MAIN
CIRCUIT
Service Manual M4 - M10
29.12.1999
Page
61
8.
MAIN
CIRCUIT
8.1 General
Main circuit consists in three main blocks; AC- to DC converter, DC bus and
DC- to AC-converter.
AC-power from line is rectified by six-pulse rectifier, filtered by three phase
line reactor and by above mentioned DC capacitors.
DC bus, based on voltage source topology, maintains stable DC voltage level
in DC capacitors for accurate motor control. Addition to the DC bus there are
precharging circuit for limiting surge current to the DC capacitors during
start-up.
DC- to AC-converter maintains proper control for motor regarding desired
speed and torque and regarding measured values of DC voltage in DC bus and
measured motor current.
8.2 AC- to DC converter
8.2.1 Description
Front end of drive consists three phase line reactor from where power is lead
to six pulse rectifier. The rectifier is either module including six diodes, three
individual modules including two diodes or alternatively in semi controlled
bridge, one diode and one SCR per module in phase.
The line reactor is wound to common core for optimised inductance value with
high enough saturation point in compact mechanical size.
Diode or semiconductor bridge is module type component, installed onto heat
sink for proper cooling. Construction of the diode module provides 2,5kV
voltage isolation from live parts to base plate of the module, if installed
correctly.
8.2.2 Operation
AC-power is lead from three phase line into drive, through three phase line
reactor. The line reactor smoothens input current and ripple current 6*fline in
DC bus to provide long service life for electrolytic DC capacitors in the DC bus.
The line reactor reduces content of harmonic current in the line to help power
distribution system to provide cleaner power. Inductance value of the line
reactors depends on power rating of the device. Average impedance value
runs in range 2,5...4%.
