3.3 Sequence of Operation
3.3.1 Rectifier and Option Cabinet
When input power is first applied to the frequency
converter, it enters through the input terminals L1, L2, L3
(for 12-pulse units, L11, L21, L31 and L12, L22, L32) and on
to the disconnect or/and RFI option, depending on the
unit's configuration (see illustration 3-4). If equipped with
optional fuses, these fuses (FU1, FU2, FU3, and on 12-pulse
additional fuses FU4, FU5, FU6) limit damage caused by a
short circuit in the power section. The SCRs, in the
combined SCR/diode modules, are not gated so current
can travel to the rectifier on the soft charge card. The SCR
and diode modules are separate. Additional fuses located
on the soft charge card provide protection in the event of
a short in the soft charge or fan circuits. Three phase
power is also branched off and sent to the power card. It
provides the power card with a reference of the main
supply voltage and provides a supply voltage for the
cooling fans.
During the charging process, the top diodes of the soft
charge rectifier conduct and rectify during the positive half
cycle. The diodes in the main rectifier conduct during the
negative half cycle. The DC voltage is applied to the bus
capacitors through the soft charge resistor. The purpose of
charging the DC bus through this resistor is to limit the
high inrush current that would otherwise be present.
Positive temperature coefficient (PTC) resistors located on
the soft charge card are in series with the soft charge
resistor. Frequent cycling of the input power or the DC bus
charging over an extended time can cause the PTC
resistors to heat up due to the current flow. Resistance of
the PTC device increases with temperature, eventually
adding enough resistance to the circuit to prevent
significant current flow. This protects the soft charge
resistor from damage along with any other components
that could be damaged by continuous attempts to charge
the DC bus.
The low voltage power supplies are activated when the DC
bus reaches approximately 50 VDC less than the alarm
voltage low for the DC bus. After a short delay, an inrush
enable signal is sent from the control card to the power
card SCR gating circuit. The SCRs are automatically gated
when forward biased, as a result acting similar to an
uncontrolled rectifier.
When the DC bus capacitors are fully charged, the voltage
on the DC bus will be equal to the peak voltage of the
input mains. Theoretically, this can be calculated by
multiplying the mains value by 1.414 (VAC x 1.414).
However, since AC ripple voltage is present on the DC bus,
the actual DC value will be closer to VAC x 1.38 under
unloaded conditions and may drop to VAC x 1.32 while
running under load. For example, a frequency converter
connected to a nominal 460 V line, while sitting idle, the
DC bus voltage will be approximately 635 VDC (460 x
1.38).
As long as power is applied to the frequency converter,
this voltage is present in the intermediate circuit and the
inverter circuit. It is also fed to the switch mode power
supply (SMPS) on the power card and is used for
generating all other low voltage supplies.
During normal operation, the power card and control card
are monitoring various functions within the frequency
converter. The current sensors provide current feedback
information. The DC bus voltage and mains voltage are
monitored as well as the voltage delivered to the motor. A
thermal sensor mounted on the heatsink for each rectifier
module.
Internal Frequency Converte...
High Power Service Manual for Modular F Frame Drives
MG90K202 - VLT
®
is a registered Danfoss trademark
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