# Measuring electro-hydraulic drive's inertia

I have an electro-hydraulic drive – a hydraulic motor with coupled to it a synchronous generator. Also there is an encoder installed on the shaft of the generator. By using my DAQ unit I will measure its position, speed and acceleration. The most important aspect for me right now is how I can measure the drive’s inertia and be as precise as possible.

I now that Torque = Inertia * Acc. Acceleration can be measured by the DAQ. But what to do with the drive’s torque? I can agree that the hydraulic motor’s torque can be calculated (I know pressure, displacement and its efficiency). There is no electrical connection of the generator to an exciter or means, so it is literally only extra load on the motor’s shaft. In this case I am just wondering, can I actually use the motor's torque as the drive's torque (motor plus generator).

So in this case, I can use the derived motor’s torque and measured acceleration in order to calculate the inertia? Is something wrong with my approach or?

• The coupled hydromotor has lot of friction, so measuring the inertia with some run test is difficult. The best solution is to find a datasheet of the motor and hydromotor. Nov 7, 2017 at 21:32
• @Marko, thank you. Well, I could potentially get the data sheets or maybe derive the value. But it is a fairly complicated electro-hydraulic system (a lot of non linear elements) and I simply can't rely on the manufacturer's data. Previous tests showed that characteristics of some of the components are not even close to their data sheets. Because of it, it's not easy to mathematically model the system as well. So I am in the position where I should verify the inertia experimentally. Otherwise, the whole mathematical model will behave inadequately... Nov 7, 2017 at 21:44
• If you know the efficiency as a function of speed, you may be able to assume that the friction torque is the only loss. The torque applied to accelerating the inertia would then be the developed torque minus the friction torque. The friction of the synchronous generator is probably quite small. Depending on the speed and size of the generator, the aerodynamic drag on the rotor may be more significant than the friction, but it may still be quite small.
– user80875
Nov 7, 2017 at 21:46
• @CharlesCowie, thank you for the comment. Yes, I have the efficiency graphs, so it is doable. But still wanted to confirm that my (and you approach) will make sense. Thank you. Nov 8, 2017 at 1:11