CHAPTER 2 – POWER ELECTRONICS AND CABINET FEATURES 33
2.3.1 DFE - Diode front end
The diode front end of the drive consists of two
identical phase modules. Each phase module
houses a 12-pulse diode rectifier bridge which add
up to a network friendly 24-pulse system. The DFE
rectifies the AC line voltage of the supply network
and connects its output to the minus, and plus side
of the DC link. The DFE allows two-quadrant opera-
tion of the drive.
2.3.2 DC link
The DC link consists of the DC capacitor, the
charging unit and the grounding switch.
DC link capacitor
The numbers of capacitors (1) present depend on
the power rating of the drive.
Charging unit
Charging
The unit charges the DC link capacitor and the
phase capacitors before the MCB is closed and the
drive is connected to the main power supply. Thus,
excessive inrush currents are prevented.
The charging unit is fed from the auxiliary power
supply and charges the DC link and the phase
capacitors to 90% of the steady state voltages.
The charging sequence is started by pressing the
SUPPLY ON push-button on the control compart-
ment door. After the charging sequence has fin-
ished, the charging unit is disconnected from the
medium voltage circuit, the MCB is closed and the
DC link reaches its nominal level.
Stand-by mode
When the DC link has been charged, it is possible to
keep the drive in this state to facilitate a more
rapid start-up procedure. The losses during stand-
by mode are approx. 1% of the nominal power and
are caused from small losses in the INU and the
auxiliaries.
Discharging
Discharging is initiated by pressing the SUPPLY
OFF pushbutton on the control compartment door.
The energy stored in the DC link is dissipated in the
resistors of the power part of the drive. When the
DC link voltage has decreased below a quarter of
2-5 DFE - Diode front end
2-5
2
1 Phase module 1
2 Phase module 2
1
2
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