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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)


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 Motors - Moment of Inertia vs. Fan Wheel Size - Imperial Units
Fan Wheel Size
Moment of Inertia (lb ft2)
Steel WheelAluminum 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 Motors - Moment of Inertia vs. Fan Wheel Size - Metric Units
Fan Wheel Size
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

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