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Number of poles

The number of pole pairs (p) or number of poles for an asynchronous motor determines the motor's synchronous speed:
Polzahl_Formel_1

Two-pole and four-pole asynchronous motors are typically used for centrifugal pumps. The rotational speed of these motors can be varied using frequency inverters.

The full-load speeds of asynchronous motors are approximately one percent below the synchronous speeds for high-output motors and up to six percent below the synchronous speeds for low-output motors, whereby synchronous speeds are almost reached at no load. 

See Fig. 1 Number of poles

Number of poles: Synchronous speeds of asynchronous motors Fig. 1 Number of poles: Synchronous speeds of asynchronous motors