A synchronous machine is an AC electrical machine that operates at a fixed speed directly linked to the supply frequency. Unlike other machines where speed varies with load, a synchronous machine always rotates at one specific speed — called the synchronous speed. This makes it unique and extremely important in power systems.
In this article, you will learn what a synchronous machine is, how it works, why we prefer rotating field construction, the synchronous speed formula, types of synchronous machines, and their real-world applications.
Table of Contents
What is a Synchronous Machine?
A synchronous machine is a rotating electrical machine whose rotor speed is exactly equal to the synchronous speed. The synchronous speed depends on two factors — the supply frequency and the number of poles of the machine.
The word "synchronous" means "in sync." The rotor of this machine rotates in perfect synchronism with the rotating magnetic field produced by the stator. There is no slip between the rotor speed and the field speed.
Synchronous machines can work as both generators and motors. When driven by a prime mover (like a turbine), they generate electricity. When connected to an AC supply, they run as motors at constant speed.
Working Principle
The working principle of a synchronous machine is based on Faraday's law of electromagnetic induction. An EMF is induced in a conductor whenever there is a relative motion between the conductor and a magnetic field.
In a synchronous machine, alternating EMF in the armature coil can be generated by two methods:
- Method 1: Rotating the armature coil in a stationary magnetic field
- Method 2: Keeping the armature stationary and rotating the field poles
In both cases, as long as there is relative motion between the armature conductors and the magnetic field, a voltage is generated. The output waveform is sinusoidal (a sine curve).
The majority of practical synchronous machines use Method 2 — rotating field with stationary armature. This design has several important advantages.
Why Rotating Field is Preferred
Almost all modern synchronous machines use a rotating field and stationary armature design. Here is why this arrangement is preferred over a rotating armature:
- Easy insulation: The stationary armature can be easily insulated for high voltages (11 kV, 33 kV or more) since it does not move
- No vibration on armature: Since armature windings are stationary, they are not subjected to centrifugal forces or mechanical vibrations
- Simple output connection: Output current is taken directly from fixed terminals on the stationary armature — no slip rings or brushes needed for the output
- Fewer slip rings: Only 2 slip rings are needed to supply DC excitation to the rotating field, compared to at least 3 slip rings (at high voltage) for a rotating armature
- Smaller machine size: The rotating field winding carries low-voltage DC, so it is lighter and more compact
- High-speed capability: The lighter rotating field can be designed for high-speed operation without structural problems
- Better cooling: Cooling the stationary armature is much easier since it is accessible from outside
Synchronous Speed Formula
The speed at which the magnetic field rotates in a synchronous machine is called the synchronous speed. It is determined by the supply frequency and the number of poles.
Where:
- Ns = Synchronous speed (in RPM)
- f = Supply frequency (in Hz)
- P = Number of poles
Example Calculation
For a 4-pole machine connected to a 50 Hz supply:
Standard Synchronous Speeds (50 Hz Supply)
Notice that as the number of poles increases, the synchronous speed decreases. Hydro generators have many poles (sometimes 40 or more) because water turbines rotate slowly.
Types of Synchronous Machines
Functionally, synchronous machines are classified into two main types:
1. Synchronous Generator (Alternator)
A synchronous generator converts mechanical energy into electrical energy. It is the most common source of electrical power in the world. Every thermal power plant, hydro power plant, and nuclear power plant uses synchronous generators.
The rotor is driven by a prime mover (steam turbine, gas turbine, or water turbine). The rotating magnetic field induces an alternating EMF in the stationary armature windings. You can learn more about the physical design in our article on construction of alternator.
2. Synchronous Motor
A synchronous motor converts electrical energy into mechanical energy at a constant speed equal to the synchronous speed. Unlike induction motors, it requires a separate DC excitation source for the rotor field winding.
Key characteristics of a synchronous motor:
- Runs at exactly synchronous speed regardless of load
- Not self-starting — needs an auxiliary starting method
- Can operate at leading, lagging, or unity power factor
- Used as a synchronous condenser for power factor improvement
Basic Construction Overview
A synchronous machine has two main parts:
Stator (Armature)
The stator carries the three-phase armature winding. It is made of laminated silicon steel to reduce iron losses. The stator core has slots where the copper windings are placed. This is where the output voltage is generated (in a generator) or where the supply is connected (in a motor).
Rotor (Field)
The rotor carries the field winding which is excited by DC current. Based on rotor construction, synchronous machines are of two types:
- Salient pole rotor: Has projecting poles. Used in low-speed machines (hydro generators). Typically has many poles (P = 12 to 60+).
- Cylindrical (non-salient) rotor: Has a smooth cylindrical shape with slots for field winding. Used in high-speed machines (turbo generators). Usually has 2 or 4 poles.
Applications of Synchronous Machines
As a Generator
- Power generation in thermal, hydro, nuclear, and gas power plants
- Standby and emergency power supply (diesel generators)
- Ship and aircraft power systems
As a Motor
- Driving constant-speed loads (compressors, pumps, mills)
- Power factor correction (synchronous condenser)
- Precision speed applications in textile and paper industries
As a Synchronous Condenser
When a synchronous motor runs at no load with over-excitation, it draws leading current from the supply. This property is used to improve the power factor of industrial loads. The V curve of synchronous motor explains this behavior in detail.
Synchronous Machine vs Induction Machine
For a detailed comparison, read our article on induction motor vs synchronous motor.
FAQs
What is the difference between synchronous and asynchronous machines?
A synchronous machine runs at exactly synchronous speed with zero slip. An asynchronous machine (induction machine) always runs at a speed slightly less than synchronous speed — it needs slip to produce torque.
Why is a synchronous motor not self-starting?
When AC supply is given, the stator field rotates at synchronous speed instantly. The rotor, due to its inertia, cannot accelerate from zero to synchronous speed immediately. The rapidly alternating torque averages to zero, so the rotor stays stationary. External starting methods like damper windings or a pony motor are needed.
Can a synchronous machine work as both motor and generator?
Yes. The same synchronous machine can work as a generator when driven by a prime mover, or as a motor when connected to an AC supply. The construction is essentially the same for both.
What happens if the load on a synchronous motor exceeds the pull-out torque?
If the load torque exceeds the maximum torque (pull-out torque), the rotor falls out of synchronism. The machine loses synchronization and stops. Protective relays typically disconnect the machine before damage occurs.
Why are synchronous generators used in power plants instead of induction generators?
Synchronous generators provide precise frequency control, can supply reactive power, and maintain stable voltage. Induction generators cannot control power factor and depend on the grid for excitation, making them unsuitable as primary power sources.
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