Why is a capacitor used in 1- phase and not in a 3- phase motor?

A capacitor is used to create a rotating magnetic field, which is necessary for the motor to start and run.

Single-phase motors lack the natural rotating magnetic field present in three-phase motors, and a capacitor helps generate a second phase, allowing the motor to self-start and improve its running performance. 

Here's a more detailed explanation:

1. Single-phase motors are not self-starting:

Unlike three-phase motors, single-phase motors don't inherently produce a rotating magnetic field.

This means the rotor (the rotating part of the motor) won't start turning on its own.

A capacitor is used to introduce a phase shift in the current, effectively creating a second phase and enabling the motor to generate a rotating magnetic field. 

2. How the capacitor works:

A capacitor is connected in series with an auxiliary winding (also called the starting winding) of the motor. 

When the motor starts, the capacitor provides a surge of current to the auxiliary winding, creating a phase difference between the current in the main winding and the auxiliary winding. 

This phase difference, along with the physical separation of the windings, creates a rotating magnetic field that allows the rotor to begin rotating. 

Once the motor reaches a certain speed, a centrifugal switch (or other switching mechanism) disconnects the capacitor and auxiliary winding, and the motor continues running on the main winding. 

3. Types of capacitors and their roles:

Start capacitor:

Used for a short period during motor startup to generate a strong starting torque. 

Run capacitor:

Used to improve the motor's efficiency and power factor during continuous operation. 

Dual run capacitor:

Combines the functions of both start and run capacitors, often found in applications like air conditioners. 

4. Benefits of using a capacitor:

Improved starting torque:

Capacitors help the motor generate enough torque to overcome inertia and start rotating. 

Enhanced running performance:

Capacitors can improve the motor's efficiency and power factor, leading to better overall performance. 

Increased reliability:

By providing a starting boost and optimizing running conditions, capacitors contribute to the motor's longevity and reliability.

Summary:

A capacitor is used in a single-phase motor because single-phase power does not create a rotating magnetic field on its own, which is necessary to start and run the motor. The capacitor provides a phase shift that creates a second, out-of-phase current in an auxiliary winding, producing a rotating magnetic field that starts the motor. In contrast, a three-phase motor naturally generates a rotating magnetic field due to the three-phase supply, eliminating the need for a starting capacitor.