Siemens A1 Manual De Usuario
Introduction International
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4
MICROMASTER Applications Handbook
The supply, which can be single phase (usually at low power) or three-phase is fed
to 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 breaks in the input supply. The voltage on the capacitors is uncontrolled
and depends on the peak AC supply voltage.
to 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 breaks in the input supply. The voltage on the capacitors is uncontrolled
and depends on the peak AC supply voltage.
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, but the output voltage is still a series of square wave pulses. Pulse
Width Modulation is shown in Figure 1-5.
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, but the output voltage is still a series of square wave pulses. Pulse
Width Modulation is shown in Figure 1-5.
0 V
Time
Voltage
Current
Figure 1-5
Pulse Width Modulation
There are many complex aspects of inverters which need to be considered during
the design:
the design:
· The control system to calculate the PWM requirements is very complex and
specially designed integrated circuits (ASICs) are needed.
· The control electronics are often connected to the DC link, which is connected
to the supply, so the customer connections, display etc. must be safely isolated
from this.
from this.
· The output current must be carefully monitored to protect the inverter and the
motor during overload and short circuit.
· At first switch on the DC link capacitors are discharged, and the inrush current
must be limited, usually using a resistor which is bypassed by a relay after a few
seconds.
seconds.
· All connections to the inverter, especially the supply and control connections,
may carry a lot of interference and must be fitted with suitable protection
components.
components.
· An internal power supply with several different output voltages is needed to
supply the control electronics.
· The inverter, especially the IGBTs and rectifier diodes, produce heat which
must be dissipated using a fan and heatsink.