Detailed Explanation of Single-Phase Induction Motors
1. Construction
Single-phase induction motors share a similar structure with their three-phase counterparts, comprising a stator and a rotor.
- Rotor: The rotor in a single-phase induction motor is most often the squirrel-cage type. This rotor features a laminated iron core with conductive bars embedded in slots around its periphery. These bars are short-circuited at both ends, creating a cage-like structure. This robust and simple design is shared with three-phase squirrel-cage induction motors.
- Stator: The stator is responsible for creating the magnetic field necessary for motor operation. It consists of a laminated iron core with slots on its inner circumference. The stator windings are housed within these slots. In single-phase induction motors, there’s a *main winding, and for self-starting capabilities, an *auxiliary winding is often added. The positioning and configuration of these windings depend on the specific type of single-phase motor.
2. Working
The working principle of single-phase induction motors hinges on the interaction between the rotating magnetic field produced by the stator and the induced currents in the rotor.
- Creation of Pulsating Magnetic Field: When a single-phase AC supply energizes the main winding of the stator, it produces a magnetic field. However, unlike the rotating magnetic field generated by a three-phase supply, this field pulsates in strength along a fixed axis, reversing its polarity every half-cycle.
- Inability to Self-Start: This pulsating field, although alternating in polarity, doesn’t possess the rotational characteristic required to induce rotation in a stationary squirrel-cage rotor. As a result, single-phase induction motors are not inherently self-starting.
- Rotation Once Started: If the rotor is given an initial spin in either direction by an external force, the relative motion between the rotating magnetic field and the rotor conductors induces currents within the rotor. These currents interact with the stator field, producing a torque that sustains the rotation. The motor then accelerates to a speed slightly below the synchronous speed, governed by the load and motor characteristics.
3. Method of Starting
The key to making single-phase induction motors self-starting lies in creating a rotating magnetic field during the start-up phase. This is typically achieved by introducing a second winding on the stator, often referred to as the auxiliary winding or starting winding, and employing various techniques to create a phase difference between the currents in the main and auxiliary windings.
- Split-Phase Motors: These motors use an auxiliary winding with a significantly higher resistance and lower reactance compared to the main winding. When energized, the current in the auxiliary winding is out of phase with the current in the main winding. This phase difference generates a rotating magnetic field, albeit a weak one, sufficient to start the motor. Once the motor reaches approximately 75% of its synchronous speed, a centrifugal switch disconnects the auxiliary winding, and the motor continues to run on the main winding alone.
- Capacitor-Start Motors: In these motors, a capacitor is connected in series with the auxiliary winding during the starting phase. This capacitor further enhances the phase difference between the currents in the two windings, resulting in a stronger rotating magnetic field and improved starting torque compared to split-phase motors. Like in split-phase motors, a centrifugal switch disconnects the auxiliary winding and capacitor once the motor reaches a sufficient speed.
- Capacitor-Run Motors: Capacitor-run motors retain the capacitor and auxiliary winding throughout their operation. The capacitor is carefully chosen to improve the motor’s power factor and running characteristics, providing smoother and quieter operation.
- Shaded-Pole Motors: Shaded-pole motors utilize a unique design where a “shading coil,” a short-circuited copper ring, is placed around a portion of each salient pole on the stator. This shading coil causes a time delay in the magnetic flux within the shaded portion of the pole, effectively creating a rotating magnetic field, though with a relatively weak starting torque. This simple construction makes them suitable for low-power applications.
4. Double-Field Revolving Theory
The double-field revolving theory provides a theoretical foundation for understanding the behaviour of single-phase induction motors, particularly their inability to self-start.
- Mathematical Basis: The theory postulates that a pulsating, alternating sinusoidal magnetic flux, represented as φ = φm cos ωt, can be mathematically decomposed into two separate rotating fluxes. These rotating fluxes have equal magnitudes (φm/2, half the maximum value of the pulsating flux) and rotate in opposite directions at the synchronous speed (Ns = 120f/P).
- Zero Net Torque at Standstill: When the rotor is stationary, the two rotating fluxes induce currents in the rotor, generating torques. However, due to their opposite directions of rotation, these torques are equal in magnitude but opposite in direction, leading to a net torque of zero. Consequently, the motor cannot initiate rotation on its own.
- Net Torque During Rotation: Once the rotor starts spinning, the interaction with the two rotating fluxes changes. The rotor “sees” different relative speeds for each rotating flux. The flux rotating in the same direction as the rotor induces currents that produce a torque that supports rotation. Conversely, the flux rotating in the opposite direction induces currents that generate a counter-torque. However, as the rotor speed increases, the forward torque becomes dominant, while the backward torque diminishes, allowing the motor to accelerate and operate.
5. Why Single-Phase Induction Motors Are Not Self-Starting
The reason behind the non-self-starting nature of single-phase induction motors directly stems from the double-field revolving theory.
- Equal and Opposite Torques: The two counter-rotating fluxes generated by the single-phase stator winding induce currents in the rotor, resulting in equal and opposite torques when the rotor is at standstill. This lack of a net starting torque prevents the motor from initiating rotation.
6. Equivalent Circuit Theory
The equivalent circuit of a single-phase induction motor serves as a valuable tool for analyzing its performance characteristics. This circuit is typically derived based on the double-field revolving theory.
- Representation as Two Motors: The equivalent circuit models the single-phase induction motor as two hypothetical motors, each associated with one of the rotating fluxes, sharing the same stator winding. The rotors of these hypothetical motors rotate in opposite directions.
- Standstill Condition: When the rotor is stationary (at standstill), the equivalent circuit resembles that of a transformer with a short-circuited secondary. This reflects the fact that the motor essentially behaves as a transformer, with energy transferred from the stator to the rotor through magnetic induction.
- Running Condition: As the motor starts rotating, the slip values for the forward and backward rotating fields become different. The equivalent circuit incorporates these slip values to reflect the changing impedance and current distribution in the rotor. As the motor accelerates, the impedance associated with the forward rotating field decreases, leading to a higher current and a more prominent forward torque.
7. Types of Single-Phase Induction Motors
Single-phase induction motors are broadly classified based on the specific techniques used to achieve self-starting, as outlined in the “Method of Starting” section:
- Split-phase motors
- Capacitor-start motors
- Capacitor-run motors
- Shaded-pole motors
Each of these types utilizes different mechanisms to generate the initial rotating magnetic field required to start the motor, with varying levels of starting torque and complexity in construction.