Motor power (kW) is determined by the load requirement of the driven machine; the motor should be selected close to, but not below, the load. Speed depends on the supply frequency and the number of poles: at 50 Hz, 2 poles correspond to about 3000 rpm, 4 poles to about 1500 rpm, 6 poles to about 1000 rpm and 8 poles to about 750 rpm synchronous speed.

How is power calculated?

Mechanical power is the product of torque and speed: P (kW) = T (Nm) × n (rpm) / 9550. If the torque the machine requires and its operating speed are known, the necessary power follows from this formula. For pumps and fans, flow, pressure and equipment efficiency are taken into account; for conveyors, load, speed and friction. Losses in transmission components (gearbox, belts) and a reasonable safety margin are added, and the next standard rating is chosen. A heavily oversized motor, however, has lower efficiency and power factor at light load.

Speed and slip

The actual speed of an induction motor is slightly below synchronous speed; the difference is called slip. A 4-pole motor, for example, turns at roughly 1450–1480 rpm at full load. Synchronous speed is given by n = 120 × f / p (f: frequency, p: number of poles). On a 60 Hz supply, speeds are about one fifth higher.

What if the speed does not match?

  • Gearbox: the most common solution where low speed and high torque are required.
  • Belt and pulleys: the pulley diameter ratio adjusts speed flexibly.
  • Frequency inverter: used when speed must change during operation.

An incorrectly selected motor can cause efficiency loss, overheating and early failure. Starting conditions, such as starting under load or a high-inertia load, must be evaluated separately.

For a correct power and speed calculation, send your application details to +90 (542) 666 11 11 (WhatsApp) or info@drgmotor.com.