Fan Motors - Starting Torques
The motor must be capable of accelerating the fan wheel to it's operating speed.
It is important that a fan motor is capable to run the fan at the operating conditions - and in addition able to accelerate the fan wheel, drive and shaft to operating speed.
For a fan transporting a large volume of air at low static pressure the motor power required during continuous operating may not be enough to accelerate the fan and a correct designed and adjusted motor protection system may stop the fan before windings overheats and insulation is damaged.
Motor torque should be checked against fan wheel torque up to 90% of synchronized speed during design.
Motor Starting Torque
Motor starting torque for a belt driven fan can be calculated as
Im = 1.1 If (nf / nm)2 (1)
where
Im = moment of inertia that the motor must be capable of turning at the motor shaft (lbm ft2, kg m2)
If = moment of inertia of the fan wheel (lbm ft2, kg m2)
nf = fan speed (rpm, min-1)
nm = motor speed (rpm, min-1)
For direct drive fan Im always exceeds If.
Backward Centrifugal Wheels and Moment of Inertia
Typical inertia of Class IV steel and aluminum backward inclined centrifugal wheels:
| Fan Wheel Size (in) | Moment of Inertia (lb ft2) | |
|---|---|---|
| Steel Wheel | Aluminum Wheel | |
| 20 | 25 | 10 |
| 22 | 40 | 16 |
| 24 | 65 | 26 |
| 27 | 95 | 38 |
| 30 | 140 | 56 |
| 36 | 380 | 152 |
| 40 | 580 | 232 |
| 44 | 870 | 348 |
| 54 | 2030 | 812 |
| 60 | 3900 | 1560 |
| 66 | 5500 | 2200 |
Typical inertia of HVAC or process standard centrifugal fans with with steel backward wheels:
| Fan Wheel Size (m) | Moment of Inertia (kg m2) |
|---|---|
| 0.40 | 0.10 |
| 0.45 | 0.17 |
| 0.50 | 0.27 |
| 0.56 | 0.53 |
| 0.63 | 0.87 |
| 0.71 | 1.80 |
| 0.80 | 3.00 |
| 0.99 | 8.10 |
