Description of the Measurement Setup

This section describes the implementation of the setup for conducted noise measurements. The ob- tained results are used further in Chapter 3 for the investigation and model validation purposes. The section gives also some details about the other parts of the AC drive system beside the VSC. Due to the high cost of aerospace components, they were replaced with standard industrial solutions. Moreover, some aspects of the applied standard, which were ignored during the measurements, are also discussed in this section.

Table 3.1.: Cable parameters.

Type Length,m Wire cross section, mm2 _{C’,pF m}−1 _{L’,nH m}−1

2-Wire, unshielded cable 5 4 13 115

3-Wire, shielded cable 1 4 117 290

The structure of the setup for conducted noise measurements according to DO-160 can be seen in Figure 1.1. The EUT is the power core, which contains the VSC, its control and EMI filters (see Section 3.1). The power supply is not predefined by DO-160 [24]. But it should be able to provide the required voltage of 540 V and power of 10 kW. The LISN with 10 µF decoupling capacitors were ordered from the selected supplier. Its resulting impedance corresponds to the impedance presented in Figure 1.2. The input and output cables, which connect the EUT with power supply and load respectively, should be taken according to the specification of the AC drive [24]. The simple industrial cables were applied. Their parameters are listed in Table 3.1.

According to the specification, the input cable should be unshielded, whereas the output three-phase cable should be equipped with a shield. The lengths of the cables were provided in the specification as well (see Table 3.1). The cross-section area was chosen to be the same for the input and output cables. The value of the capacitance and inductance per unit length were estimated using single-ended S-parameter measurements. The 6-port S-parameters of the output cable were measured connecting the shield of the cable to the ground of VNA. Then, the S-parameters were converted to Z- and Y- parameters (see Appendix B). The resulting parameters are used to obtain the values of C’ and L’. In order to provide the common ground path for the unshielded cable, it was put over the copper plate with10 cm distance. This distance is also specified by DO-160 for the EMI measurements [24]. The copper plate was connected to the ground of VNA. The 4-port S-parameters of the input cable were measured giving the possibility to obtain the values of C’ and L’. As it is expected, the value of capacitance per length of input cable is much smaller in comparison to the shielded output cable (see Table 3.1). In opposite the inductance per length is lower in case of the shielded cable. The measured values correspond to the values of typical power cables which were estimated experimentally and theoretically in [114].

The measured single-ended Z-parameters of the cables were converted further into MM by means of (2.13) for the simulation purposes. According to the presented in Subsection 2.2.2 conversion, the resulting impedances Z₁₁and Z₂₂are representing the open-circuit impedances for the CM and DM re- spectively. These impedances are often used to characterize the components of an AC drive for EMI simulation [42, 44]. The corresponding impedances for the applied power cables can be observed in Fig- ure 3.9. Both cables show capacitive behaviour in the lower frequency range. However, the resonances can be observed in both cases after the frequency of 10 MHz that corresponds to the transmission line theory [114].

Due to the higher value of the specified length for the input cable (see Table 3.1), the effect of travelling waves in Figure 3.9a is observed at15 MHz and 8 MHz for CM and DM respectively. The behaviour of the CM impedance depends on the position of the cable due to the lack of the shield. If a lower distance between cable and copper ground is applied, the value of capacitance per unit length increases. The CM and DM open-circuit impedances of the output cable are presented in Figure 3.9b. Due to the smaller

0.1 1 10 0 20 40 60 80 100 Frequency,MHz Z, dB Ω CM DM

(a)Input cable

0.1 1 10 0 20 40 60 80 100 Frequency,MHz Z, dB Ω CM DM (b)Output cable

Figure 3.9.: The CM (Z₁₁) and DM (Z₂₂) open-circuit impedances of the cables.

Table 3.2.: Motor parameters.

Type Power,kW Voltage, V Frequency, Hz Speed, min−1 _{cos φ}

ACM 160MA-2/HE 11 400 50 2930 0.88

length of the output cable, the effect of travelling waves starts to influence the CM impedance only above 20 MHz. The effect of long cables is not observed for the DM open-circuit impedance of the output cable in Figure 3.9b. Due to the presence of shield, the CM und DM impedances are almost independent from the cable position towards the copper plate.

As it is explained in Subsection 2.2.2, the MM parameters can be used to analyse and to model the DM and CM separately by combining the respective 2-port networks from the components of 4-port or 6-port networks. Moreover, the Z-parameters can be converted to Y-parameters that gives the possibility to consider short-circuit impedance of the system too. These parameters are not presented here. However, they were also analysed within the conducted research showing good accordance with the theoretical behaviour.

The last part of the EMI measurements according to DO-160 is the load which is also defined by the specification of equipment. In the case of the AC drive, the motor is assumed to be the load for the EUT (VSC). An industry-standard induction motor was considered for the experiments. Its nominal parameters are listed in Table 3.2. These parameters are valid for the delta connection of the windings. Alternatively, the same motor can be connected in star with an increased voltage. It can be also supplied with60 Hz that leads to increase of the nominal speed.

In order to implement the various loading conditions, the induction machine was mechanically coupled with a DC motor. The whole assembly is shown in Figure 3.10. The applied DC machine was supplied with a 4-quadrant rectifier. It gives the possibility to operate the DC machine in the generator mode. The DC motor is used to apply the mechanical load (torque) to the induction motor. The resulting load can be varied from 0 up to nominal power of the DC machine (15 kW). The mechanical coupler, which connects the shaft of the AC motor and DC generator, is electrically isolated. However, the DC machine

Figure 3.10.: Photo of motor-generator setup. 0.1 1 10 0 20 40 60 80 100 Frequency,MHz Z, dB Ω CM DM

Figure 3.11.: The CM (Z₁₁) and DM (Z₂₂) open-circuit impedances of the motor.

can influence the impedance of the motor. Therefore the S-parameters of the motor were measured including the whole load fixture. Similar to the cables, the S-parameters of the motor were converted to MM Z-parameters that gives the possibility to study its DM and CM impedance behaviour. In Subsection 2.2.3, it is explained how to apply MM conversion to the delta connected motors. The resulting open- circuit CM/DM impedances of the applied AC motor are shown in Figure 3.11 (Z₁₁ and Z₂₂ of MM parameters for CM and DM respectively).

As can be seen in Figure 3.11, the motor shows the capacitive behaviour in the frequency range of interest (from 150 kHz to 30 MHz) in both cases for DM and CM. Such behaviour corresponds to the measurements which were conducted in [44]. In the 1st _{frequency domain models [116], the motor}

Figure 3.12.: Photo of setup for conducted EMI measurements (top view).

several MHz. For the applied motor a resonance can be observed at DM impedance at the frequency above10 MHz.

All components of the specified AC drive system were arranged according to the DO-160 for conducted noise measurements. The photo with a top view of the constructed setup for EMI measurements is shown in Figure 3.12. On the right, two LISNs for each phase connect the DC power supply and the input cable. The cable is connected on the other side with the designed power core. The output cable is not shown in Figure 3.12. However, it is arranged in a similar manner as the input cable. All components of the system are allocated above the copper plate.

According to the requirements given in DO-160 [24], the noise is measured with the current probes on all interconnections on the distance of50 mm from the EUT. The applied current probe is 6600 from Pearson which has a bandwidth (−3 dB) from 40 Hz up to 120 MHz. The main characteristic of the current probe is the transfer impedance. It describes the relationship between the measured current and output voltage [154]. The transfer impedance is normally given as a curve in the frequency domain. This curve should be then added to the measured values acquired by the spectrum analyser. All parameters of the current probe including its transfer impedance are given in Appendix D. The signals from the current probe were applied either to spectrum analyser with settings according to DO-160 or to the scope in order to provide the time domain analysis. The time domain data was also converted to the frequency domain utilizing SWDFT. Subsection 2.4.2 explains how to consider parameters of the spectrum analyser during the conversion of time domain data to the frequency domain. The parameters of utilized spectrum analyser and oscilloscope are also presented in Appendix D.

This section describes all components of the AC drive system besides the power core which have the most significant impact on the impedance of the noise propagation path. Using components of the MM Z-parameters, the CM and DM open-circuit impedances of the cables and motor are extracted and pre- sented in this section. It is shown that the applied components have mostly capacitive behaviour below certain frequencies. However, the impedances can change in a wide range due to the transmission line effects at RF. Finally, the arrangement of the setup for the conducted EMI measurements is presented.

0.1 1 10 0 20 40 60 80 Frequency,MHz Signal, dB µA

Figure 3.13.: The noise floor achieved with the applied measurement devices.

The presented setup was constructed trying to fulfil the basic requirements according to DO-160. How- ever, the presented setup neglects some aspect of DO-160 that can cause some uncertainties. These uncertainties are discussed in the next section.