Brainscape's Knowledge GenomeTM

Browse over 1 million classes created by top students, professors, publishers, and experts.


See full index

FE ELECTRICAL > Power > Flashcards

Power Flashcards

1
Q

National Electric Code (NEC)

A
  • published by National Fire Protection Association (NFPA)
  • concerned with safety electrical practices only
  • not a guide to efficiency, convenience, or appropriateness
  • complying and proper maintenance will result in an installation that is essentially free from hazard
  • not necessarily efficient, convenient, or adequate for good service or future expansion of electrical use
  • not intended as a design specification nor an instruction manual for untrained persons
How well did you know this?
1
Not at all
2
3
4
5
Perfectly
2
Q

mandatory rules of NEC

A
  • identify actions that are specifically required or prohibited
  • characterized by the use of the terms “shall” or “shall not”
  • exceptions to rules are in italic
How well did you know this?
1
Not at all
2
3
4
5
Perfectly
3
Q

permissive rules of the NEC

A
  • actions that are allowed but not required
  • are normally used to describe options or alternative methods
  • characterized by the use of the terms “shall be permitted” or “shall not be required”
How well did you know this?
1
Not at all
2
3
4
5
Perfectly
4
Q

National Electric Safety Code (NESC)

A
  • general purpose is to protect utility personnel and the general public
  • specifically “the practical safeguarding of persons during the installation, operation and maintenance of electric supply and communication lines, and associated equipment”
How well did you know this?
1
Not at all
2
3
4
5
Perfectly
5
Q

NESC applicability & NEC applicability

A

NESC applies to everything after the generator except the end-user building
NEC only applies to the end-user building

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
6
Q

real power, P

A

P= Vrms Irms cos θ
- units of watts (W)
- dissipated as heat or converted into mechanical work
- θ is power angle between input voltage and current
- when waveform is a sinusoid:
P = (1/2) Vmax Imax cos θ

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
7
Q

purely resistive circuit

A
  • voltage and current in phase (for an AC circuit)
  • in resonance (capacitance reactance and inductive reactance cancel)
  • cosθ=1

P= Vrms Irms = Vrms^2 / R = Irms^2 R [purely resistive load; pf =1]

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
8
Q

energy

A

ability to do work

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
9
Q

power

A
  • rate of flow of energy
  • energy per unit time
  • as used on the FE exam, it is the RMS quantity unless otherwise stated
How well did you know this?
1
Not at all
2
3
4
5
Perfectly
10
Q

reactive power

A
  • power used to provide energy to the magnetic fields of coils of machines or the electric fields of cables, twice per AC cycle
  • although not consumed as energy, does represent real current so it produces voltage drop and losses
How well did you know this?
1
Not at all
2
3
4
5
Perfectly
11
Q

power triangle

A
  • real power, P
  • reactive power, Q
  • apparent power, S
  • power angel, θ
How well did you know this?
1
Not at all
2
3
4
5
Perfectly
12
Q

reactive power, Q

A
  • the power stored and returned without dissipation
  • mathematically represented as imaginary
  • units: volts-amps reactive (VAR)
  • units determine difference in reactive power and real power, real power is in watts
Q = (1/2) Vmax Imax sinθ [sinusoids]
Q = Vrms Irms sinθ
How well did you know this?
1
Not at all
2
3
4
5
Perfectly
13
Q

apparent power, S

A
  • S = Ieff Veff = (1/2) Im Vm
  • magnitude of complex sum of real and reactive power
  • units of volt-amps (VA)
  • units distinguish apparent power from reactive power (volts-amps reactive) and real power (watts)
  • values of S doesn’t show how much is real versus reactive
How well did you know this?
1
Not at all
2
3
4
5
Perfectly
14
Q

power angle θ

A
  • overall impedance angle

- same as angle between input voltage and current in the circuit

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
15
Q

complex power vector, S

A

S= V*I = P + jQ

  • units of volt-amps (VA)
  • I *= complex conjugate of the current
  • power angle, θ, and current angle have opposite signs
How well did you know this?
1
Not at all
2
3
4
5
Perfectly
16
Q

complex power vector, S continued

A
  • when the current angle is negative, the power factor is lagging
  • when the current angle is positive, the power factor is leading
How well did you know this?
1
Not at all
2
3
4
5
Perfectly
17
Q

power factor, pf

A

pf = cosθ

  • usually given as a percentage
  • positive for both positive and negative angles
  • pf=1 for purely resistive load

for inductive circuit,

  • lagging power factor
  • negative θ

for capacitive circuit,

  • leading power factor
  • positive θ
How well did you know this?
1
Not at all
2
3
4
5
Perfectly
18
Q

power factor correction

A
  • adding reactive power to system to change the system’s power factor
  • usually by adding capacitance to a system that is inductive
  • when capacitance is storing reactive power, inductance is returning reactive power, and vice versa

Reactive power flow is in power systems and should be minimized, as it

  • increases losses
  • creates voltage drop
  • reduces efficiency
  • increases line size (cost)
How well did you know this?
1
Not at all
2
3
4
5
Perfectly
19
Q

Maximum Power Transfer

A
  • A fixed network will transfer the maximum power to a load when the load is the complex conjugate of the network equivalent impedance
    –> analogous to DC circuits,
    using ZL= RL + jXL in place or RL
    –> occurs when
    Rth + jXth = RL - jXL
How well did you know this?
1
Not at all
2
3
4
5
Perfectly
20
Q

net energy exchanged

A
  • inductance and capacitance power transfer
  • one direction during part of the half-cycle
  • opposite direction during the other part
  • source for storage over a cycle is zero
How well did you know this?
1
Not at all
2
3
4
5
Perfectly
21
Q

transformer

A

transfers energy from one circuit to another

  • AC current through wire coiled around a magnetic core (the primary winding) generates changing magnetic field
  • changing magnetic field induces AC current in secondary winding
  • DC current produces constant magnetic field, can’t induce current
  • magnetic flux mostly in core (much higher permeability in free space)

used for changing voltages, matching impedances, isolating circuits

  • voltage increase of decreases according to ratio of turns in primary and secondary windings
  • for impedance matching, turns ratio is chosen so that impedance can be seen by source through transformer matches source impedance
How well did you know this?
1
Not at all
2
3
4
5
Perfectly
22
Q

turns ratio, a

A
  • ratio of primary to secondary windings
  • also called ratio of transformation

a = N1/N2

step-down transformer

  • turns ratio >1
  • decreases voltage

step-up transformer

  • turns ratio < 1
  • increases voltage
How well did you know this?
1
Not at all
2
3
4
5
Perfectly
23
Q

ideal transformer

A
  • power absorbed by primary windings equals power generated by secondary windings
    IpVp = IsVs
  • windings have neither resistance nor reactance
How well did you know this?
1
Not at all
2
3
4
5
Perfectly
24
Q

turns ratio (ideal transformer)

A

a = l Vp/Vs l = l Is/ Ip |

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
25
Q

input impedance, Z1

A
  • Real primary circuits have input impedance
  • Anything attached to the transformer, such as a microphone will contribute to input impedance
  • the input impedance contributes to the impedance seen by the source
How well did you know this?
1
Not at all
2
3
4
5
Perfectly
26
Q

power

A
  • concerned with voltage and current
  • mathematically modeled as having real and imaginary parts
  • isn’t real and imaginary; it is dissipated & stored
How well did you know this?
1
Not at all
2
3
4
5
Perfectly
27
Q

“real” power

A

dissipated in resistors or rotating machine mechanical load

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
28
Q

“imaginary” power

A
  • stored in electric and/or magnetic fields during one part of the half-cycle
  • returned during the other part of the half-cycle
How well did you know this?
1
Not at all
2
3
4
5
Perfectly
29
Q

three-phase power

A

most common way to transmit large amounts of power over long distances

  • Power that would take six conductors using single-phase systems can be transmitted with just three conductors
  • There is loss in the lines for only the direction from source to load (single-phase systems have loss both ways)
  • Three wires are used to transmit power (some systems also have a neutral wire)
30
Q

Phase Vectors

A
  • rotate counterclockwise as time (phase) goes by
  • B-phase vector passes the origin
  • followed by the C-phase vector
  • sequence is A-B-C
31
Q

balanced three-phase system

A
  • voltages and currents on three wires all of same magnitude
  • out of phase by 120 degrees
  • vector sum of voltages and currents is zero
32
Q

benefits of three- phase energy distribution system

A
  • more efficient
  • smaller conductors than single -phase
  • provides same power with three wires as single-phase systems with six wires
  • smoother waveform and less ripple to be filtered when rectified
  • can produce a magnetic field that rotates in a specified direction, simplifying design of electric motors
33
Q

benefits of three-phase motor versus single-phase

A
  • provides a uniform torque, not a pulsating one
  • does not require additional starting windings or associated switches
  • physical size is smaller, with same horsepower rating
34
Q

balanced load

A
  • three equal loads with same real and reactive parts
  • magnitude and phase of voltage and current are same in each load
  • power factor is same for each phase

Because of these traits, a balanced system can be analyzed on per-phase basis (known as one-line analysis)

35
Q

delta-wired systems

A
  • three loads each connected to two of the three phase conductors
  • no neutral wire
  • line-to-neutral voltage and current do not physically exist
36
Q

wye equivalent

A
  • can still represent the delta system

- can compare the line terminal voltage to the neutral even though the neutral does not physically exist

37
Q

three-phase delta configuration

A

has three phases and no neutral wire

  • only voltages to be considered are line-to-line voltages
  • two currents to be considered
  • -> line currents that flow in the lines
  • -> phase currents which flow in the resistors, shown as load in the diagram
38
Q

line currents

A

out of phase with the line-to-line (phase) currents

39
Q

wye-wired systems

A
  • have three loads connected together at one node
  • each load connected to the three-phase conductors
  • may or may not have a neutral wire
  • line-to-line voltage and current physically exist
40
Q

three-phase wye configuration

A
  • three phases and a neutral wire
  • line-to-neutral voltages are between the lines and the neutral: Van, Vbn, and Vcn
  • line-to-line voltages are between the legs: Vab, Vbc, and Vca
41
Q

line-to-neutral voltage

A
  • differ from each by 120 degrees (1/3 cycle)
  • go through maxima in regular order
  • led by phase voltage by 30 degrees
42
Q

line-to-line voltage

A
  • expressed in terms of phase voltages

- lags the phase voltage by 30 degrees

43
Q

Phase Sequence

A
  • Va reaches peak before Vb
  • Vb reaches peak before Vc
  • ABC sequence
44
Q

transmission lines

A
  • necessary for distributed power
  • modeled as lossless when the source and load are close together
  • modeled with lumped parameters when the source and load are further apart
45
Q

transmission line resistance

A
  • can be ignored if the conductor is good and the length of the line is short
  • increases as the frequency increases because the effective cross-section decreases
46
Q

transmission line inductance

A

exhibited externally and internally if a conductor is long enough

47
Q

external inductance

A
  • produced by a long straight wire with a current

- greatly increased by the presence of other conductors near the wire

48
Q

internal inductance

A
  • caused by the skin effect

- typically insignificant compared to external inductance

49
Q

shunt capacitance

A

capacitance between conductors

50
Q

single-phase systems

A

shunt capacitance depends on conductor diameter and spacing, also dielectric properties of insulation

51
Q

polyphase systems (three-phase)

A

shunt capacitance depends on geometric arrangement of the conductors

52
Q

short-length transmission lines

A

60 Hz lines that are less than 80 km long

53
Q

medium-length transmission lines

A
  • 60 Hz lines between 80 km and 240 km (50 mi and 150 mi) long
  • modeled as either T- or pi- models
54
Q

long transmission lines

A
  • 60 Hz lines greater than 240 km (150mi) long

- modeled with partial differential equations and distributed parameters

55
Q

short-length transmission lines

A
  • modeled as a series impedance with resistance and inductance
  • shunt reactance is excluded from model because coupling is insignificant
56
Q

medium- length transmission line

A
  • modeled as T- model, pi-model, or series of T-model and pi-model sections
  • lumps series impedance as two components and shunt admittance as single component
57
Q

generation stations

A

generate very high voltages which is stepped down for sub transmission, distribution substation, primary distribution, and secondary mains

the higher voltage:

  • the more efficient transmission
  • the greater the danger of faults
  • the greater the consequences for faults
58
Q

overcurrent protection

A
  • required by the NEC to reduce probability of fire, electrical arcing
  • many protective devices, including fuses and circuit breakers
  • multiple-level protection at facilities often includes protection such as feeder circuits, sub feeder panels, and branch circuit panels
59
Q

AC Machines

A
  • Motors convert electrical energy into mechanical energy
  • Generators convert mechanical energy into electrical energy
  • AC machines have rotating magnetic fields
  • DC machines have constant magnetic fields and commutators
  • The energy must always convert to magnetic energy first
60
Q

synchronous generators

A

constant magnetic field in the rotating part that induces currents in the stationary windings of the machine

61
Q

synchronous motors and induction motors

A

stator magnetic field rotates at synchronous speed, ns

ns= 120f/p

62
Q

induction motors

A
  • essentially constant- speed drives
  • receive power through induction – no brushes
  • rotating transformer secondary (the rotor) with a stationary primary (the stator)
  • EMF induced as stator field moves pas rotor conductors
  • rotor windings have reactance, the rotor fields lags the induced EMF
63
Q

slip

A

slip = (ns- n) / ns

  • to have a change in flux linkage, rotor must turn at less than the synchronous speed
  • typically 2% to 5%
64
Q

DC Machine

A
  • device that produces DC potential
  • constant magnetic field in the stator (called the field)
  • magnetic field of the rotor (called the armature) responds to the stator field
  • field can be a permanent magnet or can be an electromagnet powered by the current of the armature circuit
  • magnetic flux generated by the field current, If, is approximately
    Φ = KfIf
65
Q

magnetic field, B

A
  • produced either by permanent magnets or electromagnets

- rotated, wires in the field cross the lines of magnetic flux and current flows in the field

66
Q

commutator

A

insures that the polarity of the output voltage is correct with time

67
Q

DC generator

A
  • simplified model ignores the resistance loss in the armature and field windings
    -terminal voltage for simplified model
    Va= KanΦ [in volts]
    n is rotational speed [rpms]
    Φ is magnetic flux
68
Q

DC motor

A
  • very similar to a DC generator, only the direction of current is reversed
  • power (for the motor model in the DC machine equivalent circuit)
    Pe= Ph + Pm = Ia^2 Ra+ IaE
  • ignoring power dissipated as heat for the DC machine equivalent circuit
    Pm= Va Ia
69
Q

servomotor

A
  • particular type of motor
  • precise control of the position or speed
  • sensors and feedback systems allow control
70
Q

back electromotive force (emf), Eg

A
  • not a physical voltage
  • a mathematical model for electrical power converted to mechanical work

V= IR + Kew
T =kTI
KT= KE

71
Q

line regulation

A

measures the ability to maintain a constant output voltage regardless of changes in the input voltage

72
Q

load regulation

A

measures the ability to maintain a constant output voltage regardless of changes in size of the load (current draw)

FE ELECTRICAL (17 decks)

Brainscape helps you reach your goals faster, through stronger study habits.
© 2024 Bold Learning Solutions. Terms and Conditions