Short Answer:

Synchronous speed is the constant speed at which the magnetic field in the stator of an AC machine rotates. It is called “synchronous” because, in synchronous machines, the rotor turns at the same speed as the rotating magnetic field of the stator without any slip.

The value of synchronous speed depends on the frequency of the AC supply and the number of poles in the machine. It is calculated using the formula: Ns = (120 × f) / P, where f is the frequency in hertz and P is the number of poles.

Detailed Explanation:

Synchronous speed

Synchronous speed is a fundamental concept in AC electrical machines. It refers to the speed at which the rotating magnetic field produced by the stator windings moves around the stator. This speed is called “synchronous” because in synchronous motors and alternators, the rotor locks with this magnetic field and rotates at the same speed. Unlike induction motors, there is no slip between the stator field and the rotor in synchronous machines.

How Synchronous Speed is Produced:

In a three-phase AC machine, when a balanced three-phase current flows through the stator windings, it creates a rotating magnetic field. This magnetic field moves around the inside of the stator in a circular path. The speed at which this magnetic field rotates is the synchronous speed.

This rotating magnetic field interacts with the rotor. In synchronous machines, the rotor is excited using DC current, producing a steady magnetic field. When the rotor catches the rotating stator field, it locks in and rotates at the same speed—this is the synchronous operation.

Formula for Synchronous Speed:

The synchronous speed (Ns) is determined by:

Ns = (120 × f) / P

Where:

• Ns = synchronous speed in revolutions per minute (RPM)
• f = frequency of the AC supply in hertz (Hz)
• P = number of poles of the machine

Example:
If a machine has 4 poles and operates on a 50 Hz supply,
Ns = (120 × 50) / 4 = 1500 RPM

This means the magnetic field rotates at 1500 revolutions per minute.

Importance of Synchronous Speed:

1. Constant Speed Operation:
   In synchronous motors, the rotor runs exactly at synchronous speed, no matter how much the load changes (within limits). This is useful in machines that require constant speed.
2. Power Generation:
   In synchronous generators (alternators), the output frequency is directly related to the synchronous speed. So, to produce stable 50 Hz or 60 Hz electricity, the machine must maintain synchronous speed.
3. Machine Design:
   Engineers use synchronous speed while designing motors and generators. It helps determine the number of poles required to achieve a specific speed.
4. Comparison with Induction Motors:
   Unlike synchronous motors, induction motors always run slightly slower than synchronous speed. This difference is called slip.

Applications Based on Synchronous Speed:

• Synchronous generators in power stations for generating fixed frequency electricity
• Synchronous motors in industries for driving constant-speed machinery
• Synchronous condensers for power factor correction
• Used in clocks, timers, and other devices needing precise speed control

Conclusion:

Synchronous speed is the speed at which the rotating magnetic field travels in the stator of an AC machine. It is determined by the supply frequency and the number of poles. In synchronous machines, the rotor runs exactly at this speed, providing steady and reliable performance. This concept is essential for designing and operating motors and generators in power and industrial systems.