How do I accurately measure moment of inertia of a small motor rotor
How do I accurately measure moment of inertia of a small motor rotor
(OP)
I need to measure the moment of Inertia of a 20mm dianeter rotor. I have calculated the inertia at 3500gmm^2 using the following equation to sum the components (cylinders):
Inertia, I = m(r1^2 + r2^2)/2
I've measured it by timing it rolling down a slope:
Inertia, I = mgr^2t^2Sinq/2L - mr^2
but, despite taking over 30 measurements at 4 different slope angles, the measured results vary significantly from close to the calculated value to almost double. The rotor is a strong magnet which may not help.
Can anyone suggest a more accurate way of quickly and cheaply measuring Moment of Inertia?
Inertia, I = m(r1^2 + r2^2)/2
I've measured it by timing it rolling down a slope:
Inertia, I = mgr^2t^2Sinq/2L - mr^2
but, despite taking over 30 measurements at 4 different slope angles, the measured results vary significantly from close to the calculated value to almost double. The rotor is a strong magnet which may not help.
Can anyone suggest a more accurate way of quickly and cheaply measuring Moment of Inertia?





RE: How do I accurately measure moment of inertia of a small motor rotor
would appear to be your problem
Cheers
Greg Locock
Please see FAQ731-376 for tips on how to make the best use of Eng-Tips.
RE: How do I accurately measure moment of inertia of a small motor rotor
Comes from I = mk^2
Where k, radius of gyration = (0.5(r1^2 + r2^2))^0.5
and m is the mass of the component.
Is this not correct?
The calculated value also agrees with the output from the CAD model. I'm having difficulty getting consistent results from the physical measurement.
RE: How do I accurately measure moment of inertia of a small motor rotor
Your measurement does not see to take into account friction/slippage and would seem to be prone to experimental error.
TTFN
RE: How do I accurately measure moment of inertia of a small motor rotor
Having said that there is probably a feasible geometry that has that equation, just not the one I thought of.
Cheers
Greg Locock
Please see FAQ731-376 for tips on how to make the best use of Eng-Tips.
RE: How do I accurately measure moment of inertia of a small motor rotor
Wes C.
RE: How do I accurately measure moment of inertia of a small motor rotor
I = (mm)^4
Wes C.
RE: How do I accurately measure moment of inertia of a small motor rotor
Wes C.
RE: How do I accurately measure moment of inertia of a small motor rotor
There is an old method called the bifilar-suspension method.
Suspend the rotor from two strings length L, seperated by distance 2R. Displace rotor a small angle to wind up the suspension, release and measure the frequency of oscillation, f. The inertia is:
J = WR^2/((2pi*f)^2 * L)
W is weight
f is Hz
L/(2R) > 10
Good Luck,
Doug
RE: How do I accurately measure moment of inertia of a small motor rotor