3. Experimental Setup and Hardware Description
3.1.2. Additional Components
The control of the VSC requires some information about the state of the system in order to calculate the required output voltage. The information about input/output currents and voltages is also necessary for the protection proposes. Therefore, some sensors are required to provide the functionality of the VSC. Depending on the application of the AC drive number of sensors can be different [27]. For the
(a)DC voltage measurements (b)AC current measurements
Figure 3.3.: The measurements circuits of the designed power core.
current study, it was decided to implement measurements of the output AC currents and the input DC voltage. Both types of measurements are based on the same IC which provides the delta-sigma conversion of analogue signals [144]. This type of ADC transforms the input voltage into the high- frequency bit-stream using the oversampling technique [152]. The output bit-stream is then converted to the high-resolution digital signal using the digital filters directly in the control system. The analogue signals are obtained using shunt resistors and voltage dividers for the output current and input voltage measurements respectively. The galvanic isolation between the input analogue signal and the output bit-stream is realised by means of the optocoupler in the applied IC [144].
The designed measurements circuits are shown in Figure 3.3. Both circuits for the input voltage in Figure 3.3a and output current measurements in Figure 3.3b are implemented on the separate PCBs. These circuits are connected then to the VSC shown in Figure 3.2 using the power cables. The applied IC with delta-sigma ADCs should be supplied from the high potential and low potential side [144]. All delta-sigma ADCs are supplied from the respective gate drivers. The DC/DC converter of low side gate drivers is used to supply the IC for the voltage measurements. The high side gate drivers provide power for the current measurements in the respective phases. The output currents of VSC are measured only in two phases as can be seen in Figure 3.3b. The current of the 3rd phase is calculated in the control
system. The applied ICs introduce also some parasitic capacitance between the high potential and low potential (ground). Therefore, they contribute to the RF impedance of the power core as well.
As far as currents and voltages at the input/output of VSC are defined, the control system can calculate the reference voltage for PWM. However, an implementation of the real closed-loop control system will make an investigation on the EMI of VSC more difficult. This is due to the fact that in the closed-loop control, the reference voltage fluctuates near the steady-state value. It would be then difficult to ensure the value of some parameters such as modulation index during the measurements. It was decided to operate the system in the open-loop control where the motor is rotating according to the frequency and amplitude of the applied PWM voltage (U/f-mode) [27]. The U/f characteristics were measured for the applied induction machine. Such an approach ensures the value of the modulation index.
3.1.3 EMI Filters
The maximum noise levels are restricted for all input and output power lines of the EUT according to DO-160 [24]. For the studied AC drive system (see Figure 1.6), it is necessary to provide the corre- sponding levels of conducted EMI at the input DC line and output three-phase AC cable. It means that the EMI filters should be installed at the input and output of VSC. As it is discussed in Section 1.4, the application of EMI filters cannot be avoided because the other noise reduction techniques influence only a particular frequency range. The improvement of VSC design shows a lower level of noise attenuation in comparison to the EMI filters. The performance of EMI filters can be optimised using different materials, components and structures. According to the goal of the research project, it is required to investigate the influence of EMI filters topologies on the conducted noise as well as the impact of VSC design on the EMI filter. For these purposes, two elementary cells of EMI filters were designed for the installation at the input and output of VSC respectively. Using these cells, it is possible to build the basic structures of EMI filters shown in Table 1.1. The circuits of cells are shown in Figure 3.4. As it is explained in Section 1.2, CM current is the main source of EMI in the AC drives. Therefore, the EMI filters are designed only for the CM noise attenuation using the CM chokes and Y-capacitors.
The single cell of the output filter is represented by the three-phase CM choke in Figure 3.4a. It means, that it is possible to implement only one structure of EMI filter (series inductor) at the output of VSC. The lack of Y-capacitors in the output EMI filter is explained by the practice of AC drive application for the civil aircraft. It is hard to ensure the lightning protection of the system in the presence of Y-capacitors at the output of VSC [49]. If lighting will hit the motor, which can be placed in the wing for example, the high voltage pulse will be applied to the Y-capacitors causing the huge leakage currents to the ground. Hence, only the inductor can be implemented at the output of VSC. Through the chain connection of several elementary cells, it is possible to increase the value of CM inductance and attenuation of the output EMI filter. However, only one cell was applied during the measurements.
The elementary cell of the input EMI filter is shown in Figure 3.4b. This cell is based on the CM choke and two Y-capacitorsCyi, which have also additional damping resistance Rdi, where i is the number of the cell. Combining two cells in different directions, it is possible to implement all structures presented
A A
B B
C C
Converter Motor
(a)Output cell
P P N N Cyi Cyi Rdi LISN Converter (b)Input cell
(a)Output cell
(b)Input cell
Figure 3.5.: Photos of elementary cells of EMI filters.
in Table 1.1. Only 2 cells were implemented during the measurements. The following structures can be evaluated with these cells: LC, LCL, LCLC. The conditions for the lighting protection limit the amount of capacitance to ground at the input as well. The total value of Y-capacitors of the whole input EMI filter cannot exceed150 nF. The lighting pulse can cause high ground leakage currents through the input of VSC with the increased value of Y-capacitance.
The cells of input and output EMI filters are shown in Figure 3.5. The cells are implemented on separate PCBs. They are connected to the VSC using the power cables. The CM chokes of the output filter in Figure 3.5a and input filter in Figure 3.5b were made using cores with the same nanocrystalline material [153]. Nanocrystalline cores are preferable for the design of EMI filters in power electronics due to the increased values of saturation currents in comparison to ferrite. The design of the CM choke has a huge influence on the resulting attenuation of the EMI filter [61]. Optimization of the CM choke itself is already considered in some publications [13]. However, the CM chokes were not changed during the measurements in the conducted research work. The detailed parameters of the applied CM chokes are given in Appendix C.
The input cell is supplied with the CM choke, Y-capacitors and damping resistors according to Figure 3.4b. The resistors cannot be seen in Figure 3.5b because they are placed on the bottom side of the board. Two cells were built which are supplied with the same CM chokes. However, different values of Cyi andRdi were applied during the measurements for the investigation purposes. Similar to the cell of the output EMI filter, only one type of the CM choke was designed within the project using the same nanocrystalline cores. The parameters of input CM choke can be found in Appendix C as well.
Observing the output and input EMI filter cells in Figure 3.5, it can be concluded that the size of EMI filter is defined mostly by the CM choke. At the same time, the total amount of Y-capacitance is limited making the size of CM choke even bigger. The resulting size of the EMI filter is comparable with other parts of the power core: switches, gate driver and DC link capacitors.