Danfoss VLT HVAC FC 100, Service Manual, Page: 48 / 220

3.3 Sequence of Operation
3.3.1 Rectifier Section
When input power is first applied to the adjustable frequency drive, it enters through the input
terminals (L1, L2, L3) and on to the disconnect or/and RFI option, depending on the unit's con-
figuration (see Figure 3-4). If equipped with optional fuses, these fuses (FU1, FU2, FU3) 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. In E adjustable
frequency drive models, 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 pre-
vent 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 line power. Theoretically, this can be calculated by multiplying the line power
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, an adjustable frequency drive 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 adjustable frequency drive, this voltage is present in the inter-
mediate 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.
High Power Service Manual for VLT
®
FC Series
Adjustable Frequency Drives
3 Internal Adjustable Frequency Drive Opera-
tion
MG.90.L1.22 - VLT ® is a registered Danfoss trademark
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