Fan Motors and Starting Torques

A fan motor must be capable not only of driving the fan at operating speed, but also be capable of accelerating the fan wheel to the operating speed

A fan motor must be capable not only of driving the fan at operating conditions, but also be capable of accelerating the fan wheel, drive and shaft to the operating speed. For a fan transporting a large volume of air at low static pressure the motor power required during the continuous operating process may not be enough for starting the fan. Often a correct designed and adjusted motor protection system will stop the fan before the windings are overheated and insulation damaged.

The motor torque should during design be checked against the fan wheel torque up to 90% of the synchronized speed.

Motor Starting Torque

The motor starting torque for a belt driven fan can be expressed as:

I_m = 1.1 I_f  (n_f  / n_m)² (1)

where

I_m = moment of inertia that the motor must be capable of turning at the motor shaft (lb_m ft², kg m²)

I_f = moment of inertia of the fan wheel (lb_m ft², kg m²)

n_f = fan speed (rpm, min^-1)

n_m = motor speed (rpm, min^-1)

For direct drive fan I_m always exceeds I_f.

Backward Centrifugal Wheels and Moment of Inertia

Typical inertia of Class IV steel and aluminum backward inclined centrifugal wheels:

Typical inertia of HVAC or process standard centrifugal fans with with steel backward wheels:

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