1. Why is it extremely important to confirm that the delta voltage equals zero b
ID: 1766205 • Letter: 1
Question
1. Why is it extremely important to confirm that the delta voltage equals zero before the delta is closed? a. To ensure that the secondary voltage does not become too high b. To avoid possible damage because of high current. c. To avoid a short-circuit of the primary winding d. To maintain the secondary voltage at a constant level 2. In a delta-delta configuration, the line voltage on the secondary side is a. equal to the primary voltage times the inverse of the turns ratio b. times the primary voltage c. times the primary voltage times the inverse of the turns ratio d. 1/ times the primary voltage 3. The voltage across two windings in a wye-wye configuration must be a. equal to the voltage across each winding b. times the voltage across each winding c. less than the voltage across each winding d. times less than the voltage across each winding 4. The voltage across two windings in a delta-delta configuration must be a. equal to the voltage across each winding b. times the voltage across each winding c. less than the voltage across each winding d. times less than the voltage across each winding 5. A three-phase transformer can be a. a single unit with three separate sets of single-phase windings b. three single-phase transformers connected together c. a single unit with one primary and three secondary windings d. either a or bExplanation / Answer
1) ans : b
It is extremely important for the voltage across the series combination of the three windings is measured to confirm that it is zero before delta is closed because a very high short-circuits current will flow if the voltage within the delta is not equal to zero when it is closed.
2) ans : equal to primary voltage times turns ratio
We know turn ratio = the number of secondary windings NS, divided by the number of primary windings NP. ( NS/NP ) , in delta delta connection we have Vs = Vp*(NS/NP)
3) ans: root(3) times voltage across each winding
In wye-connected sources, the voltage across 2 windings called as Line voltage and across a single winding called as phase voltage and loads always have line voltages greater than phase voltages, and line currents equal to phase currents. If the Y-connected source or load is balanced, the line voltage will be equal to the phase voltage times the square root of 3.
4) ans : a
Each pair of line conductors is connected directly across a single winding in a delta circuit, hence the line voltage will be equal to the phase voltage.
5) ans: b
A 3 phase transformer can be seen as a connected circuit between 3 single phase transformers, depinding on their connection we can decide their configuration ie - delta-delta, wye-delta, delta-wye, wye-wye. hence it is 3 single phase transformer connected together.
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