Synchronous Speeds
The output frequency of an alternator depends on the number of poles and the rotational speed. The speed corresponding to a particular frequency is called the synchronous speed for that frequency. This table gives some examples:
| Poles | RPM for 50 Hz | RPM for 60 Hz | RPM for 400 Hz |
|---|---|---|---|
| 2 | 3,000 | 3,600 | 24,000 |
| 4 | 1,500 | 1,800 | 12,000 |
| 6 | 1,000 | 1,200 | 8,000 |
| 8 | 750 | 900 | 6,000 |
| 10 | 600 | 720 | 4,800 |
| 12 | 500 | 600 | 4,000 |
| 14 | 428.6 | 514.3 | 3,429 |
| 16 | 375 | 450 | 3,000 |
| 18 | 333.3 | 400 | 2,667 |
| 20 | 300 | 360 | 2,400 |
| 40 | 150 | 180 | 1,200 |
Key
- Rotational speeds are given in revolutions per minute (RPM)
- Frequencies are given in Hertz (Hz)
More generally, one cycle of alternating current is produced each time a pair of field poles passes over a point on the stationary winding. The relation between speed and frequency is, where is the frequency in Hz (cycles per second). is the number of poles (2,4,6...) and is the rotational speed in revolutions per minute (RPM). Very old descriptions of alternating current systems sometimes give the frequency in terms of alternations per minute, counting each half-cycle as one alternation; so 12,000 alternations per minute corresponds to 100 Hz.
Read more about this topic: Alternator
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