# Mechanical speed

The mechanical speed is the speed of the rotor measured in [rpm] or [1/s].

The mechanical speed can be calculated as the electrical frequency divided by the number of pole pairs $p/2$. Assuming an electrical frequency $f$ of 50 Hz and a 4-pole machine, the mechanical speed of a synchronous machine can be calculated as: $$n=\frac{f}{p/2}=25\: \frac{1}{\mathrm{s}} = 1500\: \mathrm{rpm}$$

In an asynchronous machine, there is a difference between the speed of the rotating field produced by the stator windings and the mechanical speed, called slip. Assuming a slip $s$ of 5%, the mechancial speed can be calculated as: $$n=\frac{f}{p/2}\left( 1-s\right) =23.75\: \frac{1}{\mathrm{s}} = 1425\: \mathrm{rpm}$$