Basic Electricity 2 Flashcards
Describe a simple AC generator.
Conductor or loop of wire in a magnetic field connected to slip rings tat make contact with 2 brushes. The loop rotates and induces voltage as it cuts lines of magnetic field.
Describe sine wave development in an AC generator.
At 0degrees, no lines of the field are cut and no voltage is induced. As the coil rotates more lines of field are cut until the maximum at 90 degrees (max voltage generated). The process reversed until lines cut are back to zero, then the lines are in the opposite direction (negative) until back at start.
Define Period
Time required for generator to complete one cycle.
Define Frequency
Number of cycles per second.
Define Peak Voltage
Point where max lines of field are cut and max voltage is induced. Occurs at 90 degrees. (Ep).
Define Peak Current
Occurs at the same point as Peak Voltage. (Ip).
Define Peak to Peak Voltage/Current
Max difference in Voltage/Current between top and bottom of sine wave.
Define Effective Value of AC.
Amount of AC that produces the same heating effect as an equal amount of DC. Also called Root Mean Squared (RMS) value.
How to calculate RMS
Sq rt 2/2 x peak value, or
Peak value x 0.707
What is a phase angle
Fraction of a cycle, in degrees, that has gone by since a voltage or current has passed thru a given value. 0-360 degrees. Then starts over.
Describe phase relationship
If 2 or more sine waves are being compared, the difference in their phase angle describes their phase difference. If the angle is exactly the same, they are considered to be “in phase”.
How does a coil resist change in current flow?
A self induced EMF in the coil opposes change in current flow due to constant expand and collapse of magnetic fields in the coil. The induced EMF induced current that tends to maintain the same amount of current flow.
What is Inductance?
Inductor’s (coil’s) ability to induce CEMF
(L- units are Henries).
Calculate CEMF induced I an Inductor
CEMF =-L(delta current/delta time).
Calculate Inductance in series.
Leq=L1+L2+L3+…Ln.
Calculate inductance in parallel
1/Leq=1/L1+1/L2+1/L3+…1/Ln
What is a capacitor?
2 metal plates separated by insulating material. Holds charge.
What is Capacitance?
Ability to store electrical charge.
C=Q/V
Q-Amount of charge
V-Voltage applied.
Units-Farad (F)
Calculate Capacitance in series
1/Ct=1/C1+1/C2+1/C3+…1/Cn
Opposite of Inductance.
Calculate Capacitance in parallel
Ct=C1+C2+C3+…Cn
What is Inductive Reactance?
Induced EMF opposes change in current flow and is considered a type of resistance. Measured in Ohms.
Calculate Inductive Reactance
XsubL=E/I where;
E-Effective voltage across the reactance
I-Effective current
Also XsubL=2•pi•f•L
How are voltage and current related in an inductive circuit?
Voltage leads current in an inductive circuit. The circuit is considered lagging because leading and lagging is always referring to current.
What is Capacitive Reactance?
Opposition to current flow in a circuit due to charge and discharge in an attempt to keep voltage constant. Measured in Ohms.
Xc.
Calculate Capacitive Reactance
Xc=1/(2•pi•f•C) or
Xc=E/I
How do voltage and current relate in a capacitive circuit?
Current leads voltage and is therefore a leading circuit.
What is Impedance?
Total opposition to current flow taking into account resistance and total reactance.
Calculate Impedance Z
Z=sq rt(R^2+X^2) where
R=Resistance
X=Net Reactance (Xl-Xc)
What is the power triangle?
Relationship (right triangle) between True Power, Reactive Power, and Apparent Power.
What is Apparent Power?
Total power of a circuit Tru and Reactive.
What is Impedance Ratio?
Turns ratio between primary and secondary xfmer windings establishes a relationship between primary and secondary impedances.
(Np/Ns)^2=Zp/Zs
Note that turns ration is squared.
Calculate efficiency
Efficiency=Power Out/Power In x 100
What is magnetic leakage?
Magnetic flux that passes thru only one of the coils in a xfmer and doesn’t transfer electrical energy from one coil to the other.
Describe primary to Secondary Voltage relationship in transformers
If primary windings turns>secondary windings turns, then output voltage will be lower (step down).
Vice verse for step up.
Can be solved mathematically if number of turns of both and voltage of one of the windings is known.
(Vp/Vs)=(Np/Ns).
Describe primary to secondary winding current relationship in Xfmers
Current is inversely proportional to the voltage in the windings
(Vp/Vs)=(Is/Ip).
Also (Np/Ns)=(Is/Ip).
Describe relationship between primary and secondary power in Xfmers.
Ideally, primary power=secondary power.
Pp=Ps or
Vp•Ip=Vs•Is.
Power is easy as P=IE.
Describe a distribution transformer
Hugest power/volt-amp and continuous voltage ratings. Increased size of primary and secondary windings and increased voltage rating of insulation.
Describe a power transformer.
Electronic circuits.
300v-a rating or below.
Normally provide to power supply of an electronic device.
Describe control transformers.
For electronic circuits that require constant voltage/current with low volt-amp rating.
What is an Auto Transformer?
Variable voltage output is desired.
Consists of only 1 winding in which tapping at different points will yield different voltages. (Tap Changer)
What is an Isolation Transformer?
Low power xfmer that isolates noise to/from electronic circuits.
Doesn’t pass DC voltage thru coils therefore any DC voltage present won’t pass thru.
What is an Instrument Potential Transformer?
PT- steps down voltage to a low value that can be safely used for instruments like ammeters, voltmeters, watt meters, relays.
What is an Instrument Current Transformer?
CT- steps down the current of a circuit to a lower value. Serves same equipment as PT but constructed different.
Secondary winding is wound around a primary coil which contains only a few turns. Allows measurement of hi values of current.
What is an area substation?
Receives power from the SITE distribution system for use in a PARTICULAR FACILITY.
Typically contains a switch, a transformer, and a circuit breaker.
What is a Distribution Substation?
Receives high voltage, transforms it down to a lower voltage and DISTRIBUTES it to electrical loads via circuit breakers.
What is a Main Breaker?
Used to connect bus bars of switchgear to output of transformers.
What is a Feeder Breaker?
Used to receive power from switchgear bus bar and direct power to downstream loads.
Describe ‘Bond’ in wiring scheme terminology
Permanent joining of metallic parts or circuits assuring electrical continuity and sage current conductance for any expected current.
What is a Leg in a wiring scheme?
Current carrying conductor intended to deliver power to/from a load normally at an electrical potential other than ground.
What is phase voltage in wiring schemes?
The greatest root mean squared (effective) difference of potential between any 2 legs of the circuit.
What are the 3 parts of a Basic Electrical Distribution System?
1: A generating system
2: A transmission system
3: A distribution system.
Describe a Transmission System.
Includes hi voltage cables, power poles, and switching stations forming a “grid” which extends across the industrial facility.
Describe a substation
Transforms incoming hi voltage down to a value usable by the facility and distribute it to electrical equipment throughout the facility.
Describe a Distribution System.
1 or more electrical substations where the incoming hi voltage is transformed down to a lower voltage.
Describe single phase load connections.
Each phase in 3 phase is a single phase source.
Connected in 1 of 2 ways:
1: Phase to ground
2: Phase to phase.
Advantages of 3 phase systems.
More efficient than single phase.
3 phase equipment weighs less and is smaller than single phase equipment.
Can also be used to power single phase loads.
