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CHAPTER 6: SIMULATIONS OF INPUT AND OUTPUT SYSTEM EMI
Oscilloscope
Signal generator
Spectrum Analyser
Figure 6.2: Experimental setup for spectrum measurements validation
both methods and then compared. A sawtooth signal at 500kHz has been used for this
aim with the load provided by the internal 50Ω input impedance of the spectrum ana-
lyser; a digital oscilloscope with high sampling rate to save the input signal in the time
domain was connected in parallel. This experimental setup is depicted in Fig. 6.2. The
recorded signal has been exported and successively used as a voltage source for a time
transient simulation in Saber, where the Saber FFT operator have been successively ap-
plied. This signal and the one saved with the spectrum analyser are compared in Fig.
6.3, after a scaling function had been applied to the FFT signal to make it consistent
with the one of the spectrum:
V[dBµV] = 20·log 10 (V[V])+120−3 (6.3.1)
This formula relates the voltage simulation’s values, expressed in V, to the measured
one that are expressed in dBµV: it calculates the values in dB, then it adds the 120 term
to match the µV scale (120 = 20·log 10 (1000)), and finally it removes the 3dB term to
match the rms value measured (this is because the FFT operator will provide the peak
value). In Fig. 6.3 it is possible to see the matching of the peaks in the spectrum of the
two signals, the one measured and the one simulated, for the fundamental (500kHz)
and few harmonics.
6.4 LF motor model
To simulate the whole drive system and check if the system normal operation and sta-
bility is ensured once the filter is applied, a LF model for the motor is also needed.
To simplify the circuit and speed-up the simulation a simple RL circuit has been cho-
sen, since no speed or position control are implemented, we need now to estimate the
values of R and L that better represents the current dynamics of the motor in the ex-
perimental rig. To do so a sinusoidal voltage at 100Hz and 168Vrms, generated with
a programmable power supply (Chroma 61511, 12kVA 15Hz-1.5kHz three-phase out-
put), has been applied to the IM motor to simulate the converter’s output, and stator
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