3.2.3 Power Section
The high voltage power section consists of AC input terminals, AC and DC bus bars, fusing, har-
nessing, AC output, and optional components. The power section (see Figure 3-3) also contains
circuitry for the soft charge and SCR/diode modules in the rectifier; the DC bus filter circuitry
containing the DC coils, often referred to as the intermediate or DC bus circuit; and the output
IGBT modules which make up the inverter section.
In conjunction with the SCR/diode modules, the soft charge circuit limits the inrush current when
power is first applied and the DC bus capacitors are charging. This is accomplished by the SCRs
in the modules being held off while charging current passes through the soft charge resistors,
thereby limiting the current. The DC bus circuitry smooths the pulsating DC voltage created by
the conversion from the AC supply.
The DC coil is a single unit with two coils wound on a common core. One coil resides in the positive
side of the DC bus and the other in the negative. The coil aids in the reduction of line harmonics.
The DC bus capacitors are arranged into a capacitor bank along with bleeder and balancing cir-
cuitry. Due to the requirement for higher power capacity, some adjustable frequency drives have
two capacitor banks connected in parallel.
The inverter section is made up of six IGBTs, commonly referred to as switches. One switch is
necessary for each half phase of the three-phase power, for a total of six. The six IGBTs are
contained in a single module. Due to higher current handling requirements, some models contain
two or three larger six-pack style modules. In these units, each switch (half phase) is made up of
two or three IGBTs in parallel.
A Hall effect type current sensor is located on each phase of the output to measure motor current.
This type of device is used instead of more common current transformer (CT) devices in order to
reduce the amount of frequency and phase distortion that CTs introduce into the signal. With Hall
sensors, the average, peak, and ground leakage currents can be monitored.
Figure 3.3: Typical Power Section
3 Internal Adjustable Frequency Drive Opera-
tion
High Power Service Manual for VLT
®
FC Series
Adjustable Frequency Drives
3-4
MG.90.L1.22 - VLT
®
is a registered Danfoss trademark
3
Summary of Contents for VLT HVAC FC 100
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