A capacitor assists in creating a phase shift and generating a rotating magnetic field in single-phase induction motors through a technique called "split-phase" or "phase-splitting" starting. Single-phase induction motors inherently lack a rotating magnetic field like three-phase motors, which makes them less efficient at starting and running. To overcome this limitation, a capacitor is used in conjunction with the motor's main winding to create the necessary phase shift.
Here's how it works:
Main Winding (Start Winding): The main winding is the primary winding of the single-phase induction motor. It is connected directly to the AC power supply. When the motor is powered, it produces a magnetic field that pulsates or oscillates, rather than rotating. This results in very little starting torque and poor performance.
Auxiliary Winding (Start Winding): The auxiliary winding, also known as the start winding, is placed at an angle to the main winding in the motor's stator. This winding alone won't create a rotating magnetic field either. However, when a capacitor is introduced in series with the auxiliary winding, a phase shift is created between the current in the main winding and the current in the auxiliary winding.
Phase Shift: The introduction of a capacitor in series with the auxiliary winding creates a phase shift between the currents in the two windings. This phase shift causes the magnetic fields produced by the two windings to be out of phase with each other.
Rotating Magnetic Field: The out-of-phase magnetic fields from the main and auxiliary windings combine in a way that effectively creates a rotating magnetic field. This rotating magnetic field interacts with the rotor (a squirrel-cage or shaded-pole design) and induces a current in it, producing torque and causing the rotor to start rotating. Once the rotor is in motion, it will continue to rotate due to the motor's inertia and the interaction with the rotating magnetic field.
Capacitor Value: The value of the capacitor is carefully selected to create the desired phase shift. The size of the capacitor determines the degree of phase shift and the motor's starting characteristics.
It's important to note that while a capacitor can assist in generating a rotating magnetic field and starting a single-phase induction motor, it's not used during the motor's continuous operation. Once the motor is up to speed, a switch or centrifugal mechanism disconnects the start winding and capacitor from the circuit, and the motor runs using only the main winding.
This split-phase starting method is commonly used in single-phase induction motors found in various household appliances like fans, air conditioners, refrigerators, and small pumps.