Exam Prep Flashcards
Luminous efficacy equation
Illuminance (E)=
Density
The density of luminous flux reaching a surface LUX
Luminous flux (F)=
Total
The total power of a light source in a particular direction measured in candelas (cd)
Split phase motor
A) main
B) rotor
C) start windings
D) Centrifugal switch
Capacitor start induction motor
A) Main windings
B) Rotor
C) Start windings
D) Capacitor
E) Centrifugal switch
Stop buttons on motors are always wired in
Series with each other
Start buttons are always wired in
Parallel with each other
What are the three main types of motor overload unit
Thermal
Magnetic
Electronic
Turns ratio are always written down with the number of primary turns first so if a transformer had 260 primary turns and 130 secondary turns the ratio would be written out like…
Np/Ns
260/130
2:1
A transformer has a turns ratio of 5:1 if the secondary current is 24A what is the primary current?
Ip/Is = Ns/Np
Ip/24 = 1/5
Ip= 24x1/5= 4.8A
Three types of Dc Motors
Series motors
Shunt motors
Compound motors
E =
I/d squared
How is electrical energy produced from a gas fired power station
Electrical energy is produced from a gas fire power station by the gas being burned which converts chemical energy to thermal which produces steam. The high pressure steam is fed into a steam turbine thermal energy is converted to mechanical energy, the turbine rotates an alternator which produces electrical energy
Voltage levels between a generating station and domestic premises
25kv—400kv/132kv—33kv/11kv—11kv—400V/230V
What is a UPS and why and where is it used
It is an Uninterruptible power supply and is used to prevent loss of supply in hospitals and large offices
What is meant by term heating zone
A heating zone is a designated area in a property that is usually controlled by a thermostat or a timer
Types of losses in a transformer
Heat losses, Eddy currents losses occur in a transformer
What is meant by true power
Useful
The power in an AC circuit that is transmitted to the load and does useful work it gets converted
Apparent power is
supply voltage x supply current in an AC circuit
Reactive power is
The useless power in an AC circuit. It does no useful work, but is necessary for the circuit to work.
What does the curved section of the graph represent
(Curved thermal)
The curved section represents thermal operation of the mcb
What does the straight section of the graph represent
(Straight magnetic)
The straight section represents magnetic operation of the MCB due to the solenoid
Phase and line values of voltage are the same in
Delta VL=VP
Phase and line values of current are the same in
Star IL=IP
Types of split phase motors
Permanent capacitor motor
Capacitor start motor
Capacitor start and run motor
Types of heating systems
S plan
Y plan
Underfloor heating
Why is electrical energy transmitted at high voltages
It allows the voltages to be stepped up and down using transformers
What are the advantages of electrical energy transmitted at high voltages
To improve efficiency
Less heat losses
To reduce current flow
Overcome voltage drop
VP=
VL/square root 3
Advantages of load balancing on three phase systems
Current taken by each line is also equal
Reduces current flow
Disadvantages of DOL starters
Reduced lifespan of the machine
Advantages of a soft starter
Smooth starting and stopping by torque control
Reduction in starting current
Reduce maximum demand
Three losses in a TRANSFORMER
Copper losses, eddy current losses, hysterisis losses
XL=
2PiFL
