Synchronous Machines

Constant-speed AC machines used as motors and generators in power systems

⚙️ Introduction

Synchronous machines operate at synchronous speed regardless of load.
Types:
- Alternator – works as a generator
- Synchronous Motor – works as a motor

⚙️ Construction

Stator: 3-phase winding producing rotating magnetic field.
Rotor: Salient or cylindrical pole, excited by DC supply.

⚙️ Working Principle

Rotor locks with rotating magnetic field and rotates at synchronous speed.
No slip between stator field and rotor.

Synchronous Speed (Ns) = 120 × f / P

⚙️ Synchronous Generator (Alternator)

Converts mechanical power to AC electrical power.
EMF is induced in stator due to rotating rotor field.

EMF = 4.44 × f × Φ × N × K_p × K_d

⚙️ Synchronous Motor

Runs at constant speed regardless of load.
Not self-starting. Needs auxiliary motor or damper winding to start.
Operates on leading, lagging or unity power factor – improves PF in industries.

⚙️ Starting Methods

Damper winding (acts like squirrel cage for startup)
Pony motor
Variable frequency drives (VFD)

⚙️ Applications

Alternators in power plants
Constant speed industrial motors
Power factor correction

📘 Previous Year Questions

Q: Why is a synchronous motor not self-starting?
Ans: Due to absence of starting torque.
Q: What is the function of damper winding?
Ans: To provide starting torque and dampen oscillations.
Q: Calculate frequency of 6-pole alternator running at 1000 RPM.
Ans: f = (P × N) / 120 = (6×1000)/120 = 50 Hz
Q: Which machine improves power factor in industry?
Ans: Synchronous motor