Delta and Delta/Wye Flashcards
What are the standard volt drops in a Delta system?
V AB, V BC, V CA
What is an open corner test?
It is a test to see if the secondary of a delta transformer is connected properly and if circulating currents are present
Using KVL, what should the voltage read on an open corner test?
0V
What would you read if you reversed two leads in an open corner test?
double the phase voltage
What is the voltage relationship in a delta?
V phase = V line
What is the problem when we read double the voltage when conducting an open corner test?
The polarities are reversed
Which phasor is the “reference” in a delta system?
V ab at zero degrees
What is the current relationship in a delta system?
When balanced, I line = I phase x root 3
When unbalanced, I a = I ab + I ac (phantom phasor of I ca)
Are phasors drawn for the load or for the source?
the load
What are the requirements for a delta system to be balanced?
all loads have same impedance and power factor
How do we determine each phase current?
dividing the specific phase current by the specific phase voltage
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?
I line = I phase x root 3
What is the phase angle difference between the line currents and the line voltages at unity power factor?
the line currents lag the line voltages by 30 degrees
How many degrees apart are the line currents?
120
Can I line = I phase x root 3 be used in an unbalanced delta?
no
Describe an unbalanced delta load
the load doesn’t have the same impedance and/or power factor
How do we find the line currents in an unbalanced delta?
add the phasor sum of the appropriate currents
How can we verify that we found the phasor sums correctly in an unbalanced delta?
We can add I a + I b + I c and if they do not equal zero then we did something wrong
Describe a four wire delta
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
What must we do to the high leg in a delta?
We must always make sure that the high leg is indentified
Where must the higher voltage on a four wire delt system be?
Between the phase A conductor and ground
HHow do we find the voltage between the high leg and the center tap?
E an = E ab + E bn
In a four wire delta, where should the high leg be?
between line A and N with the center tap connection between lines B and C
What are the requirements for a delta wye system to be considered balanced?
same pf and same z
When reconnecting a wye load to a delta load, line voltage will
stay the same
When reconnecting a wye load to a delta load, the voltage drop across each load will
increase by root 3
When reconnecting a wye load to a delta load, phase current will
increase by root 3
When reconnecting a wye load to a delta load, line current will
increase by 3x
What is important to remember about the windings of a wye source when reconnecting it in delta?
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.
When we have a delta and a wye on the same busbar, what must be do to the voltages?
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.
When a three phase delta is reconnected as a wye, the volt drop will be _____ across each load
root 3 less
When a three phase delta is reconnected as a wye, the current through each load will be
root 3 less
When a three phase delta is reconnected as a wye, the line current will be
3 times lower
