PHEP 18 Flashcards
Why are delta-wye connections desired?
They provide a path for the 3rd harmonic magnetizing current.
How can three-phase power still be carried if one of the single phase units is removed from service?
delta delta transformer
in which connection does line voltage equal 1.73 times phase voltage?
wye connection
what is true in a delta connection?
line voltage equals phase voltageline current equals 1.73 times phase current
what is residual current
current left over when vectorial sum of 3 phase currents is zero
in any 3 phase circuit what is equal to the apparent power
1.73 times line current times line voltage
what happens when VARS are out
current will lag voltageimpedance will consist of resistance and inductive reactance
what is the most commonly used cell in batteries
lead cell
what are the active parts of the plates in a lead cell
lead peroxide and sponge leadelectrolyte is sulfuric acid
during their chemical reaction what do the components of a lead cell create
lead sulphate
how many more plates does the negative electrode have than the positive plate
1
what is the open circuit voltage of a fully charged commercial lead cell
2.07
what happens when the voltage and specific gravity curves level off while charging takes place
gassing rises
How do you prevent violent bubbling when charging?
By reducing charging current.
What are control bus batteries used for?
control equipment,
station auxilleries,
alarms, and
light systems
What is formed during battery discharge?
lead sulphate
How do you add water to a battery?
By slowly pouring acid into the water.
describe cathodic protection
positive lead grounded, negative lead attached to pipe
What are some of the advantages of three-phase circuits over single phase?
- Generators have higher efficiency
- For power transmissions, 25% less conductor materials is needed.
- They are superior to single phase motors in cost and operating characteristics.
Vectors may be moved in a diagram provided they?
Are not changed in angular position nor in magnitude.
Reversing both the voltage connection and current connections produces?
No change in indicated angle.
The vectorial sum of the three-phase currents must equal ______ except where there is a neutral which leaves ________ current.
zero,
residual
Transformers are often connected in ________ arrangement.
delta-wye
A delta-wye arrangement in a transformer reduces the third harmonic by introducing a phase-shift ______ degrees forward or backward.
30
