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Sir i want to calculate kinetic energy of
A)100w 3000rpm 220v single phase alternator
b)100w 3000rpm 220v three phase
C)100w 3000rpm 440v three phase
D)100w 3000rpm 60v single phase
sir i want to calculate kinetic energy of above all alternators please provide me formula for this....

Hello vinu,

The formula for kinetic energy of a rotating body is
KE = (1/2)*I*w^2

The parameter I in the above formula is called rotational inertia. It has to describe the physical characteristics of the rotating parts of the alternator (generally called the rotor).  For an object that is a disk (such as a flywheel),
I = M*R^2 / 2
If the armature concentrates most of the mass at the outer radius (similar to a bicycle wheel), this formula would give an approximated value:
I = M*R^2

The parameter w is angular speed in radians/s. 3000 rpm = 314 radians/s.

Two significant problems in this task:
1) Whether the alternator is single phase or 3 phase, or 220v or 440v, probably affects the mass and the distribution of the mass. And therefore it affects the calculation of I. But I have no way to know what the affect on the value of I is for those variations.

2) I predict that if you could inspect several alternators with equivalent wattage, rpm, voltage, and phase descriptions, but from different manufacturers, you would find significant differences in the distribution of rotor mass. Those differences would cause differences in the value of I. Because I, the rotational inertia, is affected strictly by the mass involved and how it is distributed with respect to the spin axis - not by the power and voltage it is designed to produce.

I hope this helps,


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Steve Johnson


I would be delighted to help with questions up through the first year of college Physics. Particularly Electricity, Electronics and Newtonian Mechanics (motion, acceleration etc.). I decline questions on relativity and Atomic Physics. I also could discuss the Space Shuttle and space flight in general.


I have a BS in Physics and an MS in Electrical Engineering. I am retired now. My professional career was in Electrical Engineering with considerable time spent working with accelerometers, gyroscopes and flight dynamics (Physics related topics) while working on the Space Shuttle. I gave formal classroom lessons to technical co-workers periodically over a several year period.

BS Physics, North Dakota State University
MS Electrical Engineering, North Dakota State University

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