SPRUHJ1I January 2013 – October 2021 TMS320F2802-Q1 , TMS320F28026-Q1 , TMS320F28026F , TMS320F28027-Q1 , TMS320F28027F , TMS320F28027F-Q1 , TMS320F28052-Q1 , TMS320F28052F , TMS320F28052F-Q1 , TMS320F28052M , TMS320F28052M-Q1 , TMS320F28054-Q1 , TMS320F28054F , TMS320F28054F-Q1 , TMS320F28054M , TMS320F28054M-Q1 , TMS320F2806-Q1 , TMS320F28062-Q1 , TMS320F28062F , TMS320F28062F-Q1 , TMS320F28068F , TMS320F28068M , TMS320F28069-Q1 , TMS320F28069F , TMS320F28069F-Q1 , TMS320F28069M , TMS320F28069M-Q1
Before talking about the different types of resistor shunt current measurements, we will study the switching signals involved and where the measurement has to be taken to measure current. Most field oriented control (FOC) drives use the space vector modulation (SVM) technique to send the duty cycle commands to the inverter switches to power the motor. A typical SVM voltage waveform looks like the blue signal as shown in Figure 18-1. The SVM waveform is next sampled by a triangular waveform which is the red signal in Figure 18-1. Whenever the triangle is greater than the SVM, the lower switch of an h-bridge phase is turned on. The resulting PWM waveform that is used to control an h-bridge phase is also shown below.
To measure current that is flowing through a phase, the normal approach is to have a resistor located at the base of the phase. So no matter what resistor configuration there is, 1-shunt, 2-shunt or 3-shunt, current can only be measured when a lower switch is on. In the PWM waveform the lower switch is on when the square wave signal is low. To sample the current, it has to be clean. A clean current signal or representation of the current signal must have no ringing or noise. With that being said, let us start looking at the different resistor shunt current measurement techniques that are used in the field.