HELP DC Variable Speed Drive. Experimental Determination of Inertia

Hi,
I perform an experiment in the lab and i want to calculate the inertia of the drive that i test.

Below is the description of what i did and the results.

Test System Description

A1 – AC Variable Speed Drive ACS800, 5.5 kW, 415 V, 17.6 A, ‘ABB’
A2 – DC Variable Speed Drive DCS800, 400 V, 25 A, ‘ABB’
M1 – Squirrel–cage induction motor, 3-phase, 415 V, 50 Hz, 1400 rpm, 2.2 kW, 4.7 A
M2 – Separately excited DC motor, 400 V, 1990 rpm, 7.7 A, 2.6 kW.

The shafts of motors M1 and M2 are coupled directly. The AC VSD (A1) is a two-quadrant drive, and hence cannot provide braking. Motor M1, therefore, operates always in the motoring mode. The DC VSD of motor M2 can provide operation in four-quadrants. It is set for torque control (Fig. 2). If the set torque of the DC drive is of the opposite direction to the direction of rotation selected on the AC drive, the DC drive will operate in the braking mode. Consequently, M1 operates as a motor, and M2 – as a
generator.

Program of Experiment
Experimental Determination of Inertia
1. On the control panel of DC drive, switch to local control (LOC) by depressing the LOC/REM
pushbutton.
2. Start the DC drive by depressing the START p/b.
3. Take the characteristic of the motor no-load torque Tloss vs speed n in the range of 0 -1600 rpm. Vary the reference torque to vary the motor speed. Progress carefully to avoid exceeding the speed limit and tripping the drive. The torque is displayed as a percentage of the motor nominal value stored in the drive memory as parameter (4.23).
4. Plot a graph of Tloss = f. If possible, determine a speed interval in which Tloss is approximately constant. (See the graph below.)

The approximate value of inertia can be calculated from the above expression by substituting the slope of the coasting curve at ω1, with speed expressed in rad/s (see the graph below).
Note: The accuracy of the inertia calculation will be better if ω1 is selected from a speed interval in which Tloss is approximately constant. The coasting curve ω(t) in that interval resembles a straight line, and its slope is approximately constant.


This is what i get and i plotted on excel below:


Converter Output Voltage as a Function of Firing Angle
1. Depress the ON pushbutton on the AC drive panel. On the control panel, set the frequency at around 25 Hz. The selected direction of rotation is clockwise (positive).
2. Start the AC motor by depressing the START pushbutton.
3. On the control panel of DC drive, switch to local control (LOC) by depressing the LOC/REM pushbutton.
4. Start the DC drive by depressing the START p/b.
5. Set the reference torque at a level of (-50%) or more (sufficient to make the armature current continuous).
6. Using the oscilloscope, monitor the motor voltage vt(t) and a line supply voltage. On the template graph provided below, record the waveform of vt(t). Record the average armature terminal voltage Vt. Measure the firing angle of the thyristor bridge α using the zero crossing of the line voltage as
a reference point
7. Repeat the above for two other values of Vt. spread over the available range of 0 – 400 V. This requires selecting new values of frequency for the AC drive.

This is what i obtained below:








So yeah anyone got an idea how i can calculate the inertia of the variable speed drive being tested?

Thanks

Also anyone have an idea what the sign of Vt indicate?

on the waveforms that i got, anyone know how i can label the time axis?
let say for the 25Hz Frequency waveform,
is the time period of the sine wave 1/25 = 0.04s.
so this is for one cycle of the sinuisoial waveform that i got?

thx