Inertia identification - SI unit

Mojo,

Thanks for your great questions.  I'm sure there are quite a few people who are wondering why we specify Inertia this way.

The unit A*s/rpm represents the amount of torque required to accelerate the motor.  It is proportional  to the SI unit kg *m^2.  The relationship is based on the amount of torque that the motor can produce.  In the InstaSPIN-FOC & InstaSPIN-MOTION User Guide we talk about how to convert between the SI unit for inertia and the scaled value that InstaSPIN-MOTION uses.  The calculation is specified in section 11.6.  We prefer that everyone uses our Inertia Identification software to get the value for Inertia since it will more closely represent what the motor is actually capable of instead of theoretically capable of.

I hope this answers your questions.

I believe the inertia formula you reference is a bit off or at least confusing. Correct me if I am wrong, but the value of USER_MOTOR_RATED_FLUX in user.h is in units of volts/Hz, while the formula uses a flux in webers = volts/rad/sec. So the kg-m^2 value from the formula is high by a factor of 2*pi. (This is drawing from a different thread on converting the flux measurement to torque constant Nm/A.)

I haven't seen anything on the physical meaning of the USER_SYSTEM_FRICTION value, could you detail that also?

I realize that system-calculated values are best for use in-system for control purposes, but perhaps you guys have underestimated the value of this system for motor testing. It is a great way to get numbers on custom motors, or motors without data sheets, and to compare them to commercially-available motors. Much more affordable than a full motor test rig, LCR meter, etc.!

Having all three formulas clearly written out in one place would be nice (torque constant, inertia, and friction).

Here are my candidates for two of them:

Inertia (kg-m^2) = USER_SYSTEM_INERTIA / {USER_IQ_FULL_SCALE_FREQ_Hz * 2 * pi / [(USER_MOTOR_RATED_FLUX / 2 / pi) * USER_IQ_FULL_SCALE_CURRENT_A * USER_MOTOR_NUM_POLE_PAIRS] }

= 0.02533 * USER_SYSTEM_INERTIA * USER_MOTOR_RATED_FLUX * USER_IQ_FULL_SCALE_CURRENT_A * USER_MOTOR_NUM_POLE_PAIRS / USER_IQ_FULL_SCALE_FREQ_Hz

Kt (Nm/A) = 1.5 * (USER_MOTOR_RATED_FLUX / 2 / pi)  * USER_MOTOR_NUM_POLE_PAIRS

= 0.2387 * USER_MOTOR_RATED_FLUX  * USER_MOTOR_NUM_POLE_PAIRS

Thanks!

LineStream - Adam Reynolds said:

Mojo,

Thanks for your great questions.  I'm sure there are quite a few people who are wondering why we specify Inertia this way.

The unit A*s/rpm represents the amount of torque required to accelerate the motor.  It is proportional  to the SI unit kg *m^2.  The relationship is based on the amount of torque that the motor can produce.  In the InstaSPIN-FOC & InstaSPIN-MOTION User Guide we talk about how to convert between the SI unit for inertia and the scaled value that InstaSPIN-MOTION uses.  The calculation is specified in section 11.6.  We prefer that everyone uses our Inertia Identification software to get the value for Inertia since it will more closely represent what the motor is actually capable of instead of theoretically capable of.

I hope this answers your questions.

A follow-up comment on my formulas above:

I tried out lab 2a and 5c for four small brushless motors and compared the results to the data sheets, using the DRV8301-HC-69M-KIT. R, L, and Kt were close to the expected values.

The inertias measured by repeatedly running Lab 5C on a motor were very consistent with each other, within 1 percent, but when using my formula above the results in SI units were uniformly only 3/4 of the data sheet values. Something would still appear to be wrong.

The results from the GUI motor ID did not seem consistent with Lab 5c, and the friction results were also suspect - some motors gave negative frictions.

Jason,

Thanks for the great thoughts.  

The units for friction are A/krpm, and it is identifying the static friction in the system.

In looking at your formula, I think the reason you are seeing 3/4 of the datasheet values is because the inertia conversion equation is based on value identified by SpinTAC that are in the units PU/(pu/s) and not A/(krpm/s).  So you will need to scale the A/(krpm/s) value into the units of PU/(pu/s) in order to find the kg*m^2 inertia.

We have found that the project configuration of the system scaling will play a role in the estimated value of inertia.  So the GUI and lab 05c will estimate slightly different values.  We recommend that you do the inertia estimation using your final hardware in order to get the best possible value.

Hi,

I'm using  InstaSpin-Foc . With tab SpinTAc Startup , I was able to identify Inertia and friction.

How can i convert inertia value ( A*sec/RPM ) to kg*m^2 ?

I need to compare the Inertia's value identified with the value on datasheet.

Best Regards,

Dario Detomaso

Hello,

Please see the thread above that given by Adam.

He mentioned:

The unit A*s/rpm represents the amount of torque required to accelerate the motor.  It is proportional  to the SI unit kg *m^2.  The relationship is based on the amount of torque that the motor can produce.  In the InstaSPIN-FOC & InstaSPIN-MOTION User Guide we talk about how to convert between the SI unit for inertia and the scaled value that InstaSPIN-MOTION uses.  The calculation is specified in section 11.6.  We prefer that everyone uses our Inertia Identification software to get the value for Inertia since it will more closely represent what the motor is actually capable of instead of theoretically capable of.

Please check this document and follow this thread and I think you will find what you need.

Good luck!

Best regards,

Maria

it's in 10.5

Bypassing Inertia Estimation

in latest revision of SPRUHJ1