marking is shown in figure 3-17. Terminals for thehigh-voltage windings are marked H1, H2, H3, and soforth. The increasing numerical subscript designates anincreasing voltage, denoting a higher voltage betweenH1 and H3 than the voltage between H1 and H2.The secondary terminals are marked X1, X2, X3,and so forth Two types of markings maybe employedon the secondaries. When the H1 and X1 leads arebrought out on the same side of the transformer (fig.3-17, view A), the polarity is called subtractive. Thereason this arrangement is called subtractive is if & H1and X1 leads are connected and a reduced voltage isapplied across the H1 and H2 leads, the resultant voltagethat appears across the H2 and X2 leads in the seriescircuit formed by this connection will equal thedifference in the voltages of the two windings. Thevoltage of the low-voltage winding opposes thehigh-voltage winding and subtracts from it; hence theterm, subtractive polarity.When the H 1 and X1 leads are brought out onopposite corners of the transformer (fig. 3-17, view B),the polarity is additive. If the H1 and X2 leads areconnected and a reduced voltage is applied across theH1 and H2 leads, the resultant voltage across the H2 andX1 leads in the series circuit formed by this connectionwill equal the sum of the voltages of the two windings.The voltage of the low-voltage winding aids the voltageof the high-voltage winding and adds to it, hence theterm, additive polarity.Polarity markings do not indicate the internalvoltage stress in the windings. They are useful; only inmaking external connections between transformers.F’igure 3-17.-Polarity markings for large tramsformers: A.Subtractive polarity; B. Additive polarity.400-HERTZ POWERDISTRIBUTIONIn addition to the 60-Hz power supplied by theship’s service generators, ships also have 400-HZsystems. On some ships 400-Hz power is generated bymotor-generator sets and distributed via specialfrequency switchboards (fig. 3-18) to the various400-Hz equipment.These motor generators supply power to ship’sservice special frequency switchboards. Figure 3-19 isa simplified line diagram of the 400-Hz ship’s servicebus tie interconnections on an older ship. The circuitsbeing fed from the 400-HZ ship’s service switchboardsare deleted from the figure for simplicity.Newer ships get their supply of 400-Hz powerthrough the use of 60/400-HZ static converters. The400-Hz system consists of four MBT’s supplying 60-Hzpower to four 60/400-Hz static frequency convertersFigure 3-18.-400-Hz switchboard.3-12
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