Number of poles

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

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.