Phase current is smaller than bus current

In motor drives, the DC bus current is generally smaller than the phase current because the bus supplies the total power, while phase currents are amplified by the inverter switching and motor inductance.

Understanding the Relationship

The DC bus current represents the total current flowing through the DC link of the inverter, while phase currents are the currents flowing through each motor winding that the Field-Oriented Control (FOC) or torque controller regulates . The bus current is effectively the sum of the instantaneous contributions of all phase currents, averaged over the switching period, and is influenced by the PWM duty cycle and the inverter topology .

Role of the Inverter and Inductance

In a typical three-phase inverter, each phase current flows through low-side or high-side switches and the motor windings. Because motors are inductive loads, the current through each phase cannot change instantaneously. During the PWM cycle, the current through the DC bus is only present when the corresponding switch is conducting, while the phase current continues to flow through the motor and recirculation diodes even when the switch is off . This results in phase currents being higher in amplitude than the average DC bus current.

Current Sensing Implications

• Phase current sensing is usually implemented with low-side shunts under each phase leg or inline shunts in the motor leads. Accurate phase current measurement requires synchronization with the PWM switching .
• DC bus current sensing is typically done with a single shunt in the DC return path or high-side isolated sensing. It is used for over-current protection and total power estimation .
• Because the bus current is an average over the switching period and shared among phases, it is smaller than the peak phase currents, which are instantaneous and can be higher due to PWM modulation and motor inductance .

Practical Example

For a brushless DC motor or PMSM driven by a three-phase inverter:

• If the inverter applies a duty cycle of 50% to a phase, the phase current may reach its peak value during the ON period.
• The DC bus current, however, only sees the portion of current drawn during the ON time, averaged over the PWM period, making it smaller than the peak phase current .

Key Takeaways

1. Bus current is an average of phase currents, not the instantaneous sum.
2. Inductive behavior of motor windings allows phase currents to continue flowing even when the DC bus current is zero.
3. PWM switching and duty cycles create a difference between instantaneous phase currents and the measured DC bus current.
4. Accurate motor control requires synchronized phase current sensing, while bus current is mainly used for protection and power estimation . Understanding this distinction is crucial for designing current sensors, selecting shunt resistances, and ensuring proper FOC or torque control in motor drives.