Delta and Delta/Wye

Question 1

What are the standard volt drops in a Delta system?

Answer 1

V AB, V BC, V CA

Question 2

What is an open corner test?

Answer 2

It is a test to see if the secondary of a delta transformer is connected properly and if circulating currents are present

Question 3

Using KVL, what should the voltage read on an open corner test?

Answer 3

0V

Question 4

What would you read if you reversed two leads in an open corner test?

Answer 4

double the phase voltage

Question 5

What is the voltage relationship in a delta?

Answer 5

V phase = V line

Question 6

What is the problem when we read double the voltage when conducting an open corner test?

Answer 6

The polarities are reversed

Question 7

Which phasor is the “reference” in a delta system?

Answer 7

V ab at zero degrees

Question 8

What is the current relationship in a delta system?

Answer 8

When balanced, I line = I phase x root 3

When unbalanced, I a = I ab + I ac (phantom phasor of I ca)

Question 9

Are phasors drawn for the load or for the source?

Answer 9

the load

Question 10

What are the requirements for a delta system to be balanced?

Answer 10

all loads have same impedance and power factor

Question 11

How do we determine each phase current?

Answer 11

dividing the specific phase current by the specific phase voltage

Question 12

In a balanced delta system, the line current is equal to the phasor sum of its phase currents. What is a shortcut for finding this?

Answer 12

I line = I phase x root 3

Question 13

What is the phase angle difference between the line currents and the line voltages at unity power factor?

Answer 13

the line currents lag the line voltages by 30 degrees

Question 14

How many degrees apart are the line currents?

Answer 14

120

Question 15

Can I line = I phase x root 3 be used in an unbalanced delta?

Answer 15

no

Question 16

Describe an unbalanced delta load

Answer 16

the load doesn’t have the same impedance and/or power factor

Question 17

How do we find the line currents in an unbalanced delta?

Answer 17

add the phasor sum of the appropriate currents

Question 18

How can we verify that we found the phasor sums correctly in an unbalanced delta?

Answer 18

We can add I a + I b + I c and if they do not equal zero then we did something wrong

Question 19

Describe a four wire delta

Answer 19

there is a centre tap between line B and C for the neutral conductor. This “cuts” the voltage in half. If there used to be 240V from V bc, now there is 120V from V bn

Question 20

What must we do to the high leg in a delta?

Answer 20

We must always make sure that the high leg is indentified

Question 21

Where must the higher voltage on a four wire delt system be?

Answer 21

Between the phase A conductor and ground

Question 22

HHow do we find the voltage between the high leg and the center tap?

Answer 22

E an = E ab + E bn

Question 23

In a four wire delta, where should the high leg be?

Answer 23

between line A and N with the center tap connection between lines B and C

Question 24

What are the requirements for a delta wye system to be considered balanced?

Answer 24

same pf and same z

Question 25

When reconnecting a wye load to a delta load, line voltage will

Answer 25

stay the same

Question 26

When reconnecting a wye load to a delta load, the voltage drop across each load will

Answer 26

increase by root 3

Question 27

When reconnecting a wye load to a delta load, phase current will

Answer 27

increase by root 3

Question 28

When reconnecting a wye load to a delta load, line current will

Answer 28

increase by 3x

Question 29

What is important to remember about the windings of a wye source when reconnecting it in delta?

Answer 29

they do not change. If the rating of the coil in wye was 138V and 3.46A, it will be the same when reconnected in delta.

Question 30

When we have a delta and a wye on the same busbar, what must be do to the voltages?

Answer 30

We must rotate them by 30 degrees so we have V ab on 0. (V an on the wye would now be at 330 degrees)

We do this so we can have V ab from the Wye and V ab from the delta at the same placement so we can add them together easier.

Question 31

When a three phase delta is reconnected as a wye, the volt drop will be _____ across each load

Answer 31

root 3 less

Question 32

When a three phase delta is reconnected as a wye, the current through each load will be

Answer 32

root 3 less

Question 33

When a three phase delta is reconnected as a wye, the line current will be

Answer 33

3 times lower