Power Section - Danfoss VLT HVAC Drive FC100 Service Manual

Internal Frequency Converte...
Circuitry for controlling the speed of the cooling fans is
also provided on the power card.
The gate drive signals from the control card to the output
transistors (IGBTs) are isolated and buffered on the gate
3 3 drive card. In units that have the dynamic brake option,
the driver circuits for the brake transistors are also located
on this card.
For 12-pulse frequency converters, an MPIC card is
connected to the power card to control additional SCRs
present on the 12-pulse units. All other functions are
identical.

3.2.3 Power Section

The high voltage power section consists of AC input
terminals, AC and DC bus bars, fusing, harnessing, AC
output, and optional components. The power section (see
illustration below) 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.
Each unit contains one DC coil per inverter module.
Therefore the F1/F3 units contain two DC coils and the
F2/F4 units contain three. The DC coil has 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 reduces
mains harmonics.
The DC bus capacitors are arranged into a capacitor bank
along with bleeder and balancing circuitry. Each inverter
module contains two DC capacitor banks.
The inverter section is made up of six IGBTs, commonly
referred to as switches. Two switches are necessary for
each phase of the three-phase power, for a total of six
switches per IGBT module (half-phase per switch). Three
IGBT modules run in parallel are contained in each inverter
due to the high current handling requirements. Each
inverter can be paralleled with one or two additional
inverter modules to provide the required current for the
power size.
50
High Power Service Manual for Modular F Frame Drives
A Hall effect type current sensor is located on each phase
of the inverter module output to measure motor current.
This type of device is used instead of more common
current transformer (CT) devices to reduce the frequency
and phase distortion that CTs introduce into the signal.
With Hall sensors, the average, peak, and earth leakage
currents can be monitored. The current sensors from each
inverter module are summed with the same phase of the
other inverter modules by the MDCIC to provide one
current level to the control card.
Illustration 3.4 Typical F-frame Power Section
AC Line
L11
L21
L31
Sine wave
AC Line
L12
L22
L32
Illustration 3.5 Typical 12-pulse Power Section
®
MG90K202 - VLT is a registered Danfoss trademark
Recti er DC Bus Inverter
Filter reactor
IGBT
Filter capacitor
Filter reactor
Filter capacitor
Motor
T1
T2
T3
PWM waveform