Synchronous speed is calculated by a formula
Understanding Synchronous Speed in AC Machines
Synchronous speed is a fundamental concept in understanding how AC electrical machines, like induction motors and synchronous motors, operate. It represents the speed at which the magnetic field produced by the stator windings rotates.
This rotating magnetic field is created by the alternating current flowing through the stator coils. The speed of this field depends directly on the frequency of the power supply and the number of magnetic poles in the stator winding.
Synchronous Speed Formula Explained
The speed of the rotating magnetic field, known as the synchronous speed (\(N_s\)), is determined by the following formula:
\(N_s = \frac{{120f}}{p}\)
Where:
- \(N_s\) is the synchronous speed in revolutions per minute (RPM).
- \(f\) is the frequency of the AC power supply in Hertz (Hz).
- \(p\) is the number of poles per phase in the stator winding.
The factor 120 comes from converting cycles per second (Hz) to revolutions per minute (RPM) and accounting for the fact that a magnetic field completes one revolution in two poles (a pole pair).
Components of the Synchronous Speed Formula
Let's look at the terms in the synchronous speed formula:
| Symbol | Description | Units |
|---|---|---|
| \(N_s\) | Synchronous Speed (Speed of rotating magnetic field) | RPM (Revolutions Per Minute) |
| \(f\) | Frequency of AC Power Supply | Hz (Hertz) |
| \(p\) | Number of Poles in Stator Winding | (Unitless, number of poles) |
Understanding these components helps explain why a motor's synchronous speed changes if you change the frequency or the number of poles.
How Frequency and Poles Affect Synchronous Speed
From the formula \(N_s = \frac{{120f}}{p}\), we can see the relationships:
- Frequency (\(f\)): If the frequency increases, the synchronous speed increases proportionally, assuming the number of poles remains constant.
- Number of Poles (\(p\)): If the number of poles increases, the synchronous speed decreases, assuming the frequency remains constant. Motors can be designed with different numbers of poles (e.g., 2, 4, 6, 8 poles) to achieve different synchronous speeds for a given frequency.
Revision Table: Synchronous Speed Formula
Here is a quick summary of the key formula for synchronous speed:
| Concept | Formula | Variables |
|---|---|---|
| Synchronous Speed | \(N_s = \frac{{120f}}{p}\) | \(N_s\): Synchronous Speed (RPM) \(f\): Frequency (Hz) \(p\): Number of Poles |
Additional Information on Synchronous Speed
While synchronous speed is the speed of the rotating magnetic field, the actual speed of an AC induction motor rotor is always slightly less than the synchronous speed. This difference is called 'slip'. Synchronous motors, however, run exactly at synchronous speed under steady-state conditions.
The number of poles \(p\) is always an even integer (e.g., 2, 4, 6) because poles are created in pairs (North and South).
Important Questions from Alternator and Synchronous Motors
The armature current of a synchronous motor has large value for-
An alternator has 20 poles and running at 300 RPM will generate alternating voltage and current whose frequency is-
Synchronous motor when used for power factor improvement should be-
In an alternator, the _______ current is generated in the stationary stator.
The speed with which the turbo alternators operate are-
