Basic Electricity - 2 Flashcards
In what applications is a current transformer used
Steps down current of a circuit to a low value for instrumentation
Describe the relations between primary and secondary power in a transformer
Pprimary=Psecondary
Define feeder breaker
Circuit breaker used to receive power from a switchgear bus bar and direct the power to loads
What is the phase relationship between current and voltage in a conductive circuit
Current leads voltage (ICE)
Define bond
The permanent joining of metallic parts or circuits
What is the phase relationship between current and voltage in an inductive circuit
Voltage leads current (ELI)
Define frequency in relation to AC generation
The number of cycles completed per second
Define peak to peak voltage in relation to AC generation
The magnitude of voltage from maximum to minimum on the AC sine wave
Define period in relation to AC generation
The time required for the generator to complete one cycle
Define leg
A current carrying conductor intended to deliver power to or from a load
Define load center
Receives power from a substation and supplies power to motor control centers
Define conductor
Any wire, cable, or substance capable of carrying current
Define reactive power using the power triangle
Reactive power is the vertical component of the power triangle (Q=EIsinØ)
Define standby power
Power source that comes on line upon a loss of normal power
Define phase voltage
The greatest RMS difference of potential between any two legs of the circuit
Describe the development of a sine-wave output in an AC generator
The magnitude and polarity of the induced voltage caries as the conductor rotates in the field. Since the slip rings are continuously in contact with the same brush, the output varies between positive and negative as the conductor rotates.
Define switchgear
An assembly of circuit breakers electrically connected to a system of busses
Define normal power
Power received from the normal power supply
How does a capacitor oppose a change in voltage
As the voltage in a capacitive circuit decreases the energy stored in the plates of the capacitor are released to the circuit opposing the change in voltage.
How does an inductor oppose changes in current flow
As the current begins to change the established magnetic field starts to change which induces a CEMF which resist the change in current.
Define transformer efficiency
Power output divided by power input
Define neutral
A current carrying conductor normally tied to ground
Describe the construction of the iron core of a transformer
Magnetic material that the coils are wound around
Define true power using the power triangle
True power is the horizontal component of the power triangle (P=EIcosØ)
Define area substation
Receives power from the site distribution system for use in a facility
Describe the relationship between peak, average, and RMS values of voltage in AC power
- Peak is the height of the sine wave
- Average is always zero
- RMS is 0.707 times the peak
Define vital/essential loads
Loads requiring constant power to ensure a facility can operate safely
Define turns ratio
The number of turns on the primary coil divided by the number of turns on the secondary coil
In what applications is a distribution transformer used
Used in AC electrical distribution
What is inductive reactance (XL)
The component of resistance in an AC circuit due to inductive loads
What are the advantages of a three phase system
More efficient at producing and utilizing AC voltage
Describe the construction of the secondary coil of a transformer
Coil that is connected to the load
How are single phase loads connected to three phase systems
- Phase to ground
- Phase to phase
Define impedance
The total opposition to current flow in an AC circuit. It is the vector sum of capacitive reactance and inductive reactance.
Define panel boards
Small distribution panels that contain numerous molded-case circuit breakers
Describe the relations between primary and secondary voltages in a transformer
Vp/Vs=Np/Ns
Describe the phase relationship between two waves
The phase difference is the measure of the distance between two waves where they cross the the x axis
Define apparent power using the power triangle
Apparent power is the hypotenuse of the power triangle (S=EIt)
What is capacitive reactance (Xc)
The component of resistence in an AC circuit due to capacitive loads
Describe the operation of a simple AC generator
A conductor rotates through a magnetic field and transfers the induced voltage to slip rings as voltage output
In what applications is a control transformer used
Used in electronic circuits that require constant voltage or constant current w/ a low power or volt-amp rating
Define effective value of AC
The amount of AC that produces the same heating effect as an equal amount of DC
In what applications is a power transformer used
Used in electronic circuits to provide the correct voltage to a component power supply
Define mutual induction
Induction of an EMF in a coil by magnetic lines of flux generated in another coil
Define impedance ratio
The ratio of impedance in the primary coil divided by the impedance in the secondary coil.
Maximum power is transferred with the ratio is 1
Define contactor
An electro-mechanical device that controls power to a piece of equipment
Describe the relations between primary and secondary current in a transformer
Vp/Vs=Is/Ip
Define distribution substation
A substation that receives high voltage and steps it down to a lower voltage to electrical loads via circuit breakers
Define main breaker
Circuit breaker used to connect bus bars of switchgear assemblies to the output transformer
In what applications is a isolation transformer used
Low power applications used to isolate noise from or to ground
Define peak to peak current in relation to AC generation
The magnitude of current from the maximum to minimum on the AC sine wave
Describe the construction of the primary coil of a transformer
Coil that is energized by the AC source
Define power factor as it relates to true power
Power factor is ratio between apparent power and true power. cos(Ø)=True Pwr/Apparent Pwr
Describe how a capacitor stores energy
Two metal plates separated by a dielectric. Each side of the plate gets charged from the circuit either positively or negatively depending on which side of the circuit the plate is connected to.
In what applications is a potential transformer used
Steps down voltage of a circuit to a low value for instrumentation
In what applications is a auto transformer used
Used in low power applications where a variable voltage is required
Define tie-breaker
Circuit breaker used to tie two busses together
Define ampacity
The maximum sustained current that a conductor can carry while remaining within its temperature rating
Define ground
A conducting connection between a circuit and the earth which has zero electrical potential
Define motor control center
Receives power and distributes it to process loads and other electrical panels
Describe the relationship between current flow, magnetic field, and stored energy in an inductor
Current flows creating a magnetic field which stores energy in an inductor
Define peak current in relation to AC generation
The maximum current appearing on the AC sine wave
Define peak voltage in relation to AC generation
The maximum voltage appearing on the AC sine wave
