Single-Phase Induction Motor

Cheatsheet Content

### Introduction to Single-Phase Induction Motors - **Definition:** An AC electric motor that operates on a single-phase power supply. - **Principle:** Uses a rotating magnetic field created by single-phase AC to induce current in the rotor, producing torque. - **Key Feature:** Not self-starting; requires an auxiliary mechanism to initiate rotation. ### Construction - **Stator:** - Laminated steel core with slots for windings. - **Main Winding (Running Winding):** High inductance, low resistance, occupies about 2/3 of stator slots. - **Auxiliary Winding (Starting Winding):** Low inductance, high resistance, occupies about 1/3 of stator slots, placed 90 electrical degrees from the main winding. - **Rotor:** - **Squirrel Cage Rotor:** Most common type. Laminated steel core with aluminum or copper bars embedded, short-circuited at both ends by end rings. No external connections. - **Air Gap:** Small gap between stator and rotor for magnetic coupling. ### Working Principle - **Double Revolving Field Theory:** A pulsating single-phase magnetic field can be resolved into two rotating magnetic fields of equal magnitude, rotating in opposite directions at synchronous speed. - Each field produces its own torque. - At standstill, these torques are equal and opposite, resulting in zero net torque (motor is not self-starting). - **Starting Mechanism:** To make the motor self-starting, a phase difference is created between the main and auxiliary winding currents, mimicking a two-phase supply. This produces a rotating magnetic field. ### Starting Methods 1. **Split-Phase Induction Motor:** - Uses a centrifugal switch in series with the auxiliary winding. - Auxiliary winding has higher resistance and lower reactance (thin wire, more turns). - When motor reaches 70-80% of synchronous speed, the switch opens, disconnecting the auxiliary winding. - **Applications:** Fans, blowers, washing machines. 2. **Capacitor-Start Induction Motor:** - A capacitor is connected in series with the auxiliary winding and centrifugal switch. - Capacitor creates a larger phase shift between main and auxiliary winding currents, leading to higher starting torque. - Centrifugal switch disconnects the auxiliary winding and capacitor at ~75% synchronous speed. - **Applications:** Compressors, pumps, conveyors. 3. **Capacitor-Start Capacitor-Run Induction Motor (Two-Value Capacitor Motor):** - Uses two capacitors: a starting capacitor (larger value, electrolytic) in series with auxiliary winding and centrifugal switch, and a running capacitor (smaller value, oil-filled) permanently in series with auxiliary winding. - Provides high starting torque and improved power factor during running. - **Applications:** Refrigerators, air conditioners. 4. **Permanent Split Capacitor (PSC) Motor:** - Uses a single capacitor permanently connected in series with the auxiliary winding. No centrifugal switch. - Simplest design, but lower starting torque. - **Applications:** Ceiling fans, blowers, small pumps. 5. **Shaded-Pole Induction Motor:** - Simplest and cheapest. - Uses copper "shading coils" on portions of the stator poles to create a delayed flux, causing a weak rotating field. - Very low starting torque, low efficiency. - **Applications:** Small fans, toys, hair dryers. ### Operating Characteristics - **Speed-Torque Curve:** Similar to a three-phase induction motor, but with lower starting torque and efficiency, and more pulsating torque. - **Efficiency:** Generally lower than three-phase motors of comparable rating. - **Power Factor:** Can be improved by using capacitors. - **Noise and Vibration:** Often higher due to pulsating torque. ### Applications - **Household Appliances:** Refrigerators, washing machines, air conditioners, fans, blenders, vacuum cleaners. - **Small Industrial Equipment:** Small pumps, blowers, office machinery. - **Typical Power Range:** Fractional horsepower up to a few horsepower.