Other Parts Discussed in Thread: BOOSTXL-3PHGANINV, 2MTR-DYNO
Hi Yanming Luo
We noticed this when running the dual axis sensor-ed FOC project and wanted to get your 2 cents on this.
Let me give you a brief overview on how we had setup the hardware/firmware.
Motor_1 is configured to run in torque control , Level 3(buildLevel3_M1() ;) And motor_2, is run in speed mode, Level 4 (buildLevel46_M2() ;). Both the inverters (BoostXL-3PhGanInv) are connected to the same DC power supply(28v).And both the motors are connected back to back (2MTR-DYNO).
Motor_2 is configured to run at a constant speed rotating in clockwise direction(motorVars[1].speedRef = 0.2 pu). Then we vary the torque (Iq) on motor_1 and observe the stability of speed on motor_2. So basically, regardless of any changes in torque, motor_2 should be able to maintain a relatively constant speed.
We apply torque from motor_1 in both clockwise(+Iq) and anticlockwise(-Iq) directions. But we are more interested when the torque applied by motor_1 is applied in the same direction at which the motor shaft rotates.
Readings are tabulated below:
|
Motor 2 Speed Mode |
|
Motor 1 Torque Mode |
|||
|
DC Bus Voltage |
DC Bus Current |
Speed Setpoint |
Speed Feedback |
|
Torque setpoint |
|
27.789135 |
0.27 |
0.2 |
0.200393096 |
|
0 |
|
27.789135 |
0.27 |
0.2 |
0.199591011 |
|
-0.05 |
|
27.8960438 |
0.31 |
0.2 |
0.199325338 |
|
-1 |
|
27.8779526 |
0.41 |
0.2 |
0.200237542 |
|
-1.15 |
|
27.9667721 |
0.68 |
0.2 |
0.200537324 |
|
-2 |
|
27.8236809 |
0.89 |
0.2 |
0.198650047 |
|
-0.25 |
|
27.8598633 |
1.14 |
0.2 |
0.203093405 |
|
-0.3 |
Now when we repeat the same experiment with both the inverters having a separate DC bus sources connected to them, if we keep increasing the torque supplied from motor_1, after a certain point, we notice that the DC bus voltage connected to the inverter 2(Inverter running in speed mode) keeps on increasing, all the while maintains the same constant speed!!
|
Motor 2 Speed Mode |
|
Motor 1 Torque Mode |
||||
|
DC Bus Voltage |
DC Bus Current |
Speed Setpoint |
Speed Feedback |
|
Torque setpoint |
DC Bus Voltage |
|
28.0407715 |
0.14 |
0.2 |
0.200034171 |
|
0 |
28.239769 |
|
28.0226803 |
0.04 |
0.2 |
0.199829102 |
|
-0.05 |
28.1690426 |
|
33.250927 |
0.03 |
0.2 |
0.20011726 |
|
-1 |
28.2216797 |
|
41.4279785 |
0.03 |
0.2 |
0.200382635 |
|
-1.12 |
28.2035885 |
|
50.3319321 |
0.03 |
0.2 |
0.203077078 |
|
-1.15 |
28.1674061 |
Why is this happening? This cannot be because of the Back emf, because the speed is constant so there should be no change in the back emf at all??
We are facing a similar issue on our custom design, which runs with two separate DC sources, similar to the seconds case mentioned above.
How do we negate this excess DC Bus voltage, do we need to implement some kind of a brake chopper to dump all the excess energy, whenever DC bus exceeds a certain range?
Thanks,
AK
