PHEP 18

Question 1

Why are delta-wye connections desired?

Answer 1

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

Question 2

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

Answer 2

delta delta transformer

Question 3

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

Answer 3

wye connection

Question 4

what is true in a delta connection?

Answer 4

line voltage equals phase voltageline current equals 1.73 times phase current

Question 5

what is residual current

Answer 5

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

Question 6

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

Answer 6

1.73 times line current times line voltage

Question 7

what happens when VARS are out

Answer 7

current will lag voltageimpedance will consist of resistance and inductive reactance

Question 8

what is the most commonly used cell in batteries

Answer 8

lead cell

Question 9

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

Answer 9

lead peroxide and sponge leadelectrolyte is sulfuric acid

Question 10

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

Answer 10

lead sulphate

Question 11

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

Answer 11

1

Question 12

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

Answer 12

2.07

Question 13

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

Answer 13

gassing rises

Question 14

How do you prevent violent bubbling when charging?

Answer 14

By reducing charging current.

Question 15

What are control bus batteries used for?

Answer 15

control equipment,
station auxilleries,
alarms, and
light systems

Question 16

What is formed during battery discharge?

Answer 16

lead sulphate

Question 17

How do you add water to a battery?

Answer 17

By slowly pouring acid into the water.

Question 18

describe cathodic protection

Answer 18

positive lead grounded, negative lead attached to pipe

Question 19

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

Answer 19

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.

Question 20

Vectors may be moved in a diagram provided they?

Answer 20

Are not changed in angular position nor in magnitude.

Question 21

Reversing both the voltage connection and current connections produces?

Answer 21

No change in indicated angle.

Question 22

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

Answer 22

zero,

residual

Question 23

Transformers are often connected in ________ arrangement.

Answer 23

delta-wye

Question 24

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

Answer 24

30

Question 25

The polarity of a transformer refers to?

Answer 25

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

Question 26

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

Answer 26

30 degrees
150 degrees
210 degrees, or
330 degrees.

Question 27

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

Answer 27

a delta-delta connection.

Question 28

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

Answer 28

Harmonics

Question 29

Which harmonic is the strongest?

Answer 29

The third harmonic

Question 30

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

Answer 30

It short circuits the harmonic.

Question 31

What other arrangement has essentially the same effect of a delta-delta configuration?

Answer 31

A delta-wye arrangement.

Question 32

How do you control the third harmonic in a wye-wye configured transformer?

Answer 32

By adding a fourth winding, called a tertiary winding, and connecting it in delta arrangement.

Question 33

AC voltmeters and ammeters do not indicate?

Answer 33

Polarity

Question 34

In a three-phase system, what controls the amount of current and its phase angle relationship to voltage?

Answer 34

The load impedance

Question 35

Power factor =

Answer 35

power divided by the apparent power.

Question 36

The polarity of a transformer is indicated by the position of the ___________ terminals.

Answer 36

low-voltage