Engineering ToolBox - Resources, Tools and Basic Information for Engineering and Design of Technical Applications!

This is an AMP page - Open full page! for all features.

Fan Motors - Starting Torques

Sponsored Links

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
Sponsored Links

Related Topics

Ventilation Systems

Design of systems for ventilation and air handling - air change rates, ducts and pressure drops, charts and diagrams and more.

Related Documents

Belt driven Fans - Fan vs. Motor Speed

Fan vs. motor speed in belt driven fans.

Electrical Induction Motors - Torque vs. Speed

Full load operating torque vs. break down, pull up and locked rotor torque.

Electrical Motors - Heat Loss

Heat loss from an electrical motor to the surroundings.

Electrical Motors - Shaft Power vs. Voltage and Current

Calculate electric motors shaft power.

Fans - Efficiency and Power Consumption

Power consumption and typical efficiencies of fans

Fans - Troubleshooting

Guide to troubleshooting fan problems.

Fans - Volume Flow, Pressure Head and Power Consumption vs. Air Temperature and Density

The temperature and density of the air influences on the volume flow, pressure head and power consumption in a fan.

Torque or Moment of Force - Online Converter

Torque or moment - the tendency of a force to rotate an object.

Sponsored Links

Search Engineering ToolBox

  • the most efficient way to navigate the Engineering ToolBox!

SketchUp Extension - Online 3D modeling!

Add standard and customized parametric components - like flange beams, lumbers, piping, stairs and more - to your Sketchup model with the Engineering ToolBox - SketchUp Extension - enabled for use with the amazing, fun and free SketchUp Make and SketchUp Pro . Add the Engineering ToolBox extension to your SketchUp from the Sketchup Extension Warehouse!


We don't collect information from our users. Only emails and answers are saved in our archive. Cookies are only used in the browser to improve user experience.

Some of our calculators and applications let you save application data to your local computer. These applications will - due to browser restrictions - send data between your browser and our server. We don't save this data.

Google use cookies for serving our ads and handling visitor statistics. Please read Google Privacy & Terms for more information about how you can control adserving and the information collected.

AddThis use cookies for handling links to social media. Please read AddThis Privacy for more information.