GE Industrial Solutions AF-600 FP High Power Unit Sizes 6x - Manual (Page 36)

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) 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) 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.
High Power Service Manual for Unit Sizes 6x
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