Centrifugal Fans — Motor Speed & Key Engineering Notes

Synchronous speed formula:

$$N_s = rac{120 imes f}{P}$$

• $f$: Power frequency (Hz)

• $P$: Number of motor poles

• Example: A 4-pole motor runs at 1800 rpm on 60 Hz, and 1500 rpm on 50 Hz.

1. Common Frequencies

• United States: 60 Hz (most common)
  Typical motor speeds:

• 2-pole: 3600 rpm

• 4-pole: 1800 rpm (most common for centrifugal fans)

• 6-pole: 1200 rpm

• Most regions outside the U.S.: 50 Hz
  Motor speed is about 17% lower than at 60 Hz.

2. Key Engineering Notes for Centrifugal Fans

1. Frequency affects fan performance:
   60 Hz → higher speed → higher flow, pressure, and power
   50 Hz → lower speed → reduced fan performance

2. Affinity laws:

• Flow ∝ Speed

• Pressure ∝ Speed²

• Power ∝ Speed³
  → Small speed increases cause large power increases.

3. Impeller tip speed limits:
   Ensure tip speed stays within safe aerodynamic and structural limits (commonly < 100 m/s).

4. Actual motor rpm ≠ synchronous rpm:
   Induction motors have slip, so actual speed is slightly lower than the theoretical value.

5. VFD usage is common:
   Provides flexible control and energy savings; consider low-speed cooling and harmonics.

6. For export or international projects:
   Equipment designed for 60 Hz will deliver lower flow and pressure at 50 Hz unless re-selected or used with a VFD.