Electrical Theory The Basics

References Required Introduction to Naval Engineering – (Ch 16)

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

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 – Think “pressure” created by a “pump” to cause “flow” of water through a “pipe”

Basic Terminology Current (I) – Flow of electric charges – electrons – through a conductor (circuit) per increment of time – Unit is ampere (number of charged particles passing a point each second) – 1 amp = 1 coulomb/sec = 6.02x10 23 electrons/sec – Think “flowrate” of water through a “pipe”

Basic Terminology Resistance (R) – An electrical circuit’s opposition to the flow of current through it – Measured in ohms (  ) – Think “headloss” which lowers pressure in a pipe Conductor – All materials will conduct electricity, but at varying resistances – Good conductors have little resistance (ie: silver, copper, aluminum, iron) – Think “pipes”

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)

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)

Electrical Devices Diode – Designed to have small resistance to current flow in one direction & large resistance in opposite direction – Allows current flow in one direction – Think “check valve”

Electrical Devices Rectifier – Converts AC DC – Special arrangement of diodes such that current flows in the same direction with respect to a resistor.

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: V in V out – Step-down: V in V out – Only works with AC – Think “reducer valve”

Ohm’s Law & Applications Law: current of a circuit is directly proportional to the applied voltage and inversely proportional to circuit resistance I  V, I  1/R V =IR Power P = VI P = (IR)I = I 2 R

Applications Resistors in Series R T = R 1 + R 2 + R Resistors in Parallel.. Examples: should be able to find total current flow in circuit, current flow through each resistor, voltages, power dissipated, etc.

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

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

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).

Electromagnetic Induction Faraday (1831): Electricity is generated using Electromagnetic Induction (Maxwell-Faraday equation) – A changing magnetic field creates an electric field – A changing electric field creates a magnetic field

When pole of magnet entered coil, current flowed in one direction When direction of magnet reversed, current flowed in opposite direction

Electromagnetic Induction Results in: – Generator action: generator converts mechanical to electrical energy – Motor action: motor converts electrical to mechanical energy

Generator Action For emf/current (electricity) to be induced: – Magnetic Field – Conductor – Relative Motion between the two Voltage produced: “induced emf/voltage” Current produced: “induced current” Left-hand rule for generator action

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 DC- powered electromagnet

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  NI DC

Electromagnet RHR for Conductors – Thumb in direction of current flow – Fingers curl in direction of magnetic field RHR for Coils – Curl fingers in direction of current flow – Thumb points to north pole

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

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

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)

Standard Terminology Commutator- segmented metal ring which mechanically reverses the armature coil connections.

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 (rarely used) – Revolving field (Used in SSTG’s, GTGS, DG)

AC Generators Two types: – Revolving armature (rarely used) – Low power apps to avoid arc-over – Revolving field (Used in SSTGs, GTGs, DGs) – High power apps

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

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

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

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

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

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

Three Phase (3  ) AC Power

Take Aways Give the requirements for motor/generator action. Apply the RHR/LHR to a given problem. Describe the relationship between input and output voltage in various transformers Apply Ohm’s law and Kirchoff’s Laws to SIMPLE ckts Determine the speed, number of poles or frequency of a given machine.

Questions?