MIL-HDBK-1004/5
Section 3: MAXIMUM SERVICE CABLE LENGTH
FOR A 100-AMPERE UNIT LOAD
3.1
Variations. The length of the service cable used to supply the
100-ampere 0.8-power-factor loads will affect the voltage drop at the aircraft
interface point, which must not be less than 113 volts to meet criteria. Case
B analyzes the effects of service-cable lengths according to this requirement.
Cases B1 through B5 analyze the drop through the service cable only. Cases
Bll through B15 analyze the drop through the combined service cable and
aircraft cable to the aircraft interface point, the point where
minimum-voltage criteria must be met.
3.2
Discussion. The maximum length of the service cable is determined by
MIL-STD-704's steady state voltage requirement for the aircraft's internal
operating voltage, which is 108 volts minimum. The minimum voltage at the
aircraft interface point has to allow for a 0- to 5-volt drop in the aircraft.
Therefore, the minimum voltage at the aircraft interface connector is 113
volts RMS. The four parameters that determine the voltage at the connector
are: the load, the service cable impedance, line drop compensation provided,
and the voltage at the feeder cable where the utilization service assembly is
connected. Cases Bl through B15 depict voltage drops for several lengths of
service cables. The series of runs from Cases B1 through B5 establish a
maximum length of service cable for one set of cable parameters with a fixed
12-percent line compensation.
For Cases B1 to B5 (Figures A-17 through A-21, respectively), the
service cable length was varied from 40 to 200 feet. The service cable
characteristics are:
Resistance = 0.0807 ohms per 1,000 feet
Inductance = 0.1853 ohms per 1,000 feet
A 100-ampere, 0.8-power-factor load is assumed at bus 6 (load end of
service cable). This set of runs is preliminary and does not show a detailed
distribution to the load.
Service cable lengths are as follows: Case B1 - 40 feet: Case B2 -
80 feet:Case B3 - 120 feet: Case B4 - 160 feet: and Case B5 - 200 feet. Case
B figures are similar to Case A figures, except that the effects of the
service cable lengths have been indicated. This adds a sixth bus at the end
of the service cable. The two important voltages are the per-unit voltages at
bus 3 and bus 6. Bus 3 per-unit voltage determines the no-load voltage on all
utilization service assemblies on this feeder cable, and bus 6 voltage
indicates the steady state load voltage at the end of the service cable. To
meet the MIL-STD-704 specification for steady state voltage, the voltage at
the aircraft interface connection must be kept above 113 volts RMS.
Using the system parameters given and a service cable length of 200
feet (Case B5), the steady-state voltage at bus 6 with a dedicated feeder and
a 100-ampere 0.8-power-factor load is 115.6 volts, as shown on Figure A-21.
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