In this topic, you study Synchronous Speed & Frequency of Induced EMF.
N in the Expression (3.1) is called the synchronous speed and can be defined as the speed at which the machine must be run in order to generate the required frequency. It is normally denoted by NS. Thus,
Since the alternators operate at only one speed namely the synchronous speed, determined by the number of poles and the required line frequency, they are also called as synchronous generators. Consider frequency f = 50 hertz. For this frequency, speed of the alternators with different number of poles should be as given in the following table:
| No. of poles | Speed (r.p.m.) |
| 2 | 3000 |
| 4 | 1500 |
| 6 | 1000 |
| 8 | 750 |
| 10 | 600 |
| 12 | 500 |
| 14 | 250 |
FREQUENCY OF INDUCED EMF
In the case of an elementary two-pole, single-phase alternator shown in Fig. 3.1, we have seen that the emf generated in each conductor of the armature completes one cycle as it moves past both the poles (North and South). In actual practice, the alternator has more number of poles. Therefore, in general, it can be said that the emf generated in each conductor completes one cycle as it moves past one pair of poles. If
N = Speed, in revolutions per minute (r.p.m.)
P = Total number of poles, then
Number of cycles produced per revolution =
Hence, frequency of generated emf (i.e. number of cycles per second),
f = (No. of cycles per revolution) x (No. of revolutions per second)
= x
= hertz
Thus, there exists a definite relationship between the speed of rotation of the rotor (N), the frequency of the induced emf (f) and the number of poles (P).