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
The single shunt current measurement technique measures the power supply current and with knowledge of the switching states recreates each of the three phase currents of the motor. Figure 18-2 illustrates where the single shunt is located in the inverter circuit.
There are eight different switch options in SVM. Table 18-1 explains each one and shows the direction of the voltage space vector and what current can be measured in that state. With the switches in states 0 and 7 only circulating current is present and there is no possibility to measure current with the single shunt technique. To properly measure current with the single shunt technique, the current measurement and switching state have to both be considered.
Switch State | AH | BH | CH | Vector | Measure |
---|---|---|---|---|---|
0 | 0 | 0 | 0 | ● | offsets |
1 | 1 | 0 | 0 | → | Ia |
2 | 1 | 1 | 0 | ↗ | -lc |
3 | 0 | 1 | 0 | ↖ | Ib |
4 | 0 | 1 | 1 | ← | -la |
5 | 0 | 0 | 1 | ↙ | Ic |
6 | 1 | 0 | 1 | ↘ | -lb |
7 | 1 | 1 | 1 | ● | offsets |
Figure 18-3 shows a SVMPWM waveform and the current measurement signal that result. In this case the current conduction times for IC and IA are on long enough so that the slew rate of the op-amp and settling time of the whole measurement system have enough time to go to steady state so that the ADC can have enough time to sample the current. As we will see shortly, when using the single shunt technique, it is mandatory to be able to measure current in the smallest time possible.
In Figure 18-4, imagine the voltage space vector traversing counter clockwise around the circle. As the space vector points toward the corners of the hexagon, the time window for sampling current completely disappears. There are zones located at 0, 60, 120, 180, 240, and 300 degrees where only one current can be measured and the other two currents must be found in another fashion.
In Figure 18-5, the space vector is pointing too close to π/3 and is causing the current measurement window to shrink for IA. Because of slew rate of the op-amp and a long settling time, IA will be missed and will cause an error in the FOC controller. One way of fixing this problem is to force a measurement window opening that lasts long enough to accommodate slew rate and settling time. An illustration of this technique is shown in Figure 18-6. The maximum duty cycle waveform is shifted to the right and the minimum duty cycle waveform is shifted to the left. The advantage of phase shifting the PWMs like this is that there is no distortion in the voltage waveform result per phase. Software still has to be written to compensate for the resulting current waveform and even though a current measurement window can be made as large as needed, it is best to keep the window as small as possible. As the space vector reaches the voltage limit set by the DC bus, there will be less room to shift the signals. So to get the best utilization of the DC bus and still have the ability to measure current requires that the chosen op-amp have very high slew rate and low settling time.
Current ripple is another problem that arises when using the single shunt technique. The motor is an inductive and resistive circuit element and therefore has an R/L time constant. On the current waveform that is shown in Figure 18-3, the motor's electrical time constant is large thus causing the current to be very level and the measured currents IC and IA can be considered the average current going to the motor. If the motor's R/L time constant is smaller, then the current will look more like a saw tooth wave. Now the current has to be sampled as close to the center of the total conduction time as possible to obtain the average motor current. This will cause an even stricter performance requirement for the chosen op-amp.
Let's run through a quick calculation to see what types of op-amp parameters are needed. First, a normal PWM frequency is around 20 kHz which is a period of 50 µs. At 20 kHz when causing a deadtime or any non-symmetric adjustment of the PWM that is 0.5 µs or greater, current distortion will occur. The C2000 F2805xF and F2806xF family of processors has the capability of 90 MHz clock speeds which translates to a 45 MHz ADC clock. The minimum sampling window is 7 ADC clock cycles or 156 ηs. The worst case time delay when considering slew rate delay is during the maximum voltage transition in this case 3.3 V. Ignoring settling time, the slew rate that will keep the signal measurement below 0.5 µs is 3.3/344 ηs or 9.6 V/µs. Settling time will take up about half of the time, so to be safe the op-amp slew rate should be chosen at 20 V/µs.