Siemens Midimaster Eco Reference Manual page 12

ECO REFERENCE MANUAL
The three phase supply is fed into a full wave
rectifier which supplies the DC link capacitors.
The capacitors reduce the voltage ripple
(especially on single phase supplies) and supply
energy for short mains breaks. The voltage on
the capacitors is uncontrolled and depends on the
peak AC supply voltage.
MICROMASTER Eco &
MIDIMASTER Eco inverters are
available as three phase.
DIAGRAM AND
TEST RESULTS
OV
Pulse Width Modulation
The DC voltage is converted back to AC using
Pulse Width Modulation (PWM). The desired
waveform is built up by switching the output
transistors (Insulated Gate Bipolar Transistors;
IGBTs) on and off at a fixed frequency (the
switching frequency). By varying the on and off
time of the IGBTs the desired current can be
generated. The output voltage is still a series of
square wave pulses and the inductance of the
motor windings results in a sinusoidal motor
current. Pulse Width Modulation is shown in the
figure below.
Voltage
Current
Time
3-5