Synchronous Motor

Synchronous motors are AC motors that run at synchronous speed, where the rotor speed is synchronized with the frequency of the supply current. They are widely used in applications requiring precise and constant speed control.

Key Components and Structure

1. Stator:

   • Function: Produces a rotating magnetic field when supplied with AC power.
   • Construction: Made of laminated steel cores with embedded three-phase windings.

2. Rotor:

   • Salient Pole Rotor: Used in low-speed applications with a large diameter and poles that project outward.
   • Non-Salient Pole (Cylindrical) Rotor: Used in high-speed applications with a smooth cylindrical shape.
   • Field Windings: Supplied with DC power to create a constant magnetic field.
   • Damper Windings: Copper bars placed in the pole faces to help in starting and reduce oscillations.

3. Exciter:

   • Function: Supplies the DC power to the rotor field windings.
   • Types:
     • Static Exciter: Uses solid-state devices to provide excitation.
     • Rotary Exciter: A small DC generator mounted on the same shaft as the motor.

4. Slip Rings and Brushes:

   • Function: Transfer DC power from the exciter to the rotor field windings.
   • Construction: Slip rings are metal rings mounted on the rotor shaft, with brushes making sliding contact.

Operating Principle

Synchronous motors operate based on the principle of magnetic locking between the stator's rotating magnetic field and the rotor's constant magnetic field.

1. Magnetic Locking:

   • The stator windings create a rotating magnetic field when connected to a three-phase AC supply.
   • The rotor, excited with DC power, produces a constant magnetic field.
   • The interaction between these fields causes the rotor to lock in synchronism with the rotating magnetic field of the stator.

2. Constant Speed Operation:

   • The motor operates at synchronous speed given by the formula: $$N_s = \frac{120 \times f}{P}$$ where $N_s$ is the synchronous speed in RPM, $f$ is the supply frequency in Hz, and $P$ is the number of poles.

Starting Methods

Starting a synchronous motor is challenging because it cannot start on its own from standstill. Various methods are employed to start synchronous motors:

1. Direct-On-Line (DOL) Starting:

   • Suitable for small synchronous motors.
   • Motor starts directly connected to the supply but faces high inrush current.

2. Using Damper Windings:

   • The motor starts as an induction motor using damper windings, which help the rotor accelerate to near-synchronous speed before DC excitation is applied.

3. Using a Separate Starting Motor:

   • An auxiliary motor is used to bring the synchronous motor to synchronous speed, after which the rotor excitation is applied.

4. Variable Frequency Drive (VFD):

   • Gradually increases the supply frequency from zero to the rated frequency, allowing smooth acceleration to synchronous speed.

Advantages

1. Constant Speed:

   • Maintains precise speed irrespective of load variations.

2. High Efficiency:

   • Excellent at converting electrical power to mechanical power.

3. Power Factor Correction:

   • Can be adjusted to operate at leading, lagging, or unity power factor.

4. Stability:

   • Provides stable operation with minimal speed fluctuations.

5. Durability:

   • Robust construction leads to long operational life.

Disadvantages

1. Complex Starting Mechanism:

   • Requires auxiliary devices or methods to start.

2. Higher Initial Cost:

   • More expensive than induction motors due to the excitation system.

3. Maintenance:

   • Requires regular maintenance of slip rings and brushes.

Applications

1. Industrial Drives:

   • Used in conveyors, compressors, crushers, and other constant speed applications.

2. Power Generation:

   • Employed as synchronous generators in power plants.

3. Power Factor Correction:

   • Improves overall power factor in power systems by operating in a leading power factor mode.

4. Pumps and Blowers:

   • Suitable for applications needing precise speed control and high efficiency.

5. Mills and Rolling:

   • Used in steel mills and rolling applications for constant speed requirements.

Detailed Steps for Connecting and Operating a Synchronous Motor

1. Selection:

   • Choose a synchronous motor based on the speed, load, and power requirements.
   • Verify the motor’s voltage and frequency ratings match the power supply.

2. Installation:

   • Mounting: Securely mount the motor on a stable base.
   • Alignment: Properly align the motor with the load to prevent mechanical stress.

3. Power Connections:

   • Stator: Connect the stator windings to the three-phase AC power supply.
   • Rotor: Connect the rotor field windings to the DC exciter via slip rings and brushes.

4. Control Circuit:

   • Starting Method: Implement the chosen starting method (damper windings, auxiliary motor, or VFD).
   • Excitation Control: Set up control circuits for adjusting the field excitation.

5. Starting the Motor:

   • Initial Start: Use the auxiliary method to bring the motor to near-synchronous speed.
   • Synchronization: Apply DC excitation to the rotor to synchronize with the stator’s rotating magnetic field.

6. Operation and Monitoring:

   • Load Connection: Gradually apply the load and monitor motor performance.
   • Speed and Stability: Ensure the motor runs at synchronous speed with stable operation.
   • Power Factor: Adjust field excitation to maintain the desired power factor.

7. Maintenance:

   • Regular Inspection: Inspect the motor for wear, overheating, and vibrations.
   • Brushes and Slip Rings: Check and maintain brushes and slip rings for good electrical contact.
   • Cooling System: Ensure the cooling system is functioning properly to prevent overheating.

Additional Technical Details

1. Excitation System:

   • Manual Excitation: Allows manual adjustment of the rotor field current to control the motor’s power factor.
   • Automatic Voltage Regulator (AVR): Automatically adjusts the field current to maintain constant voltage output in generators.

2. Power Factor Control:

   • By varying the field excitation, synchronous motors can be made to operate at leading, lagging, or unity power factor. This flexibility helps in power factor correction in industrial plants.

3. Load Characteristics:

   • Constant Torque Applications: Suitable for applications where the load torque remains constant with varying speeds.
   • Variable Torque Applications: Used in applications where torque varies with speed, such as centrifugal pumps and fans.

4. Speed Control:

   • Although synchronous motors run at a constant speed, precise speed control can be achieved using VFDs by varying the supply frequency.

Summary

Synchronous motors are essential in applications requiring constant speed, high efficiency, and power factor correction. Their operation involves magnetic locking between the stator's rotating magnetic field and the rotor's constant magnetic field. With various starting methods and precise control capabilities, synchronous motors are widely used in industrial drives, power generation, and power factor correction. Proper selection, installation, and maintenance are crucial for their optimal performance and longevity.