I know the equation for slip is

          s= (Ns-Nr/ns)x100

and that to find Ns is

         Ns= f/p.

I have been given running speed (16.2 rev/s), total number of poles (18), frequency (50Hz).

Can someone help explain how to get a value for Nr?

  • \$\begingroup\$ Perhaps a bit of internet searching could help. For instance I quickly found Link. On that page there is a definition for Nr. Which is "nr = rotor rotation speed (rpm)". \$\endgroup\$ – Tyler May 31 '19 at 17:27
  • \$\begingroup\$ If Nr > Ns, it's not motoring. I suspect one of your parameters is incorrect. \$\endgroup\$ – Phil G May 31 '19 at 19:58

\$N_R\$ is your rotor rotational speed represented in revolutions per minute (rpm). In the question you've been given a rotational speed in revolutions per second. Should be a easy as:

$$ N_R \text{ [rpm]} = 16.2 \text{ [rev/s]} * 60 \text{ [s/min]} = 972 \text{ [rpm]} $$

Also, the equation for synchronous rotational speed, \$N_S\$ is:

$$N_S \text{ [rpm]} = \frac{120f \text{ [Hz]}}{p \text{ [poles per phase]}} = \frac{120 * 50}{6} = 1000 \text{ [rpm]} $$

Where \$p\$, your poles per phase, will be eighteen divided by three, for a three-phase motor.

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