What is a Synchronous Machine?
A synchronous machine is an AC machine whose rotor rotates at the same speed as the rotating magnetic field of the stator. It can function either as:
A synchronous generator (alternator), converting mechanical power to electrical power.
A synchronous motor, converting electrical power to mechanical power.
It runs strictly at synchronous speed and does not fall out of step under normal load variations.
1. Construction of Synchronous Machine
A synchronous machine consists of two main parts:
A. Stator (Stationary Part)
Houses the three-phase armature winding.
Constructed using laminated steel cores to reduce eddy current losses.
Connected to the AC power system in case of a motor or to the load in case of a generator.
Creates a rotating magnetic field when supplied with 3-phase AC.
B. Rotor (Rotating Part)
The rotor carries the field winding which is supplied with DC to create a magnetic field.
Rotor types:
1. Salient Pole Rotor (for low-speed applications like hydro-generators)
Projected poles.
Large diameter, small axial length.
2. Cylindrical (Non-Salient) Rotor (for high-speed applications like turbo-generators)
Smooth cylindrical shape.
Small diameter, long axial length.
Other rotor components:
Field winding: Excited with DC to produce the magnetic field.
Slip rings and brushes: Used to supply DC (in older machines).
Damper windings: Used to help start synchronous motors and for damping oscillations.
2. Working Principle of Synchronous Machine
The synchronous machine operates on the principle of electromagnetic interaction between the stator’s rotating magnetic field and the rotor’s magnetic field.
Synchronous Speed Formula
The speed of the rotating magnetic field is given by:
Ns= 120f/p
Where:
Ns = synchronous speed (RPM)
F = frequency of AC supply (Hz)
P = number of poles on the stator
Generator Mode (Alternator)
Mechanical energy (e.g., from a turbine) rotates the rotor.
The DC-excited rotor produces a magnetic field.
As it rotates, the rotor’s field cuts the stator windings, inducing EMF per Faraday's Law.
The frequency of the generated voltage is directly related to the speed of the rotor.
The stator is supplied with 3-phase AC to produce a rotating magnetic field.
The rotor, excited with DC, locks in synchronism with the stator’s field.
The torque is produced due to alignment of magnetic poles.
The rotor rotates at exactly synchronous speed.
4. Phasor Diagram of Synchronous Generator
In generator mode:
Ef: internal generated EMF
V: terminal voltage
Ia: armature current
Xsla: voltage drop across synchronous reactance
Ef = V + jXsIa
Depending on the power factor, the phasor diagram changes shape.
5. Advantages of Synchronous Machine
Constant Speed Operation: Speed remains constant regardless of load (suitable for precision drives).
Power Factor Correction: Can be operated at leading power factor to improve system power factor.
Used in Large Power Applications: Very efficient and cost-effective for large-scale generation.
Stable Parallel Operation: Good stability when connected in parallel with other generators.
6. Disadvantages of Synchronous Machine
Not Self-Starting (in motors): Needs external means (induction motor or damper windings) to bring rotor up to speed.
Complex Construction: Requires DC excitation system and control.
Cost: Higher initial and maintenance cost compared to induction machines.
Sensitivity to Load Changes: Not ideal for fluctuating load unless properly controlled.
Applications of Synchronous Machines
Power Plants: As generators in thermal, hydro, and nuclear power stations.
Industrial Plants: Synchronous motors for driving large compressors, pumps, and crushers.
Power Factor Correction: Synchronous condensers in power systems.
Grid Support: Voltage and frequency regulation.
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