GD2 Value of a Centrifugal Blower Rotor

Recall the formula " T = I * Alfa ; where T is the driving torque, I is moment of inertia and alfa is angular acceleration.

For a driver such as a motor or a turbine, the max. torque is a limited value; so as "I" is increased, the angular acceleration will decrease. For this reason, the time taken to accelerate rotor from zero to its full speed will increase.

Large size industrial fans, such as power plant main exhaust fans as large as 4-5 MW, take very long time (20-30 seconds) to reach to full speed; also they are equipped with starting devices (e.g. Liquid Resistance Start) to limit the initial high current due to high slip.

Hope this is what you were looking for ! Any different opinion?

Difficult to calculate accurately, and anyway blower manufacturer should be able to tell you.

But to get an idea, could take the mass of the rotor M and estimate the radius of gyration k. Then Moment of inertia I = M*k². As a guide, for a solid cylinder I = M*r⁴/4 = M*(r/2)², so k = r/2.

This gives I as M*r² and this is the right value to use in calculations like angular acceleration = torque/I. In my opinion GD² causes confusion.

Cheers..........Codey

Sorry, should have written - "for a solid cylinder I = M*r²/4 "

ACCORDING TO WHICH CODE THIS FORMULA

It doesn't come from a code, it's the result of a fairly simple calculation involving integration.

Codey

• RPM = ω x 2 x ∏ / 60 (rounds per minute = angular speed x 2 x 3.1415 divides bij 60 seconds )
• ω = α x t (angular speed = angular acceleration x acceleration time. (we accelerate from stand still)
• M = T x α (accelerating torque = moment of inertia x angular acceleration)

This means if you accelerate the blower rotor with a know torque, and measure the rotating speed after a know time, you can calculate the moment of inertia

<...Moment of inertia...> does not affect <...starting torque...>; it only affects acceleration time.