Theory Flashcards
Factors on EMF generation/ Faraday’s Law
-density of magnetic flux
-length of conductor within magnetic field
-rate at which conductor cuts lines of magnetic force
Maximum cutting rate
90 degrees
Components of AC sine wave
-One positive alternation (0 to 180)
-One negative alternation (180 to 360)
Frequency
-number of electrical cycles per second
-symbol (f)
-unit of measurement (hertz)
-symbol of hertz (hz)
-time to complete one cycle (time period)
Frequency formula
f= (p x n)/120
f=(pp x r/min)/60
Maximum values at sine waves
+V at 90 degrees
-V at 270 degrees
Instantaneous values of a cycle
e=Emax x sine of angle
Effective Value/RMS value
Emax = E RMS / 0.707
Impedance on AC circuit
Resistance
Inductive Reactance
Capacitive Reactance
Inductance
Property of a circuit that opposes change of current
through a counter EMF
(L)
unit of measurement: Henry
Inductive reactance
- Current limiting property of an inductor of a circuit
(XL) - measured in Ohms
XL = 2 pi f L
Factors on AC resistance
- DC ohmic value
- Iron losses
- Dielectric losses
- Skin Effect
Iron Losses
- Hysteresis loss and Eddy current loss
Hysteresis loss
- energy it takes to overcome molecular friction due to reversing polarity
- use an alloy that contains Silicon for the core and armature
Eddy Current loss
- circulating current induced in magnetic material that is adjacent to an AC circuit
- use laminations in the core and armature, less induced voltage
Dielectric loss
- energy loss to circuit due to voltage stress of a conductor
- greater at high voltage
Skin Effect
- tendency of AC to flow towards conductors surface
- effective area of conductor is reduced, resistance increases
- problem at high frequency
- allows transmission lines to have a steel core
Inductor
- an insulated wire wrapped around a core
- coil, solenoid, reactor
Factors that affect Inductance
L= (N^2 u A)/l
N(number of turns of a conductor)
u(permeability of core material)
A(cross-sectional area)
l(length of core’s magnetic path)
Mutual Induction
- emf generated in one conductor by a change of flux around a separate circuit
Lenz’s Law
- induced voltage attempts to force electron in one direction
Self Induction
- emf generated by a change of flux around a conductor
Left Hand Rule
- thumb points towards core
- forefinger points to direction of magnetic field
- middle finger points of of page, cemf
Factors that affect CEMF
- inductance of coil
- rate of change of current
cemf= (change in current / change in time) x inductance
Time Constants of inductive circuits
T= L/R
- takes 5 time constants to reach steady value
- one time constant = 63.2%
Discharge Resistor
- placed in parallel to coil
- reduces inductive kick
- reduces arcing
Inductive Circuits
Current lags voltage by 90 degrees
Reactive Power/ Quadrature Power
Qxl = Vx x Ix
Qxl = I^2 XL
Capacitor Construction
2 plates of conducting material seperated by a insulating material called dielectric
Non polarized Capacitors
Air
Oil filled
paper, tubular
plastic film
mica
ceramic disc
Capacitor applications
Timer
Rectifier smoothing and filtering
Power factor correction
Motor starting
Capacitance
capacitor ability to store charge
measured in micro farads
Factors that affect Capacitance
Area of plate
distance between plates
type of dielectric material
Dielectric Constant
Rating of dielectric insulating material’s ability to polarize and store charge compared to air
Dielectric Strength
Ability to withstand volatge
kV/mm
Electric Charge
Q= I x t
C= Q / V
Time Constants in Capacitive circuits
T= R x C
Capacitive reactance
Xc = 1/( 2 pi f C)
Capacative phasor relationship
current leads voltage by 90 degrees
Series AC Circuits
Total Resistance Rt =R1 + R2 +R3 …
Total Inductance Reactance XLt = XL1 + XL2 + XL3 …
Total Capacitive Reactance XCt = XC1 + XC2 + XC3 ..
