4. SYNCHRONOUS MOTOR notes in english

 

4. SYNCHRONOUS MOTOR

A synchronous motor is an alternating current (AC) motor that operates at a constant speed, known as synchronous speed, regardless of the load on the motor. It is widely used in applications requiring precise speed control, such as in clocks, large machines, and industrial drives.

4.1 Principle of Working / Operation:

The working principle of a synchronous motor is based on the interaction between a rotating magnetic field produced by the stator and the magnetic field produced by the rotor.

• Stator: The stator of the motor is powered by three-phase AC supply, which produces a rotating magnetic field. This rotating field is at a constant speed, known as the synchronous speed.

• Rotor: The rotor of a synchronous motor is made to rotate at the same speed as the rotating magnetic field produced by the stator, hence the term "synchronous." The rotor can be of two types: salient pole or cylindrical pole.

• Excitation: The rotor of the synchronous motor is excited with DC current, which produces a magnetic field. This rotor magnetic field locks in synchrony with the stator's rotating magnetic field, causing the rotor to rotate at the same speed (synchronous speed).

• Synchronous Speed: The motor runs at a speed determined by the frequency of the AC supply and the number of poles in the stator, given by the formula:

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

  Where:

  • $N_s$ = Synchronous speed in RPM
  • $f$ = Supply frequency in Hz
  • $P$ = Number of poles in the motor

At synchronous speed, the rotor rotates in perfect synchrony with the stator's rotating magnetic field, and the motor does not slip like an induction motor.

4.2 Significance of Load Angle:

The load angle (also known as the power angle or torque angle) refers to the angle between the rotor's magnetic field and the stator's magnetic field in a synchronous motor. This angle plays a significant role in determining the torque and power delivered by the motor.

• Load Angle and Torque: As the load on the motor increases, the load angle increases. The motor generates torque to oppose the load, and the load angle increases proportionally. The motor produces the required torque to meet the load demand by increasing the load angle.

• Power Transfer: The power delivered by the synchronous motor depends on the load angle. If the load angle becomes too large, the motor may lose synchrony, and it could stall. For a motor to remain in synchronous operation, the load angle must be within a certain range, which is typically 0 to 90 degrees.

• Stability: A large load angle indicates that the motor is under heavy load. If the angle exceeds a critical value, the motor may lose synchronization and begin to slip. The load angle is thus essential for maintaining stable operation of the synchronous motor.

4.3 Torque: Starting Torque, Running Torque, Pull-in Torque, Pull-out Torque:

Synchronous motors have unique torque characteristics compared to induction motors.

1. Starting Torque: Synchronous motors do not produce starting torque in the same way as induction motors. To start a synchronous motor, external methods such as a damper winding or an auxiliary motor are typically used. Upon reaching synchronous speed, the rotor locks into the rotating magnetic field of the stator, and the motor begins to run.

2. Running Torque: Once the synchronous motor reaches synchronous speed, it produces constant running torque to meet the load requirements. The torque is almost independent of the speed, as the motor operates at a constant speed under normal conditions.

3. Pull-in Torque: The pull-in torque is the torque required to pull the motor into synchrony. When the motor is started, it initially operates at sub-synchronous speed and gradually speeds up. The pull-in torque is the torque needed to bring the rotor up to synchronous speed and lock it with the rotating magnetic field.

4. Pull-out Torque: The pull-out torque is the maximum torque that the motor can generate while maintaining synchrony. If the load exceeds the pull-out torque, the motor will lose synchrony and start to slip. It is a critical value because it determines the maximum load the motor can handle before it loses synchronization.

4.4 Synchronous Motor on Load with Constant Excitation (No Numerical):

When a synchronous motor is loaded, the amount of excitation (DC current in the rotor) remains constant. In such cases:

• The motor operates at a constant synchronous speed, but the torque increases or decreases based on the load applied to the motor.
• The load angle (or power angle) increases with the load. The motor generates torque to meet the load, and this increase in the load angle indicates a higher load.
• Under constant excitation, the motor will produce a constant magnetomotive force (MMF) in the rotor, and the power delivered to the load is determined by the torque and the speed.
• The motor continues to operate at synchronous speed, but any variations in load are accommodated by variations in the load angle.

The synchronous motor is highly efficient when operating at a constant load and with constant excitation because it does not experience the losses associated with slip, unlike induction motors.

4.5 Methods of Starting of Synchronous Motor:

Synchronous motors cannot start directly under normal conditions because they require the rotor to reach synchronous speed to lock into the stator's magnetic field. Therefore, special methods are employed to start the motor:

1. Using an Auxiliary Induction Motor: An induction motor is used to bring the synchronous motor up to near synchronous speed. Once the motor reaches synchronous speed, the excitation is applied to the rotor, and it locks in synchrony with the stator field.

2. Damper Windings: Damper windings are placed in the rotor to allow the motor to start as an induction motor. These windings help the rotor reach synchronous speed. Once the motor reaches synchronous speed, the excitation is applied to the rotor, and the motor continues to run synchronously.

3. Auto-Starting using Variable Frequency Drive (VFD): A VFD is used to control the speed of the motor. The VFD adjusts the frequency of the AC supply to gradually bring the synchronous motor up to synchronous speed. The excitation is applied once the motor reaches the desired speed.

4. Using a Star-Delta Starter: For motors with low power ratings, a star-delta starter can be used to start the motor. The motor is initially connected in star configuration to reduce the voltage and current, and once the motor reaches a certain speed, it is switched to delta configuration for full voltage operation.

4.6 Losses in Synchronous Motors and Efficiency (No Numerical):

Like all electric motors, synchronous motors experience various losses that affect their efficiency:

1. Stator Copper Loss: The current flowing through the stator windings generates resistance losses, known as copper losses. These losses are proportional to the square of the current.

2. Rotor Copper Loss: In synchronous motors with salient pole rotors or wound rotors, there are copper losses due to the resistance of the rotor windings.

3. Core Losses: The magnetic core of the stator and rotor experiences losses due to hysteresis and eddy currents. These losses are present even if the motor is not under load.

4. Friction and Windage Losses: The bearings and rotor shaft produce frictional losses. Additionally, the movement of the rotor through the air causes windage losses due to the friction between the air and the rotor.

5. Excitation Losses: In synchronous motors with separate excitation systems (DC excitation), there are losses in the excitation system itself, including losses in the exciter and associated components.

Efficiency: The efficiency of a synchronous motor is determined by the ratio of the mechanical power output to the electrical power input. The motor is typically very efficient under constant load conditions, but its efficiency can decrease if the motor operates off the synchronous speed or if the excitation is not optimized.