18
JOHNSON CONTROLS
FORM 201.21-NM3 (616)
PRODUCT DESCRIPTION
The power section of the drive is composed of four
major blocks consisting of an AC to DC rectifier section
with accompanying pre-charge circuit, a DC link filter
section, a three phase DC to AC inverter section, and
an output suppression network.
The AC to DC rectifier
utilizes a semi-converter formed
by the connection of three SCR/diode modules (1SCR-
3SCR) in a three phase bridge configuration. The mod
-
ules are mounted on a liquid cooled heatsink. Use of the
semi-converter configuration permits implementation
of a separate pre-charge circuit to limit the flow of cur
-
rent into the DC link filter capacitors when the drive is
switched on and it also provides a fast disconnect from
the power mains when the drive is switched off. When
the drive is turned off, the SCRs in the semiconverter
remain in a non-conducting mode and the DC link filter
capacitors
remain uncharged. When the drive is com-
manded to run, the DC link filter capacitors are slowly
charged via the semi-converter. The SCR’s are then
gated fully on.
Three power fuses (1FU - 3FU), an optional circuit
breaker (1SW) and a standard 5% impedance minimum
3 phase line reactor connect the AC to DC converter to
the incoming power. Very fast semiconductor power
fuses are utilized to ensure that the SCR/diode module
packages do not rupture if a catastrophic failure were to
occur on the DC link. The SCR Trigger board provides
the gating pulses for the SCR’s as commanded by the
VSD Logic board.
The DC Link filter
section of the drive consists of a
group of electrolytic filter capacitors (C1-C6). This ca
-
pacitor bank effectively smooths the ripple voltage from
the AC to DC rectifier while simultaneously providing a
large energy reservoir for use by the DC to AC inverter
section of the drive. In order to achieve the required
voltage capability for the capacitor portion of the filter,
filter capacitor “banks” are formed by connecting two
groups of parallel capacitors in series to form a capaci-
tor “bank”. In order to assure an equal sharing of the
voltage between the series connected capacitors and to
provide a discharge means for the capacitor bank when
the VSD is powered off, “bleeder” resistors (1RES and
2RES) are connected across the capacitor banks.
The DC to AC inverter
section of the VSD serves to
convert the rectified and filtered DC back to AC at the
magnitude and frequency commanded by the VSD Logic
board. The inverter section is actually composed of two
to four identical inverter output phase assemblies. These
assemblies are in turn composed of 3 pairs of Insulated
FIG. 2 - PWM VOLTAGE WAVEFORM
LD10480
Variable Speed Drive (VSD)
The VSD (variable speed drive) is a liquid cooled,
transistorized, PWM inverter, which provides speed
control to vary the speed of 2, 3 or 4 compressor motors.
The VSD changes the duration of the voltage pulses
supplied to the motor to enable control of compressor
speed to match the system load. A PWM generator, on
the VSD Logic Board, with a switching frequency of
3125 Hz modulates the voltage signal to provide a rela-
tively constant V/F ratio. In some cases, the V/F ratio
is slightly modified to provide additional torque to the
motor. Sample 3 phase current waveforms are shown
in FIG. 1 to show the sinusoidal characteristics of the
current drawn by the compressor motors.
FIG. 1 - PWM CURRENT WAVEFORM
LD10479
A Sample PWM voltage waveforms is shown in FIG.
2. The pulses near the sides of the rectangular groups
of waves are notably narrower, representing the lower
voltage of a sinusoidal waveform as it rises or falls from
the “0” crossing.
Summary of Contents for YCAV0267E
Page 61: ...61 JOHNSON CONTROLS FORM 201 21 NM3 616 This intentionally left blank 6 ...
Page 73: ...73 JOHNSON CONTROLS FORM 201 21 NM3 616 This page intentionally left blank 6 ...
Page 89: ...89 JOHNSON CONTROLS FORM 201 21 NM3 616 6 035 20890 009 REV LOCATION LABEL CON T LD11141 ...
Page 146: ...146 JOHNSON CONTROLS FORM 201 21 NM3 616 TECHNICAL DATA This page intentionally left blank ...
Page 147: ...147 JOHNSON CONTROLS FORM 201 21 NM3 616 6 This page intentionally left blank ...
Page 247: ...247 JOHNSON CONTROLS FORM 201 21 NM3 616 This page intentionally left blank ...
Page 269: ...269 JOHNSON CONTROLS FORM 201 21 NM3 616 This page intentionally left blank 8 ...
Page 289: ...289 JOHNSON CONTROLS FORM 201 21 NM3 616 8 This page intentionally left blank ...
Page 317: ...317 JOHNSON CONTROLS FORM 201 21 NM3 616 8 This page intentionally left blank ...
Page 348: ...348 JOHNSON CONTROLS FORM 201 21 NM3 616 MAINTENANCE NOTES ...
Page 349: ...349 JOHNSON CONTROLS FORM 201 21 NM3 616 9 NOTES ...