# Culver Armature and Motor

## Motor RPM and Slip

#### Synchronous speed

Understanding the behaviour of induction motors, it's useful to understand the differerences from a synchronous motor. That type of motor always runs at a synchronous speed- a shaft rotation frequency that is an integer fraction of the supply frequency. The synchronous speed of an induction motor is the same fraction of the supply.

It can be shown that the synchronous speed of a motor is determined by the following formula:

$n_s={120times{f}over{p}}$

where ns is the (synchronous) speed of the rotor (in rpm), f is the frequency of the AC supply (in Hz) and p is the number of magnetic poles per phase.

For example, a 6 pole motor operating on 60 Hz power would have a speed of:

$n_s={120times{60}over{6}}=1200 mathrm{rpm}$

Note on the use of p - some texts refer to number of pole pairs per phase instead of number of poles per phase. For example a 6 pole motor, operating on 60 Hz power, would have 3 pole pairs. The equation of synchronous speed then becomes:

$n_s={60times{f}over{P}}$

with P being the number of pole pairs per phase.

#### Slip

The slip is a ratio relative to the synchronous speed and is calculated using:

$s = Bigl(frac{n_s-n_r}{n_s}Bigr)$

Where

s is the slip, usually between 0 and 1
nr = rotor rotation speed (rpm)
ns = synchronous rotation speed (rpm)