capacitor Run Motor 2 capacitors are connected in parallel with the start winding Once started, the first capacitor cuts out and leaves the second to continue to maintain an out of balance magnetic field

Capacitor Start – Capacitor Run Motor This type is similar to the last but has: Better torque Improved power factor