Fuses ratings

Mike O'Dell mo at 131.ccr.org
Wed Jul 25 18:00:26 CDT 2012

i worked with a company which used the HVDC approach for marine systems.
since everything has a switching PS today, driving one with an inverter
simply puts two in series plus the bulk rectifier. So all the big loads
were designed to use BLDC motors with wide-range controllers. the air
conditioning didn't have a bang-bang thermostat - it simply spun at
the right speed to maintain thermostasis. likewise the refrigeration
and othere big loads.

the thing that made it *really* interesting, however, was the
feed-forward control loop done with a CAN-bus. the large loads would
announce to the generators how they would be ramping power and
then after an ack from the generator controller, they could start
the run-up. before the ack, however, the controller goosed the
diesel(s) so they could run-up smoothly with the load ramp.
the generators were DC output in the 200-700VDC range.

this is in considerable distinction to the classic genset which
responds to the reduction in RPM caused by increased load on the
generator (alternator) and throttles-up to maintain RPM. variable
speed drives allow big loads to ramp up without hammering the
generators, but they still react to the increasing load.

the feed-forward control system is good for a 15-20% improvement
in fuel economy in heavy applications.

they built a power and air conditioning system for the modern
version of the Army MASH unit operating rooms. It provided electric power
for all the normal loads via inverters from the HVDC bus but
then supplied refrigeration and A/C with the variable-speed
compressors and BLDC motors. the resulting package was about
1/2th the size and weight of just the genset it replaced
(the normal A/C units were carried on separate trucks!) 
and the fuel consumption was down about 40%. this is a big issue
when you have to supply fuel out in the middle of
Nowhere, Iraq.

the system was called a "load-following generator" which is
inaccurate from a control-system viewpoint, but the fact that
the generator didn't have to run harder than necessary to
maintain load was "load-following".


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