Short Answer:

Synchronous machines are classified mainly based on their operation and construction. Based on operation, they are divided into synchronous generators (alternators) and synchronous motors. Based on rotor construction, they are classified into salient pole and non-salient (cylindrical) pole types.

This classification helps in selecting the right type of synchronous machine for different applications. Salient pole machines are used in low-speed applications like hydroelectric plants, while cylindrical pole machines are used in high-speed applications like thermal power stations and industrial drives.

Detailed Explanation:

Classification of synchronous machines

Synchronous machines are a type of AC machine that operates at constant speed regardless of load. They are very important in power generation and industrial processes. To understand their use better, we classify synchronous machines based on their function and rotor construction.

1. Classification Based on Operation:

Synchronous machines are first divided into two categories depending on how they are used:

• Synchronous Generator (Alternator):
  These machines convert mechanical energy into electrical energy. When the rotor is driven by a turbine or engine, and DC excitation is given, they generate AC voltage in the stator. Alternators are used in power stations for electricity generation.
• Synchronous Motor:
  These machines convert electrical energy into mechanical energy. When supplied with three-phase AC to the stator and DC to the rotor, the rotor locks in with the rotating magnetic field and rotates at synchronous speed. These are used in industries where constant speed is needed under varying load.

2. Classification Based on Rotor Construction:

Depending on the design of the rotor, synchronous machines are further classified as:

• Salient Pole Rotor (Projected Pole Type):
  This type has large poles projecting out from the rotor surface. The poles are made of laminated steel and are spaced around the rotor.
  Features:
  • Used in low-speed machines (up to 1000 RPM)
  • Found in hydroelectric generators
  • Easier to construct and cool
  • High pole number (more than 4)
  • Non-uniform air gap
• Non-Salient Pole Rotor (Cylindrical or Smooth Pole):
  In this type, the rotor has a smooth cylindrical shape with field windings placed in slots. There are no distinct projecting poles.
  Features:
  • Used in high-speed machines (1500 RPM and above)
  • Common in steam and gas turbine alternators
  • Strong and well-balanced for high-speed rotation
  • Uniform air gap
  • Lower pole number (usually 2 or 4)

3. Other Classifications (Optional/Advanced):

Although not commonly required, synchronous machines can also be classified by excitation method:

• Electrically Excited: Using external DC supply
• Self-Excited or Brushless: Using rotating rectifiers
• Permanent Magnet Synchronous Machines (PMSM): Rotor contains permanent magnets

Importance of Classification:

Understanding the classification helps engineers and designers choose the right machine for a specific application. For example:

• Hydropower → Salient pole alternator
• Thermal power → Cylindrical rotor alternator
• Constant speed motor → Synchronous motor
• High-efficiency appliances → PMSM

Conclusion:

Synchronous machines are mainly classified based on their function (as generator or motor) and rotor type (salient or cylindrical). This classification helps in selecting machines suitable for specific speed, construction, and performance needs in power plants and industries. Knowing these types ensures proper usage and better efficiency of the system.