DC bus current in motor drives

I was looking at some of the SimPowerSystems examples for motor drives and noticed strange behavior in the DC bus currents. For example in ac7_example (AC7 - Brushless DC Motor Drive During Speed Regulation), the bus voltage is well regulated but the current fluctuates rapidly from positive to negative:

This doesn't seem physical, as I would expect the sign of the current to follow the torque. Does anyone have an explanation for this?

Additionally, in ac7_example_comparison (AC7 - Comparison Between Detailed and Simplified Models), bus currents are plotted and look correct, but the output signals were passed through a discrete transfer function:

(1-exp(-Ts/.005))/(z-exp(-Ts/.005))

The transfer function makes the output look right, but the actual current still fluctuates.

Answer 1

[Joel Van Sickel]

As this is a three phase machine being driving by an inverter, is not unexpected to see these currents on the dc/capacitor. The rectifier is constantly dumping current in while the inverter is constantly pulling current out, but they are not synchronized in any way. It will also look different depending on where you measure the current. This is between the inverter and the dc cap, so this makes sense. If you measured between the rectifier and capacitor, it would also stay above zero (diode blocking).