AC Circuits & Transformers Lecture 7 PDF

IM 115 AC CURRENT, MUTUAL INDUCTANCE & TRANSFORMERS DC & AC Currents  DC (Direct Current)  Current flows from one pole to another and never changes direction  AC (Alternating Current)  Current flows first from one pole to the other and then reverses and flows in the opposite directi...

IM 115 AC CURRENT, MUTUAL INDUCTANCE & TRANSFORMERS DC & AC Currents  DC (Direct Current)  Current flows from one pole to another and never changes direction  AC (Alternating Current)  Current flows first from one pole to the other and then reverses and flows in the opposite direction  This changing current flow offers improved characteristics Easy energy transformation Efficient transfer over long distances Induction and capacitance  The Sinusoidal (Sine) Wave  Thesine wave is the wave generated when AC current is produced  Itvaries in intensity as the wave goes from zero to maximum and back through zero to the negative maximum  This pattern follows the “sine” of an angle The Sine Wave Positive alternaton i + - i negative alternation AC Generator: One revolution of the wire loop generates one cycle of the sinusoidal voltage. Thomas L. Floyd Copyright ©2003 by Pearson Education, Inc. Principles of Electric Circuits, 7ed. Upper Saddle River, New Jersey 07458 All rights reserved. Mutual Inductance  As the sine wave fluctuates, the magnetic field grows and ebbs  When two coils are placed close to each other, a changing electromagnetic field produced by the current in one coil will cause an induced voltage in the second coil because of mutual inductance A coil of wire forms an inductor. When there is current through it, a three-dimensional electromagnetic field is created, surrounding the coil in all directions. As the current grows and ebbs, the magnetic field expands and collapses Thomas L. Floyd Copyright ©2003 by Pearson Education, Inc. Principles of Electric Circuits, 7ed. Upper Saddle River, New Jersey 07458 All rights reserved. The Basic Transformer A transformer is simply two coils placed so that there is mutual inductance between the two coils Primary Secondary Winding Winding  Transformer Windings  The driving winding is the primary (power input)  The driven winding is the secondary (power output)  Primary and secondary are application specific and there is no physical difference between the two The Transformer Core  The windings are formed around a core  Common core materials are: Air core Ferrite core Iron core Air Ferrite Iron Core type and Shell type  Step-Up Transformers  Ifthe number of turns in the secondary is greater than the number of turns in the primary the output voltage will be greater than the input voltage  This is referred to as a step-up transformer as the voltage is increased in the secondary output Step-Up Transformer 1:4 30 Vrms 120 Vrms 4 30Vrms  = 120Vrms 1 According to the turns ratio and given information, what will the voltage on the secondary coil be? Thomas L. Floyd Copyright ©2003 by Pearson Education, Inc. Principles of Electric Circuits, 7ed. Upper Saddle River, New Jersey 07458 All rights reserved. Step-Down Transformers  Ifthe number of turns in the secondary is less than the number of turns in the primary the output voltage will be less than the input voltage  This is referred to as a step-down transformer because the voltage is decreased at the secondary output Step-Down Transformer 4:1 120 Vrms 30 Vrms 1 120Vrms  = 30Vrms 4 What will the voltage on the secondary be? Thomas L. Floyd Copyright ©2003 by Pearson Education, Inc. Principles of Electric Circuits, 7ed. Upper Saddle River, New Jersey 07458 All rights reserved. Current in a transformer  Ideally, current will also be reflected across the transformer according to the turns ratio.  This reflection is opposite, or the reciprocal of the voltage. This must happen to maintain the same power in and power out.  There is one other condition that applies with current A certain amount of current is required to operate the inductors. This is called “excitation current”.  Thiscurrent use is present any time the transformer is connected to a source, even if the secondary load is not being used. Utility-pole transformer in a typical power distribution system. Thomas L. Floyd Copyright ©2003 by Pearson Education, Inc. Principles of Electric Circuits, 7ed. Upper Saddle River, New Jersey 07458 All rights reserved.