RE: [PSUBS-MAILIST] RE: DC Motors

["Jay K. Jeffries" <bottomgun@mindspring.com>]

Carsten,
You might consider a motor-generator (MG) setup...a linked combo of a DC
unit that can operate as a motor or generator and a AC unit that can operate
as a motor or generator.  These were used on older diesel boats to generate
the high voltage and or frequency required to operate some electronics.  On
present day nuclear subs, the MG resides between the steam turbine
generators (SSTG) and the batteries.  When charging the batteries, the AC
unit is a motor and the DC unit the generator.  When drawing power from the
batteries, the DC unit is the motor and the AC unit the generator.  Under
normal ship's electrical load with the SSTG on line, the MG set would be
slowly discharging the batteries to keep them in condition.  On a loss of
SSTG power, the MG sets convert the DC power to AC.  The system was aligned
in a port & starboard cross connected system for redundancy.  I'll try to
lay out an ASCII schematic; the "X" indicate isolation switches for spitting
systems or isolating faulty components/wiring:

 (DC load side)    (AC load side)
          _____ MG ____X______ SSTG
         |                 |
         X                 |
         |                 |
BATTERIES                  X (normally open)
         |                 |
         X                 |
         |_____ MG ____X___|__ SSTG


Your boat could be wired in a similar manner where the diesel generator is
substituted for the SSTG.  Your main motor(s) could then utilize the
efficiency of using high voltage and lower amperage units.
Respectfully,
Jay K. Jeffries



Hi Stephen I got this way an run in trouble.. 
On euronaut I build a 400 V 40 kw DC Motor in - and use them as motor
underwater and generator on the surface - direct to the 400 V DC
batteries. That was the idear - But first highpower Dc technologies is
old technologies and not so much parts are available anymore. 

The real trouble beginns when you realise that both  bilge Pumps have
400 V AC - both compressors have 400 V AC and other subsystem like
chargers etzc also.  

The converter to make from 400 V Dc just 400 volt AC ist not off the
shelf..  To make 400 AC you need at least a DC Voltage of 600 Volt.. 
So I need now a highpower DC-Dcconverter which make from 350-450 V DC a
600-700 V DC and than a DC-Ac converter to make 400 V 3-phase AC.. 

Maybe it is easier to go with a AC system today. 

Make shure that you have all your subsystem on your list before you
decide which way your go. To rebuild all subsystem back from AC to DC
will be a nightmare. 

48 V DC for a 75 kw output are very, very strong cables.. 

75000 watt / 48 V = 1563 ampere.. 

best regards Carsten







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