|Electrical Machines - Basic vocational knowledge (Institut für Berufliche Entwicklung, 144 p.)|
|5. Asynchronous motors|
|5.3. Operational behaviour|
Many drives, for example in the textile or wood-working industries, require speeds considerably greater than 3000 rpm.
Conversely, sometimes low speeds are similarly necessitated. Thus, speed control of the asynchronous motor becomes essential. Speed control possibilities for asynchronous motors can be derived from the following equation:
n = nD (l - s)
The speed established given a certain torque can therefore be influenced by:
- frequency change in the supplied three-phase alternating current through the employment of rotating frequency converters of through alternating or converter circuits with adjustable frequencies.
- altering the number of pole pairs through pole changing. This can ensue in two ways. Either by means of two or several separate stator windings which can be switched on as desired or by switching over parts of a single stator winding (Dahlander pole-changing circuit).
- altering the slip by changing the voltage application UI by means of series resistors or adjustable transformers, resp. by changing the ohmic resistance R2 in the rotor circuit (slip ring rotor).
The most commonly employed adjustment procedures are those of circuit engineering as cited in section 5.4.2.