Low Cost Phase Current Sensing in DSP Based AC Drives

This paper presents the implementation of two techniques for phase current sensing based on a low cost last generation pC DSP (TMS320F24x). The first one is based on the measurement of the current flowing in a shunt resistor put in the dc bus of the inverter, the second one is based on the measurement of the current flowing in shunt resistors put in series to the emitter of the IGBT for each lower leg of the inverter. The information provided by these sensors, together with the pulse width modulation (PWM) information, is used to reconstruct the motor phase currents. After a description of the two methods and the limitations they present, an application in case of an induction motor drive is illustrated in which the Adjacent Vector - Space Vector Pulse Width Modulation (AV SVPWM) technique is used to drive the inverter. Some aspects involving the non ideal behaviour of different parts of the system (presence of dead time, power switches driver delay, rise and settling time of the current, A/D conversion module sampling time) are investigated in details and the specific hardware implementation is discussed. Experimental results and comparisons between the two methods are presented.