Real-time current estimation in brushless DC drives using a single DC link current sensor

The classical method of current control in a three phase inverter is to compare current demand with actual winding current, where the winding current is obtained by direct measurements on the motor windings. This method of current measurement relies upon total uniformity of current sensors, to achieve a balanced output. The problem of current sensor imbalance can lead to unacceptable torque ripple at low speeds. By using a single sensor located on the DC link, there is inherent balance. Due to the chopping action of the inverter, current cannot always be directly measured at the DC link. This paper describes an observer based system that uses a motor/converter model to estimate this flywheeling current. When the winding current can be directly measured at the DC link, the observer updates its system model in a closed loop manner. The observer is implemented in real-time using a DSP (digital signal processor) based system. Full steady-state and transient results are described