Electrical Theory I - The Basics

1. Electrical Theory I - The Basics Let there be light….

2. Introduction • Basic Terminology • Ohm’s Law • Kirchhoff’s Laws & Applications • Basic Circuit Analysis • Transformers & Rectifiers

3. How is Electricity Produced? • Friction: “static electricity” from rubbing (walking across a carpet) • Pressure: piezoelectricity from squeezing crystals together (quartz watch) • Heat: voltage produced at junction of dissimilar metals (thermocouple) • Light: voltage produced from light striking photocell (solar power) • Chemical: voltage produced from chemical reaction (wet or dry cell battery) • Magnetism: voltage produced using electromotive induction (AC or DC generator).

4. Basic Terminology • Electromotive Force (E or V) • Force which causes electrons to move from one location to another • Known as emf, potential difference, or voltage • Unit is volt (V) • Source: • Generator • Battery • Like pump that moves water through “pressure”

5. Basic Terminology • Current (I) • Flow of electric charges - electrons (or holes) - through a conductor or circuit per increment of time • Unit is ampere (number of charged particles passing a point each second) • 1 amp = 1 coulomb/sec = 6x1018 electrons/sec • Like rate of flow of water through a pipe

6. Basic Terminology • Resistance (R) • An electrical circuit’s opposition to the flow of current through it • Measured in ohms (W) • Conductor • All materials will conduct electricity, but at varying resistances • Good conductors have little resistance (ie: silver, copper, aluminum, iron)

7. Basic Terminology • Insulator • Substances which offer high resistance to current flow (ie: wood, rubber, plastics) • Circuits made of wires covered with insulator • Power (P) • Rate at which work is performed • Measured in watts (W)

8. Ohm’s Law & Applications • Law: current of a circuit is directly proportional to the applied voltage and inversely proportional to circuit resistance • I a V, I a 1/R V =IR • Power • P = VI P = (IR)I = I2R

9. Kirchhoff’s Laws • Kirchhoff’s Current Law (KCL) • A node is any junction in a circuit where two or more elements meet • Currents into a node sum to zero OR • Current entering a junction is equivalent to the current leaving a junction

10. Kirchhoff’s Laws • Kirchhoff’s Voltage Law (KVL) • A loop is any path in a circuit that current can take so that it meets back up to where it starts • Voltages around a CLOSED loop sum to zero

11. Applications • Resistors in Series • RT = R1 + R2 + R3 + . . . • Resistors in Parallel • 1/RT = 1/R1 + 1/R2 + 1/R3 + . . . • Examples: should be able to find total current flow in circuit, current flow through each resistor, voltages, power dissipated, etc.

12. Questions?

13. Electrical Theory II –The Applications Harnessing the Power…

14. Power Generation • Chemical Cells • Electrodes • Electrolyte • Conductor • Wet/Dry Batteries • Magnetism

15. Electromagnetic Induction • Faraday (1831): • Showed that an emf is induced in a conductor if a magnet passes by a conductor • When pole of magnet entered coil, current flowed in one direction • When direction of magnet reversed, current flowed in opposite direction

16. Electromagnetic Induction • Magnitude of induced current can be increased by: • Increasing strength of magnetic field • Increasing speed of relative motion • Positioning of field & conductor to increase number of magnetic lines of flux cut • Magnetic field usually produced by electromagnet

17. Electromagnet • Soft iron core wound with coils of wire • When current present (excitation current), core becomes magnetized • Field strength determined by number of turns and magnitude of current: B a NIDC

18. Electromagnetic Induction • Results in: • Generator action: generator converts mechanical to electrical energy • Motor action: motor converts electrical to mechanical energy

19. Generator Action • For emf/current (electricity): • Magnetic Field • Conductor • Relative Motion b/t the two • Voltage produced: “induced emf/voltage” • Current produced: “induced current” • Left-hand rule for generator action

20. Motor Action • For motor action (torque/motion): • Magnetic Field • Conductor • Current flow in conductor • Torque produced: “induced torque” • Right-hand rule for motor action

21. Standard Terminology • Stator: stationary housing of the generator or motor • Rotor: rotating shaft inside the stator • Field windings: conductors used to produce electromagnetic field • Armature windings: conductors in which output voltage is produced (or input is provided)

23. Basic Terminology • Direct Current (DC) • Current flow is unidirectional and of constant magnitude (battery) • Alternating Current (AC) • Magnitude & direction of current flow periodically change • Each sequence called a cycle • Frequency is cycles per second (Hz)

24. AC Generators • Most electrical power used is AC made by AC generators • Basic principle: rotating magnetic field “cutting through” a conductor • Regardless of size, all AC generators work on same principle • Two types: • Revolving armature (NOT used) • Revolving field (Used in SSTG’s, GTGS, DG)

25. AC Generators • Two types: • Revolving armature (NOT used) • Revolving field (Used in SSTG’s, GTGS, DG)

26. AC Generators • Field windings on rotor • DC current provided for field via slip rings and brushes (vice commutator rings) • Rotor turned by prime mover creates rotating magnetic field • Armature windings on stator • As field rotates, AC current produced in armature • Since stationary contacts, no arc-over

27. AC Generators • Determining speed of AC machine: f= P(RPM)/120 RPM = 120f/P • Must maintain constant 60Hz output use speed governor to maintain constant RPM (independent of loading) • Must also regulate voltage output • Since constant RPM, must control field excitation (DC current) to control output voltage

28. Three Phase (3f) AC Power • Phases: number of sets of armature windings on stator • 3f has three sets of armature windings • Voltage induced is 120o out of phase for each • Output: 3 sinusoidal voltages and currents • Allows more power to be delivered with a smaller design generator

29. Three Phase (3f) AC Power

30. AC Motors • Use AC current as input to produce work • Many different types depending on number of phases of AC input & construction • Ex: induction motor • Input AC current on stator produces rotating field • Current produced in conductors on rotor produces torque

31. DC Generators • Basic Principle: rotate a conductor within a magnetic field to induce an EMF • Field windings located on stator & receive current from outside source

32. DC Generators • Armature windings on rotor • Commutator rings used to mechanically reverse the armature coil connection to the external circuit • EMF developed across the brushes becomes a DC voltage/current (pulsating and unidirectional)

33. DC Motors • Essentially the same in construction as DC generator • Based on principle that current carrying conductor placed at a right angle to a magnetic field tends to move in a direction perpendicular to magnetic lines of flux • Only need to change relative voltage to go between generator motor

34. AC vs DC power • Many different voltages required on board ship • Easier to transform AC power for each application

35. Electrical Devices • Transformer • Device w/o moving parts that transfers energy from one circuit to another by electromagnetic induction • Consists of ferromagnetic core & sets of windings • Step-up: Vin Vout • Step-down: Vin Vout • Only works with AC

36. Electrical Devices • Rectifier • Converts AC DC • Designed to have small resistance to current flow in one direction & large resistance in opposite direction • Typically called a diode or rectifier

37. Questions?