PlantFlow 3D › Guides › Single-phase capacitor motors
Single-phase capacitor motors
Why a single-phase motor needs a capacitor, how to reverse it, and how to find a failed capacitor.
The starting problem
A single winding fed from single-phase supply produces a magnetic field that pulses back and forth but does not rotate, so the motor has no starting torque. It only hums. Once it is turning, it keeps running on the main winding alone.
The capacitor solves it
A second, auxiliary winding is connected in series with a capacitor. The capacitor shifts the current in that winding in time compared with the main winding, and together the two windings produce a rotating field. A capacitor-run motor keeps the capacitor connected all the time; a capacitor-start motor disconnects it with a switch after starting.
Reversing
Swapping the two ends of the auxiliary winding (Z1 and Z2) with respect to the main winding reverses the direction of the rotating field and therefore the motor. Swapping only L and N does not reverse a single-phase motor.
Fault finding
A motor that hums, does not start, but runs if you spin the shaft by hand usually has a failed capacitor or an open auxiliary winding. It draws locked-rotor current, so the overload will trip after a while. Always isolate and discharge a capacitor before testing it, and replace it with one of the same capacitance and voltage rating from the motor nameplate.
Try it yourself
Choose “Single-phase motor” in the simulator, run it, then swap Z1 and Z2 in the 2D diagram. Try a troubleshooting challenge with a failed capacitor.
Open the PlantFlow 3D simulator →Safety: this guide explains principles for learning. Real installations must follow the applicable codes, the equipment manufacturer's instructions and the approval of a competent, qualified person. Isolate, lock off and prove dead before touching any circuit.