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CHAPTER 1: INTRODUCTION
Phase Current [A]
3
2
1
0
−1
−2
−3
0 2 4 6 8 10
Time [ms]
12 14 16 18 20
Figure 1.2: Output current of matrix converter
1.2 EMI related to power converters
The introduction of power converters, especially voltage source PWM inverters, gran-
ted a powerful control for motor drives, making them the common choice for motor
drive systems. Still, they introduced new issues typical of their nature both at the input
and the output side, that can mainly be summarized with:
• Bad influence on other electrical systems with the radiated and conducted emis-
sions
• High current harmonics injection on the input, creating power pollution
• Motors insulation failure due to surge voltage at the windings
• Premature bearing failure due to current induced by common mode voltage[11,
12]
All of these problem are due to converters switch-mode operation, that creates ripple
current both at the input and at the output. An example to illustrate this behaviour can
be seen in Fig. 1.2 which shows the output current of a matrix converter supplied at
240Vrms, 50Hz feeding a 1.5kW asynchronous motor with 170Vrms at 100Hz. Fig. 1.3
shows the spectrum of wavefrom in Fig. 1.2 where high frequency components are vis-
ible at the multiples of the switching frequency, in this case 12.5kHz. The rapid changes
of the switched voltages in power converters, because of the parasitic capacitive cou-
pling, generate currents in the earth path, so they do at the motor side for the same
reason. The frequency of interest included in EMI normative are 0.15 – 30MHz, to pro-
tect the bandwidth of radio and TV broadcasts. In this range mainly the conducted EMI
3