MIL-STD-1399-300 Electric Power, Alternating CurrentThis military standard section establishes electrical interface characteristics for shipboard equipment utilizing AC electric power to ensure compatibility between user equipment and the electric power system. Characteristics of the electric power system are defined and tolerances are established, as well as requirements and test methods for ensuring compatibility of shipboard user equipment with the power system. The policies and procedures established by MIL-STD-1399 are mandatory. This section and the basic standard are to be viewed as an integral single document for use in the design and testing of electric power systems and user equipment.

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MIL-STD-1399-3008

3.6  Power factor (pf).  Pf is the ratio of the real power in watts to the product of the rms voltage and rms current.  For voltage waveforms with minimal distortion, pf can be approximated as the product of the displacement pf (dpf) and the distortion ( ).  This is shown in Equation 9.

Pf =    P (watts)  �   dpf

Vrms Irms

EQUATION 9

3.6.1  Displacement power factor (dpf).  The dpf is defined as the cosine of the angle difference between the fundamental frequency component of the input voltage and the fundamental frequency component of the current, shown in Equation 10.  The dpf is the same as the pf in linear circuits with sinusoidal voltages and currents.  The angle determines whether the pf is leading or lagging.  A positive value of the angle means that the current lags the voltage (lagging pf, inductive load).  A negative value of the angle means that the current leads the voltage (leading pf, capacitive load).

Where:

dpf = cos (φV  − φI)

φv  is the angle of the fundamental frequency component of the input voltage

φI  is the angle of the fundamental frequency component of the current

EQUATION 10

3.6.2  Distortion component ( ) of power factor.  The distortion component ( ) of power factor is the ratio of the rms magnitudes of the fundamental frequency current to the total current, shown in Equation 11.

Where:

  =  Ifundamental

Itotal

Ifundamental  is the rms value of the fundamental frequency current

Itotal  is the rms value of the total current

EQUATION 11

3.7  Power.  Quantity that consists of real, reactive, and apparent power.

3.7.1  Real power.  Real power, or average power, is defined as the product of the rms voltage and rms current multiplied by the power factor (pf).  The unit of real power is the watt.  Real power provides work over time.

3.7.2  Reactive power.  Reactive power is defined as the product of the rms voltage and rms current multiplied by a reactive factor.  The unit of reactive power is volt-ampere reactive (VAR).  Reactive power provides no net energy transfer over time; the average reactive power is zero.

3.7.3  Apparent power.  Apparent power is defined as the product of the rms voltage and the rms current.  The unit of apparent power is volt-ampere (VA).  Apparent power can be calculated as the square root of the sum of the squares of real and reactive power.

3.8  Pulse.  A pulse is a brief excursion of power lasting longer than 1 cycle at nominal frequency and less than

10 seconds.

11

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