A 4-pole motor with 1,460 or 1,465 on its nameplate is not faulty. 1,500 rpm is the speed of the rotating magnetic field in the stator: ns = 120 × f / p = 120 × 50 / 4 = 1,500 rpm. If the rotor reached this speed, no current would be induced in its bars and no torque would be produced; so an induction motor always runs slightly below synchronous speed.

The difference is slip: s = (ns − n) / ns. Our 7.5 kW 4-pole model has a rated speed of 1,460 rpm; slip is (1,500 − 1,460) / 1,500 ≈ 2.7%.

PolesSynchronous speed (50 Hz)Example rated speed
23,000 rpm2,930 rpm (7.5 kW)
41,500 rpm1,460 rpm (7.5 kW), 1,465 rpm (11 kW)
61,000 rpm970 rpm (7.5 kW)
  • Slip is larger in small motors: the 0.25 kW 4-pole model runs at 1,340 rpm, while large ratings approach 1,490 rpm.
  • An unloaded motor runs very close to synchronous speed; speed drops slightly as load rises.
  • Pulley, gearbox and pump calculations should use the rated speed on the nameplate, not 1,500.

The effect of speed choice on the machine is illustrated in our 1,500 versus 3,000 rpm selection article.