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AC motors, specifically single – phase induction motors, often need a capacitor for starting due to the following reasons:
1. Magnetic Field Requirement for Rotation
A three – phase AC motor can generate a rotating magnetic field directly from the three – phase supply, which interacts with the rotor to produce torque and make the motor rotate. However, a single – phase AC supply produces a pulsating magnetic field (a field that only varies in magnitude but doesn’t rotate). A pulsating magnetic field can’t generate the necessary starting torque to get the rotor moving from a standstill.
2. Role of the Capacitor in Creating a Rotating Field
To create a rotating magnetic field in a single – phase motor, we need to introduce a phase difference between two windings (typically the main winding and the auxiliary winding). The capacitor is connected in series with the auxiliary winding.
When AC current flows through the circuit, the capacitor causes the current in the auxiliary winding to lead the voltage (in contrast, the current in the main winding lags the voltage to some extent). This phase difference between the currents in the two windings results in the production of two magnetic fields that are out of phase. The combination of these two out – of – phase magnetic fields creates a rotating magnetic field.
3. Starting Torque Generation
This rotating magnetic field cuts through the rotor conductors, inducing currents in them. According to Lenz’s law and the principle of electromagnetic induction, the interaction between these induced currents and the rotating magnetic field generates a starting torque, which is sufficient to overcome the static friction and inertia of the rotor, causing the motor to start rotating.
4. Types of Capacitor Usage
- Capacitor – start motors: The capacitor is only used during the starting phase. Once the motor reaches a certain speed (usually around 70 – 80% of the synchronous speed), a centrifugal switch disconnects the auxiliary winding and the capacitor from the circuit. The motor then runs on just the main winding.
- Capacitor – start – run motors: A capacitor is used both for starting and running. There are two capacitors: a larger one for starting (disconnected by a centrifugal switch after start – up) and a smaller one that remains in the circuit for improved running performance, providing better power factor and efficiency during operation.
In summary, the capacitor is essential in single – phase AC motors to create the necessary rotating magnetic field for starting by introducing a phase shift between winding currents.
