4.1 Synchronous machine construction and principles

Synchronous machines are the backbone of power systems, converting mechanical energy to electrical and vice versa. They consist of a stationary stator with armature windings and a rotating rotor with field windings, working together to generate or consume electrical power.

Understanding synchronous machine construction and principles is crucial for grasping their dynamic behavior in power systems. From rotor types to excitation systems, these elements shape machine performance, stability, and control strategies in real-world applications.

Synchronous machine components

Stator construction and armature winding

• Synchronous machines consist of a stationary part called the stator, which contains the armature winding connected to the external electrical system
• The stator is constructed using a cylindrical core made of laminated steel sheets to reduce eddy current losses (hysteresis losses)
• The stator core has slots along its inner periphery to accommodate the distributed three-phase armature winding displaced by 120 degrees electrical
• The armature winding is made of insulated copper conductors to minimize electrical losses and improve efficiency

Rotor types and field winding

• The rotating part of a synchronous machine is called the rotor, which contains the field winding excited by a DC current to create a magnetic field
• Rotors can be of two types: salient-pole or cylindrical
  • Salient-pole rotors have projecting poles with concentrated field windings and are used in low-speed machines (hydroelectric generators)
  • Cylindrical rotors have a smooth cylindrical surface with distributed field windings and are used in high-speed machines (turbo-generators)
• The field winding on the rotor is supplied with DC current through slip rings and brushes or a brushless excitation system using a rotating rectifier
• The rotor is supported by bearings at both ends, allowing it to rotate freely within the stator, and the shaft is coupled to the prime mover (generators) or the mechanical load (motors)

Operation of synchronous generators and motors

Synchronous generator operation

• Synchronous generators convert mechanical energy into electrical energy through electromagnetic induction
• When driven by a prime mover, the rotating magnetic field produced by the rotor's field winding induces an alternating voltage in the stator armature winding (Faraday's law)
• The induced voltage frequency depends on the rotation speed and the number of poles, with synchronous speed (rpm) given by: $N_s = 120f / P$, where $f$ is the frequency (Hz) and $P$ is the number of poles
• The induced voltage magnitude depends on the rotor magnetic field strength, controlled by the field current, and the rotation speed

Synchronous motor operation

• Synchronous motors convert electrical energy into mechanical energy by creating a rotating magnetic field in the stator that interacts with the rotor's magnetic field
• When a three-phase voltage is applied to the stator armature winding, it creates a rotating magnetic field that causes the rotor to synchronize and rotate at synchronous speed
• The developed torque in a synchronous motor depends on the magnitude of the stator and rotor magnetic fields and the angle between them, known as the power angle or torque angle
• In both generators and motors, the stator and rotor magnetic fields must be synchronized for effective operation, and any disturbance to this synchronism can lead to stability issues

Factors affecting synchronous machine performance

Excitation current and power factor

• The excitation current supplied to the rotor field winding determines the rotor magnetic field strength, directly impacting the induced voltage (generators) or developed torque (motors)
• The power factor, the ratio of active power to apparent power, depends on the excitation current and load characteristics
  • Overexcited machines have a leading power factor, while underexcited machines have a lagging power factor
• Armature reaction, caused by the magnetic field produced by stator currents, can distort the main magnetic field and affect performance, but can be compensated by adjusting the excitation current

Machine parameters and losses

• The saliency of the rotor, the difference between direct-axis and quadrature-axis reactances, influences the machine's performance, particularly its torque characteristics and power factor
• Stator winding resistance and leakage reactance cause voltage drops and affect the machine's terminal voltage
• Mechanical losses (friction and windage) and electrical losses (copper losses in windings and core losses in the magnetic circuit) impact the machine's efficiency
• The cooling system employed (air, hydrogen, or water cooling) affects the machine's power density and thermal performance

Mechanical vs electrical aspects of synchronous machines

Power balance and torque

• The prime mover or mechanical load is directly coupled to the rotor shaft, establishing a mechanical connection, with torque and speed determining the mechanical power input (generators) or output (motors)
• Electrical power output (generators) or input (motors) is determined by the induced voltage, stator current, and power factor, given by: $P = \sqrt{3} \times V \times I \times \cos\phi$, where $V$ is the terminal voltage, $I$ is the stator current, and $\cos\phi$ is the power factor
• The power balance equation governs the relationship between mechanical and electrical power, considering machine losses, and in steady-state operation, the mechanical power input (generators) or electrical power input (motors) must equal the sum of the electrical power output (generators) or mechanical power output (motors) and the total losses
• Developed torque is proportional to the product of the stator and rotor magnetic fields and the sine of the power angle, which depends on the balance between the mechanical and electrical torques acting on the rotor

Stability and excitation control

• Any mismatch between the mechanical and electrical torques will cause the rotor to accelerate or decelerate, leading to a change in the power angle and a corresponding change in the electrical power output (generators) or mechanical power output (motors)
• Synchronous machine stability depends on its ability to maintain synchronism between the rotor and stator magnetic fields under various operating conditions and disturbances
• The machine's inertia, damping, and synchronizing torque play crucial roles in maintaining stability
• The excitation system, which controls the rotor field current, acts as a link between the mechanical and electrical aspects of the machine, regulating the terminal voltage (generators) or reactive power (motors) and helping to maintain stability