The method most used in present shipboard equipment is multipulse
rectification, in which the secondary of a power line transformer is
interconnected to give 12- , 18-, or 24-pulse rectification. The first
harmonic in 24-pulse rectification is the 23rd (1380 Hz for 60 Hz power) and
has an amplitude of 4.3% of the fundamental, fairly close to the 3% limit. The
problem is that a power line transformer is needed which, for 60 Hz power, is
larger than an off-line switching-mode power supply. Also, the transformer VA
rating (ratio of volt- amperes to watts) is greater than 2 because of the poor
utilization of the secondary windings (Reference 18).
The most popular multipulse rectification is a parallel six-phase,
full-wave arrangement with a wye and delta secondary. This eliminates the 5th
and 7th harmonics, making the 11th the first harmonic of concern (9%
amplitude). This increases the transformer's VA rating from 1.05 to 1.22 for a
conventional bridge. Better configurations are available, such as the 6-phase,
center-tapped, ring-connected transformer which decreases the transformer's VA
rating to 1.03 rather than increasing it (Reference 19).
If isolation is not required, then an auto-transformer configuration can be
used with a considerable savings in size, reduced acoustic hum and reduced
stray magnetic field. A discussion of the merits of this approach is in
The best solution using the multipulse method is to exploit some
characteristics of the ship electrical system combined with the combat system.
The present state-of-the-art in off-line switching-mode power supplies
strongly favors using 115 V rather than 440 V as the input power to the power
supply. Since there is a transformer someplace in the system to make this
transformation, it can be used with multipulse rectification to provide 155
VDC to the off-line switching-mode power supplies. These power supplies will
operate from 115 VAC or 155 VDC equally well. If a transformer-rectifier
scheme such as the 6-phase, center- tapped, ring-connected transformer can be
used, the 3% limit can be met with no size or weight penalty to the combined
ship and combat system.