1.30.2013

Rotor Inertia Measurement

How do we measure rotor inertia?
We can calculate it out of free run.

Method:
• Connect motor to the battery;
• Connect oscilloscope to the terminals;
• Disconnect the battery;
• Let the single measurement appear in the oscilloscope.

Results:
We measured free run BEMF dynamic process, which is equivalent for speed.
We can approximate the achieved graph to linear, which will allow us to use approximated differential dynamics:
1. Main dynamic formula (M_din - dynamic torque, M_t.v. - no load torque).
2. Useful torque.
3. Dynamic Torque equal to No Load Torque.
5. EMF and angular velocity link.
6. Voltage balance equation.
Moment of inertia is equal to:

Where I_t.v. - no load current, k - motor constant, T - time, E_bat - battery EMF, r_a - armature resistance, r_bat - battery resistance.

Simulation:
Simscape electrical in Matlab, helps us to have this approximated dynamic process and calculate the rotor inertia, which is equal to 1,081e-4 kgm^2:
Conclusion:
This is quite accurate method for rotor inertia calculation, if the gearmotor has static load (friction), which linearised the exponential graph of the speed.