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
Slide 6
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
Slide 7
Basic Terminology Resistance (R) An electrical circuits
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
Slide 8
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)
Slide 9
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)
Slide 10
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
Slide 11
Electrical Devices Rectifier Converts AC DC Special arrangement
of diodes such that current flows in the same direction with
respect to a resistor.
Slide 12
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
Slide 13
Ohms 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
Slide 14
Applications Resistors in Series R T = R 1 + R 2 + R 3 +...
Resistors in Parallel.. Examples: should be able to find total
current flow in circuit, current flow through each resistor,
voltages, power dissipated, etc.
Slide 15
Kirchhoffs Laws Kirchhoffs 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
Slide 16
Kirchhoffs Laws Kirchhoffs 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
Slide 17
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).
Slide 18
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
Slide 19
When pole of magnet entered coil, current flowed in one
direction When direction of magnet reversed, current flowed in
opposite direction
Slide 20
Electromagnetic Induction Results in: Generator action:
generator converts mechanical to electrical energy Motor action:
motor converts electrical to mechanical energy
Slide 21
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
Slide 22
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
Slide 23
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
Slide 24
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
Slide 25
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
Slide 26
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
Slide 27
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)
Slide 28
Standard Terminology Commutator- segmented metal ring which
mechanically reverses the armature coil connections.
Slide 29
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 SSTGs, GTGS, DG)
Slide 30
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
Slide 31
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
Slide 32
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
Slide 33
Motor Action For motor action (torque/motion): Magnetic Field
Conductor Current flow in conductor Torque produced: induced torque
Right-hand rule for motor action
Slide 34
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
Slide 35
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
Slide 36
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
Slide 37
Three Phase (3 ) AC Power
Slide 38
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 Ohms
law and Kirchoffs Laws to SIMPLE ckts Determine the speed, number
of poles or frequency of a given machine.
