PHEP 18 Flashcards
(48 cards)
Why are delta wye connections desired?
They provide a path for the 3rd harmonic magnetizing current.
What is true in a delta connection?
Line voltage equals phase voltage, line current equals 1.73 times phase current.
What is residual current?
Current left over when vectorial sum of 3 phase currents is zero.
Wye connections, line voltage = _______ x phase voltage. And the Line current equals_______.
1.73; phase current.
Delta Connection, Line current = _____ x phase current.
1.73.
In an Inductive circuit, when the VARs are out, the current will lag the Voltage and the Impedance will consist of resistance and ________.
Inductive reactance.
The 3 phase coils of machines can be connected A to B, B to C, and C to A to form a _______ connection.
Delta (triangle).
The polarity of a transformer is: Subtractive when X1 & H1 terminals are__________ and Additive when X1 & H1 terminals are ________ to each other.
Adjacent; Diagonal.
In a 3 phase AC circuits, the circuits differ in phase position and are usually combined so that fewer _______ are required.
Conductors.
3 phase power can still be carried or transformed by a Delta-Delta transformer even if… It is called an…….
-one of the single phase units is removed from service -open delta or V transformation.
What are some of the advantages of three-phase circuits over single phase?
- 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.
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 backwards.
30 degrees.
The polarity of a transformer refers to?
The relative direction of the induced voltages in the primary and secondary windings.
The displacement angle caused by the delta-wye arrangement can cause phase-shifts of?
30 degrees, 150 degrees, 210 degrees, or 330 degrees.
A delta-wye connection cannot be connected in parallel with?
A delta-delta connection.
Any apparatus which has an iron magnetic circuit in which there is no air gap is likely to produce?
Harmonics.
Which harmonic is the strongest?
The third harmonic.
What happens to the third harmonic in a delta-delta configuration?
It short circuits the harmonic.
What other arrangement has essentially the same effect of a delta-delta configuration?
A delta-wye arrangement.
