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21-5 Generators

21-5 Generators. An electric generator, or dynamo, converts mechanical energy to electrical energy. An ac generator consists of many loops of wire wrapped around an armature that can rotate in a magnetic field… it produces an ac current.

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21-5 Generators

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  1. 21-5 Generators An electric generator, or dynamo, converts mechanical energy to electrical energy. An ac generator consists of many loops of wire wrapped around an armature that can rotate in a magnetic field… it produces an ac current. The emf formula is e=NBAwsinwt…where A=lh and w=2pf (See page 629) Over 99% of the electricity in the USA is produced by generators. See Example 21-6 p630.

  2. 21-5 Generators (cont’d) • DC generators are much like ac generators, but the slip-ring is replaced by a split-ring commutator. The AC current is reversed every ½ turn to give a + only current. (see p 630) • A capacitor can be placed in series with the generator, or wrap many more windings to smooth the current. • In an alternator, dc current is produced by semiconductor diodes.

  3. 21-6 Counter EMF and Torque; Eddy Currents. • Because of Lenz’s law, a generator produces a counter emf or back emf to oppose mottion. • See Example 21-7 p 632 • Torque that opposes the motion of a generator is called counter torque. • Smaller counter-currents in a conductor moving in a magnetic field are called eddy currents.

  4. 21-7 Transformers; Transmission of Power • A transformer is a device for increasing or decreasing an ac voltage. • A transformer consists of 2 coils a primary coil and a secondary coil. • Iron transformers are about 99% efficient. • A step-up transformer increases voltage… a step-down transformer decreases voltage. • VpIp=VsIs • See Example 21-9 on p 635

  5. 21-7 (cont’d) • There is always some power loss in transmission of electricity, but it is minimized by transmission at high voltage and low current. P=VI and P=I2R • Voltage is stepped-up to 2,400 V and then stepped-down to 240V or 120V. • See p636 Example 21-10

  6. HOMEWORK • P655 pr#19-23, and 24-26, and pr#30-35 due Wednesday BOP. • Packet 3B due Monday BOP.

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