🔹 Why Capacitor Is Used in Single-Phase Motors:
Single-phase supply does not create a rotating magnetic field on its own — it only produces an alternating field that pulsates.
As a result, a single-phase motor cannot start by itself — it needs help to generate a rotating magnetic field to start spinning.
The capacitor creates a phase shift in the auxiliary (start) winding, producing a second current out of phase with the main winding
This phase shift mimics a second phase, creating a rotating magnetic field strong enough to start the motor.
✅ So, the capacitor in single-phase motors:
Helps the motor start
In some motors (like capacitor-run motors), it also improves running efficiency and torque
🔹 Why Capacitor Is Not Needed in Three-Phase Motors:
A three-phase supply naturally produces a rotating magnetic field due to the three voltages being 120° out of phase.
This means a three-phase motor starts on its own, without needing a capacitor.
The built-in phase difference between the three lines ensures smooth and efficient rotation.
✅ So, no capacitor is needed because:
The power supply already provides the rotating magnetic field
The motor can self-start and run efficiently without external phase-shifting components .