 Description
and Application
SMC transformer/rectifiers convert the three-phase supply voltage to the specified dc
voltage. SMC provides rectifiers in a wide range of sizes and configurations to meet
your specific requirements. Typically these range from 500KW through 5000KW and
250VDC through 1500VDC.
SMC rectifiers are available to meet the requirements of your duty cycle. Ranging
from 100% continuous duty to Extra-Heavy Traction Duty as defined by NEMA RI-9.
To assure maximum system reliability, SMC rectifiers are convection cooled for all duty
cycles, except for applications requiring a dust-tight enclosure. This capability
includes "n-1" operation where full rated output is possible with one diode per
leg removed from the circuit.
Some typical applications for which SMC power silicon rectifiers are well suited are:
 | Traction power  |
| Chemical processes |
| Electromagnets |
| DC motor power supplies |
| Other applications requiring DC power |
SMC also custom designs silicon power rectifiers for special applications.
Electrical Configurations
A rectifier unit may be configured into a wide variety of connections to meet
requirements of output ripple. The most common these connections are six-pulse (ANSI
Circuit 25 or 26) and twelve-pulse (ANSI Circuit 31). A connection with more pulses on the
output voltage has lower ripple and harmonic content, however; they are proportionally
more costly.
Typical 6-Pulse Rectifier
(ANSI Circuit 25)
A six-pulse rectifier is composed of six or multiple of six diodes
configured to form a three-phase double-way bridge for AC to DC conversion. The output of
this configuration has six pulse per ac cycle and a ripple content of 4.17%. |
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Typical 12-Pulse Rectifier
(ANSI Circuit 31)
A twelve-pulse rectifier is comprised of essentially two six-pulse bridges connected in
parallel through an interphase transformer to insure proper current balance between the
two bridges. The input to the two bridges are provides by two seperate windings of the
rectifier transformer. One of the windings is connected delta while the orhte is connected
wye, this provides the necessary phase shift to produce the six phases. when these six
phases are fullwave rectified this produces the twelve pulse output. This connection
provided a ripple voltage of 1.02%. |
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Because of the high currents in the larger rectifiers, two or more diodes may be
paralleled to share the current. The actual number of diodes is determined by the output
current, duty cycle and the application requirement such as "n-1". To assure
acceptable current sharing among parallel diodes, SMC rectifiers are designed with
geometric symmetry and commutation bus bars.
 Diode Heatsink Assemblies (Stacks)
The disc type (Hockey-puk) diodes are mounted between two silver plated copper bus bars
to which the extruded aluminum heatsinks are mounted. To maintain proper mounting
pressure, an insulating clamp is used which extends through the heatsinks and copper bars,
sabdwiching the diode in the center. This clamp is designed to allow accurate tightening
without the use of special tools. This configuration not only removes the aluminum
heatsink from the current carrying path but also increases the thermal mass of the stack
allowing the rectifier to ride through short duration overloads without exceeding its
maximum temperature. |
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Protective Devices
SMC rectifiers are selectively protected by fast acting semiconductor fuses. If a diode
fails, this series connected fuse will interrupt the curent in less than half a cycle to
isolate the faulted diode.
They are protected from voltage surges by high energy surge suppessors and an RC
snubber network to limit all switching and lightning surges to an admissible level
according to NEMA RI-9. The surge suppessor acts to limit the voltage magnitude to alevel
less than the peak reverse voltage of the diodes, while the RC snubber network acts to
provide a low impedance path to fast rise time transient voltages.
This combination of design and manufacturing factors provide a rectifier that will give
years of uninterrupted service.
Enclosure
SMC rectifier enclosures are self-supporting and freestanding. Typical units have
section housing the diode stacks and connection bus bars, a section housing the controls
and metering, and section(s) housing output disconnect switch(es) if required. The
diode and disconnect switch sections are equipped with access doors with large viewing
windows. The enclosure is mounted on a welded structural steel tubing base with a
bolted skeletal steel frame. The access doors, side and top panels are bolted to
this framework.
Non dust-tight enclosures are ventilated by a gride of small openings located along the
bottom and the top of the enclosure. Dust-tight enclosures fitted with exhaust
fan(s) mounted in the top of the enclosure with filters fitted inside the ventilation
openings along the lower portion of the enclosure.
These rectifiers may be configured with front and rear access or front access only.
If front and rear access is used then doors with viewing windows are provided for
both locations.
Typical Rectifier Arrangement
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