57 Frequency Converters 1.00 U-V U-W W-U 0.866 0.50 0 0 180 360 -0.50 -0.866 -1.00 Switching pattern of phase U Phase voltage (0-point & half the intermediate circuit voltage) Combined voltage to motor Fig. 2.14 Output voltage PWM Low stator frequencies result in longer periods. The period can increase to such an extent that it is no longer possible to maintain the frequency of the triangular waveform. This makes the voltage-free period too long, causing the motor to run irregularly. To prevent this, the frequency of the triangular waveform can be doubled at low frequencies. The low switching frequency leads to an increase in acoustic motor noise.
To limit the amount of noise produced, the switching frequency can be increased. This has been made possible thanks to advances in the fi eld of semiconductor technology, which mean that modulation of an approximately sinusoidal output voltage and generation of an approximately sinusoidal current are now achievable. A PWM FC that relies exclusively on sinusoidal reference modulation can provide up to 86.6% of the rated voltage (see Fig. 2.14 Output voltage PWM). The phase voltage at the FC output terminals corresponds to half the intermediate circuit voltage divided by 2, and is thus equal to half the mains supply voltage.
The mains voltage of the output terminals is equal to 3 times the phase voltage and is thus equal to 0.866 times the mains supply voltage. 60 120 240 300