1. THREE PHASE INDUCTION MOTOR Notes in English

 

1. THREE PHASE INDUCTION MOTOR:

1.1 Working Principle:

A three-phase induction motor operates on the principle of electromagnetic induction. When a three-phase current is supplied to the stator windings, it produces a rotating magnetic field. This rotating field induces a current in the rotor, which in turn generates its own magnetic field. The interaction between the rotating stator field and the rotor field produces torque, causing the rotor to rotate.

1.2 Production of Rotating Magnetic Field:

The rotating magnetic field in an induction motor is created by the three-phase currents that are 120° out of phase with each other. These three currents produce a magnetic field that rotates in space at a constant speed, which is called the synchronous speed. The speed of this rotating magnetic field is determined by the frequency of the supply current and the number of poles in the motor.

1.3 Synchronous Speed:

The synchronous speed is the speed at which the magnetic field produced by the stator rotates. It is given by the formula:

$$N_s = \frac{120 \times f}{P}$$

Where:

• $N_s$ = Synchronous speed (in RPM)
• $f$ = Frequency of the supply (in Hz)
• $P$ = Number of poles in the motor

The synchronous speed is constant and depends only on the supply frequency and the number of poles in the motor.

1.4 Rotor:

The rotor is the rotating part of the motor. It is situated inside the stator and experiences the induced electromotive force (emf) from the rotating magnetic field. The rotor can be of two types:

• Squirrel Cage Rotor: The most common type, with a simple structure of copper or aluminum bars short-circuited at both ends.
• Slip Ring Rotor: Has external brushes and rings that allow for external resistances to be added for controlling the motor's speed and torque.

1.5 Slip:

Slip refers to the difference between the synchronous speed and the actual rotor speed, expressed as a percentage of the synchronous speed:

$$Slip = \frac{N_s - N_r}{N_s} \times 100$$

Where:

• $N_s$ = Synchronous speed
• $N_r$ = Rotor speed

Slip is essential because it ensures the relative motion between the rotating magnetic field and the rotor, which is required to induce current and generate torque.

1.6 Construction of 3-Phase Induction Motors:

• 1.6.1 Squirrel Cage Induction Motor: The squirrel cage rotor consists of laminated iron cores with bars of conducting material (copper or aluminum) placed in the slots. The ends of the bars are short-circuited by rings, forming a cage-like structure. This is the most widely used type due to its simplicity and ruggedness.
• 1.6.2 Slip Ring Induction Motor: This rotor has windings instead of conducting bars. The windings are connected to external resistances via slip rings and brushes, which allow for control of starting current and torque.

1.7 Rotor Qualities:

• 1.7.1 Frequency: The frequency of the rotor current is the difference between the supply frequency and the frequency of the rotor’s rotation. The rotor current frequency is less than the stator supply frequency and is given by:

  $$f_r = f \times (1 - s)$$

  Where $f_r$ is the rotor frequency and $s$ is the slip.

• 1.7.2 Induction emf: The induction emf is the voltage induced in the rotor due to the rotating magnetic field from the stator. This emf is responsible for inducing current in the rotor, which interacts with the magnetic field to produce torque.

• 1.7.3 Power Factor at Starting and Running Condition: At starting, the power factor is low due to the high slip and the high current drawn by the motor. When the motor reaches running conditions, the power factor improves as the slip decreases, and the motor operates closer to synchronous speed.

1.8 Characteristics of Torque versus Slip (Speed):

• At synchronous speed, the slip is zero, and no torque is produced.
• As the motor accelerates and slip increases, torque increases. The torque reaches its maximum value when the slip is around 0.2-0.3 (i.e., the rotor speed is about 80-70% of the synchronous speed).
• At higher slips (when the motor is running at lower speeds), torque decreases again.

1.9 Torques: Starting, Full Load, and Maximum with Relations Among Them:

• Starting Torque: This is the torque developed when the motor is first started and is typically high for squirrel cage motors.
• Full Load Torque: This is the torque developed when the motor is operating under its rated load, and the motor’s slip is generally small.
• Maximum Torque: This is the maximum torque the motor can produce, typically occurring at a slip of around 0.2-0.3.

The relationship between these torques can be summarized as:

• The starting torque is higher than the full-load torque, especially in squirrel cage motors.
• The maximum torque occurs at a certain value of slip, and beyond that, torque starts to decrease.

1.10 Induction Motor as a Generalized Transformation with Phasor Diagram:

An induction motor can be viewed as a generalized transformer where the stator is the primary and the rotor is the secondary. The power delivered to the rotor is transformed into mechanical power. The phasor diagram of an induction motor helps visualize the relationships between the applied voltage, the stator current, and the rotor current, showing the phase differences and power flow at various operational conditions.

1.11 Starters:

• 1.11.1 Need and Types: A starter is required to limit the starting current, which can be several times the full-load current. Without a starter, this high starting current could damage the motor or the power supply.
• 1.11.2 Stator Resistance: In this type of starter, resistance is added in series with the stator windings to limit the starting current. It is commonly used for small motors.
• 1.11.3 Auto Transformer: An auto-transformer reduces the voltage supplied to the motor during startup, thus reducing the starting current. Once the motor reaches a certain speed, the full voltage is applied.
• 1.11.4 Star Delta: This starter connects the motor windings in a star configuration during startup (which reduces voltage and current), and then switches to the delta configuration once the motor reaches full speed.
• 1.11.5 Rotor Resistance: In this starter, additional resistance is added to the rotor circuit to control the starting current and torque.

1.12 Maintenance of Three-Phase Induction Motors:

Regular maintenance ensures the reliable operation of induction motors. Common practices include:

• Checking insulation resistance to prevent winding faults.
• Monitoring vibration and noise to detect mechanical issues.
• Lubricating bearings to reduce wear.
• Ensuring proper cooling to prevent overheating.
• Cleaning air vents and ensuring airflow for ventilation.

This maintenance helps prolong the life of the motor and maintain efficient operation.