PHEP 18 Flashcards Preview

RWD OP 1ST 6 MONTHS > PHEP 18 > Flashcards

Flashcards in PHEP 18 Deck (36)
Loading flashcards...
1

Why are delta-wye connections desired?

They provide a path for the 3rd harmonic magnetizing current.

2

How can three-phase power still be carried if one of the single phase units is removed from service?

delta delta transformer

3

in which connection does line voltage equal 1.73 times phase voltage?

wye connection

4

what is true in a delta connection?

line voltage equals phase voltageline current equals 1.73 times phase current

5

what is residual current

current left over when vectorial sum of 3 phase currents is zero

6

in any 3 phase circuit what is equal to the apparent power

1.73 times line current times line voltage

7

what happens when VARS are out

current will lag voltageimpedance will consist of resistance and inductive reactance

8

what is the most commonly used cell in batteries

lead cell

9

what are the active parts of the plates in a lead cell

lead peroxide and sponge leadelectrolyte is sulfuric acid

10

during their chemical reaction what do the components of a lead cell create

lead sulphate

11

how many more plates does the negative electrode have than the positive plate

1

12

what is the open circuit voltage of a fully charged commercial lead cell

2.07

13

what happens when the voltage and specific gravity curves level off while charging takes place

gassing rises

14

How do you prevent violent bubbling when charging?

By reducing charging current.

15

What are control bus batteries used for?

control equipment,
station auxilleries,
alarms, and
light systems

16

What is formed during battery discharge?

lead sulphate

17

How do you add water to a battery?

By slowly pouring acid into the water.

18

describe cathodic protection

positive lead grounded, negative lead attached to pipe

19

What are some of the advantages of three-phase circuits over single phase?

1. Generators have higher efficiency
2. For power transmissions, 25% less conductor materials is needed.
3. They are superior to single phase motors in cost and operating characteristics.

20

Vectors may be moved in a diagram provided they?

Are not changed in angular position nor in magnitude.

21

Reversing both the voltage connection and current connections produces?

No change in indicated angle.

22

The vectorial sum of the three-phase currents must equal ______ except where there is a neutral which leaves ________ current.

zero,
residual

23

Transformers are often connected in ________ arrangement.

delta-wye

24

A delta-wye arrangement in a transformer reduces the third harmonic by introducing a phase-shift ______ degrees forward or backward.

30

25

The polarity of a transformer refers to?

the relative direction of the induced voltages in the primary and secondary windings.

26

The displacement angle caused by the delta-wye arrangement can cause phase-shifts of?

30 degrees
150 degrees
210 degrees, or
330 degrees.

27

A delta-wye connection cannot be connected in parallel with?

a delta-delta connection.

28

Any apparatus which has an iron magnetic circuit in which there is no air gap is likely to produce?

Harmonics

29

Which harmonic is the strongest?

The third harmonic

30

What happens to the third harmonic in a delta-delta configuration?

It short circuits the harmonic.