Revision Flashcards
Why is three-phase and neutral supply used in industrial installation?
Higher power out put, reduced cable size and more split-phases
Transformer cooling methods
Oil cooled, air cooled and Gas cooled
In 230v domestic, what is the max fuse rating? When does installation start?
How many live conductors?
Fuse rating = 100a
Installation starts after the meter
There are 2 live conductors = line and neutral
2 losses that occur in the core of the transformer
eddycurrents and hysteresis
What can be done to stop losses in a transformer
Thin insulated laminations
An inductive load has a power factor of 0.5 lagging
How can the power factor be improved to be closer to unity?
What effect does the improvement have on the power factor?
To improve the power factor you need t add a capacitor, this will cause the current to lead the voltage.
How is the split-phase effect achieved in an induction start single-phase motor
It has a separate start winding which is connected to the main supply then to a centrifugal switch, the separate winding causes a phase-shift by having a different resistance this causes rotation
How many connections are required in a three-phase motor when using a star-delta starter?
Reason that the motor is started by being put into the star connection
How much power is available in the star connection compared to the delta connection?
6 connection cables
To reduce high current start up
It is reduced by a third
Halogen cycle
Tungsten atoms are driven off the filament attached to halogen molecules, by convection it returns back to the filament
Immersion heater
Water is heated inside of the tank by an electric current going through the element, this causes heat and heats the water around it
Storage heater
Storage heaters come on late at night when electricity is at its cheapest, this is also known as economy 7, an electric current is passed through the element which heats up the bricks around it, the bricks hold the heat and releases it during the day
State the uses of high pressure mercury lamps, high pressure sodium lamps and metal halide lamps
High pressure mercury = street lighting
High pressure sodium = industrial and security lighting
Metal halide = sports arena flood lights
Breaker tripping process
Biometallic strip heats and bends which disconnects the circuit
Two advantages of AC induction motors
Maintenance free and lower power demand
Start and run motor
Fan
