Single-phase AC motors typically require an additional component, such as a capacitor, to assist in achieving sufficient starting torque. These motors are commonly found in appliances like air conditioners, refrigerators, washing machines, and small pumps.
Single-phase AC motors have a problem during startup because the voltage and current waveforms are not in sync due to the single-phase power supply. Unlike three-phase motors, which have a balanced and rotating magnetic field, single-phase motors rely on a pulsating magnetic field. This results in lower starting torque and sometimes even motor stalling.
A capacitor is added to the motor circuit to create a phase-shifted auxiliary winding. This effectively generates a second phase in the motor, which helps overcome the limitations of the single-phase power supply and improves starting torque. The capacitor assists in two main ways:
Phase Shift: The capacitor is connected in series with an auxiliary winding in the motor. This arrangement introduces a phase difference between the main winding and the auxiliary winding. This phase difference creates a rotating magnetic field, even though the power supply is single-phase. This rotating field produces a higher starting torque, helping the motor overcome inertia and start its motion.
Imbalance Compensation: The capacitor helps balance the current imbalance between the main and auxiliary windings during startup. Without the capacitor, the motor would draw uneven currents from the power supply, leading to inefficiency, reduced starting torque, and potential overheating. The capacitor's reactive impedance helps balance out these currents, allowing the motor to start more smoothly and efficiently.
The size (value) of the capacitor used is crucial. It needs to be properly selected based on the motor's specifications to achieve the desired phase shift and starting torque. If the capacitor's value is too low, the motor might not start properly, and if it's too high, the motor might overheat or even fail to start due to excessive phase shift.
It's important to note that once the motor is up and running, the capacitor's effect diminishes, as it's primarily intended to assist during startup. There are different types of single-phase motor starting methods, such as split-phase, capacitor-start, and capacitor-start capacitor-run (two capacitors), each using different capacitor configurations to achieve the necessary phase shift and starting torque.