3phase(2)

# 3phase(2) - Unit Three-Phase Circuits objectives A fter...

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Unit 12 Three-Phase Circuits A fter studying this unit, you should be able to: Discuss the differences between three-phase and single-phase voltages. Discuss the characteristics of delta and wye con- nections. Compute voltage and current values for delta and wye circuits. Connect delta and wye circuits and make mea- surements with measuring instruments. Most of the electrical power generated in the world today is three-phase. Three-phase power was first conceived by Nikola Tesla. In the early days of electric power generation, Tesla not only led the battle concerning whether the nation should be powered with low-voltage direct current or high-voltage alternating current, but he also proved that three-phase power was the most efficient way that electric- ity could be produced, transmitted, and consumed. Three-Phase Circuits There are several reasons why three-phase power is superior to single- phase power. objectives

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262 Electrical Studies for Trades 1. The horsepower rating of three-phase motors and the KVA (kilo-volt- amp) rating of three-phase transformers is about 150% greater than for single-phase motors or transformers with a similar frame size. 2. The power delivered by a single-phase system pulsates, Figure 12-1 . The power falls to zero three times during each cycle. The power delivered by a three-phase circuit pulsates also, but it never falls to zero, Figure 12-2 . In a three-phase system, the power delivered to the load is the same at any instant. This produces superior operating characteristics for three-phase motors. 3. In a balanced three-phase system, the conductors need be only about 75% the size of conductors for a single-phase two-wire system of the same KVA rating. This helps offset the cost of supplying the third conductor required by three-phase systems. A single-phase alternating voltage can be produced by rotating a magnetic field through the conductors of a stationary coil, as shown in Figure 12-3 . Figure 12-1 Single-phase power falls to zero three times each cycle. Figure 12-2 Three-phase power never falls to zero. Figure 12-3 Producing a single-phase voltage. N S A
Since alternate polarities of the magnetic field cut through the conduc- tors of the stationary coil, the induced voltage will change polarity at the same speed as the rotation of the magnetic field. The alternator shown in Figure 12-3 is single phase because it produces only one AC voltage. If three separate coils are spaced 120° apart, as shown in Figure 12-4, three voltages 120° out of phase with each other will be produced when the magnetic field cuts through the coils. This is the manner in which a three-phase voltage is produced. There are two basic three-phase connec- tions, the wye or star connection and the delta connection.

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## This note was uploaded on 05/21/2011 for the course CGN 3710 taught by Professor Bloomquist during the Summer '10 term at University of Florida.

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3phase(2) - Unit Three-Phase Circuits objectives A fter...

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