ELECTRICAL PROBLEMSThe formula is ns = 60 x f/p where ns = synchronous speed; f = supply frequency & p = pairs of poles per phase. The actual running speed is the synchronous speed minus the slip speed.

For a 50 Hz three phase supply:-

2 poles or 1 pair of poles = 3,000 RPM (minus the slip speed = about 2,750 RPM or 6 -7% ns)4 poles or 2 pairs of poles = 1,500 RPM etc6 poles or 3 pairs of poles = 1,000 RPM etc (never used one)8 poles or 4 pairs of poles = 750 RPM etc10 poles or 5 pairs of poles = 600 RPM etc (never used one)12 poles or 6 pairs of poles = 500 RPM etc (never used one)16 poles or 8 pairs of poles = 375 RPM etc

For a 60 Hz three phase supply:-

2 poles or 1 pair of poles = 3,600 RPM (minus the slip speed = about 2,750 RPM or 6 -7% ns)4 poles or 2 pairs of poles = 1,800 RPM etc6 poles or 3 pairs of poles = 1,200 RPM etc (never used one)8 poles or 4 pairs of poles = 900 RPM etc10 poles or 5 pairs of poles = 720 RPM etc (never used one)12 poles or 6 pairs of poles = 600 RPM etc (never used one)16 poles or 8 pairs of poles = 450 RPM etc

To determine the number of poles, you can read the data plate directly OR calculate it from the RPM stated on the data plate OR you can count the coils and divide by 3 (poles per phase) or by 6 (pairs of poles per phase).

Where the power of the motor is constant, the torque increases at the rate that the speed decreases.