2. SINGLE-PHASE INDUCTION MOTORS notes in english

 

2. SINGLE-PHASE INDUCTION MOTORS:

2.1 Double Field Revolving Theory:

The Double Field Revolving Theory explains how single-phase induction motors work. According to this theory, when a single-phase current is applied to the stator windings, it produces two equal and opposite rotating magnetic fields. One field rotates in the same direction as the applied current, and the other field rotates in the opposite direction.

These two fields combine to produce a rotating magnetic field, which induces a current in the rotor, thus enabling it to start rotating. However, because a single-phase current is not sufficient to create a constant rotating magnetic field, auxiliary mechanisms such as capacitors or extra windings are used to provide the necessary starting torque.

2.2 Principle of Making These Motors Self-Start:

Single-phase induction motors are not self-starting because a single-phase supply does not create a rotating magnetic field. To make these motors self-starting, various methods are used:

• Split-phase motors: These motors have two windings (main and auxiliary) that are connected in such a way that they create a phase difference, which results in a rotating magnetic field.
• Capacitor motors: These use a capacitor to create a phase shift between the two windings, which provides a starting torque.
• Shaded-pole motors: These motors use a shaded coil to create a rotating magnetic field in the stator to help start the motor.

By using these mechanisms, the motor is able to generate the initial torque required for starting.

2.3 Construction, Working, and Torque-Speed Characteristics of Following Motors:

• 2.3.1 Resistance Start Induction Run:

  • Construction: This motor uses a main winding and an auxiliary winding in which the auxiliary winding is connected with a resistance. The resistance is used to reduce the current in the auxiliary winding and create a phase difference, resulting in a rotating magnetic field.
  • Working: The motor starts with the help of the resistance connected in series with the auxiliary winding. Once the motor reaches a certain speed, the resistance is cut off, and the motor runs with just the main winding.
  • Torque-Speed Characteristics: This motor has a low starting torque and moderate running torque. The speed-torque curve is similar to that of a standard induction motor, but the starting torque is limited due to the use of resistance.

• 2.3.2 Capacitor Start Induction Run:

  • Construction: Similar to the resistance start motor, but instead of a resistor, a capacitor is connected in series with the auxiliary winding. The capacitor helps to create a larger phase difference between the two windings.
  • Working: The capacitor helps create a higher starting torque by improving the phase shift between the main and auxiliary windings. Once the motor reaches a certain speed, a centrifugal switch disconnects the capacitor, and the motor continues to run on the main winding.
  • Torque-Speed Characteristics: This motor has a high starting torque compared to the resistance start motor, and it provides better performance under load conditions.

• 2.3.3 Capacitor Start Capacitor Run:

  • Construction: This motor has two capacitors: one for starting (capacitor start) and one for running (capacitor run). Both capacitors are connected in series with the auxiliary winding.
  • Working: The starting capacitor is used to generate a high starting torque, while the running capacitor remains in the circuit throughout the operation of the motor to improve running efficiency and power factor.
  • Torque-Speed Characteristics: This motor provides both high starting torque and high efficiency during running. The torque-speed curve is similar to that of a three-phase motor, but with a lower starting current.

2.3 Introduction of:

• 2.3.4 Shaded Pole Motor:

  • Construction and Working: A shaded-pole motor has a simple construction with a stator having shaded poles (a part of the pole is wound with a copper coil). These shaded poles create a weak rotating magnetic field, which is enough to start the motor.
  • Applications: Shaded pole motors are generally used for small appliances like fans and small pumps due to their simplicity and low cost, though they have low efficiency and starting torque.

• 2.3.5 Repulsion Type Motor:

  • Construction and Working: The repulsion type motor is a special type of single-phase motor that uses a commutator and brushes along with the stator and rotor windings. The motor works on the principle of the repulsion of like magnetic poles. The interaction of the fields between the rotor and stator windings creates torque.
  • Applications: These motors are used where high starting torque is required, but they are generally less efficient than other types.

• 2.3.6 Series Motor:

  • Construction and Working: A series motor has the field winding and armature winding connected in series, meaning the same current flows through both. This motor is used for applications requiring high starting torque and variable speed operation.
  • Applications: Common in devices like electric trains, cranes, and other machinery requiring variable speed and high torque.

• 2.3.7 Universal Motor:

  • Construction and Working: A universal motor is a type of motor that can operate on both AC and DC supply. It has a wound armature and field windings, and it can run on single-phase AC or DC, which makes it versatile for various applications.
  • Applications: Commonly used in small household appliances, power tools, and other devices where high speed and portability are needed.

2.4 Motor Selection for Different Applications as Per the Load Torque-Speed Requirements:

The selection of the right single-phase motor depends on the torque-speed characteristics required by the application. For example:

• High starting torque: Motors like capacitor start induction run or repulsion motors are ideal.
• Low-cost, low-torque applications: Shaded pole motors are suitable for small devices like fans.
• Variable speed and high torque: Series motors or universal motors are good choices.

By analyzing the load torque-speed characteristics, engineers can choose the motor type that best fits the application requirements, ensuring optimal performance and efficiency.

2.5 Maintenance of Single-Phase Induction Motors:

Maintenance of single-phase induction motors includes:

• Regular inspection of windings: To prevent faults and ensure good insulation.
• Lubrication of bearings: To reduce friction and wear, improving motor lifespan.
• Checking for overheating: Ensuring the motor runs at appropriate temperatures to prevent damage.
• Cleaning: Regular cleaning of dust and debris from vents and windings to maintain proper airflow.
• Checking capacitor health: For motors with capacitors (such as capacitor start motors), it is essential to check the condition of the capacitors regularly to ensure proper functioning.

Regular maintenance helps to prevent downtime and extends the operational life of the motor.