Bus current is small but phase current is large

The DC bus current can appear small while the motor phase current is large because the inverter and motor inductance store and release energy, causing phase currents to peak locally while the average bus current remains lower.

Understanding Bus vs. Phase Current

Bus current represents the total current flowing through the DC link of the inverter, essentially the current drawn from the power supply. Phase current is the current flowing through each motor winding, which is what the motor actually uses to generate torque and is regulated by the controller (e.g., FOC or torque control) . In PWM-controlled inverters, the phase current can have high peaks due to the switching of transistors, while the bus current is an average over the DC link. The bus current is influenced by the total energy drawn from the supply, which is smoothed by the DC link capacitors, whereas the phase current reflects instantaneous energy delivered to the motor windings .

Role of Inductance and Switching

Motors are inductive loads, meaning the current through the windings cannot change instantaneously. During the ON phase of a transistor, the inductor stores energy, and during the OFF phase, the current recirculates through diodes or other paths. This results in phase currents that can be much higher than the average bus current, even though the total energy is conserved . Essentially, the inverter acts like a step-down DC/DC converter, where the product of bus voltage and bus current equals the product of phase voltage and phase current, ignoring losses.

Current Sensing Considerations

Phase currents are typically measured using low-side or inline shunts for each phase, while bus current is measured with a single shunt in the DC link. Because of this, the bus current measurement reflects an average over all phases and switching cycles, whereas phase current measurements capture the instantaneous peaks needed for torque control . This explains why phase currents can appear large even when the bus current is relatively small.

Summary

• Bus current: Average current drawn from the DC supply; smoothed by DC link capacitors.
• Phase current: Instantaneous current through motor windings; can have high peaks due to PWM switching and inductive energy storage.
• Reason for difference: Inductance stores energy during transistor ON periods and releases it during OFF periods, causing phase currents to peak while bus current remains lower.
• Measurement: Bus current is a single-point average; phase currents are measured per phase for accurate control and torque regulation. Understanding this distinction is crucial for designing current sensors, selecting shunt placement, and interpreting motor drive measurements accurately.