# Please explain the answers to these, only three of them...Thanks!

Discussion in 'Homework Help' started by ElecTechCan, Jun 26, 2010.

1. ### ElecTechCan Thread Starter New Member

Jun 26, 2010
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Ac Generator Theory Question #12
Question 12:

Suppose we have an alternator with two sets of windings, A and B:

Each pair of windings in each set is series-connected, so they act as just two separate windings:

If one end of each winding pair were connected together at a common ground point, and each winding pair output 70 volts RMS, how much voltage would be measured between the open winding pair ends?

Delta and Wye 3-Phase circuits Question # 15
Question 15:

One of the conductors connecting the secondary of a three-phase power distribution transformer to a large office building fails open. Upon inspection, the source of the failure is obvious: the wire overheated at a point of contact with a terminal block, until it physically separated from the terminal.

What is strange, though, is that the overheated wire is the neutral conductor, not any one of the "line" conductors. Based on this observation, what do you think caused the failure?
After repairing the wire, what would you do to verify the cause of the failure?

Last edited: Jun 26, 2010

Mar 6, 2009
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3. ### ElecTechCan Thread Starter New Member

Jun 26, 2010
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I did try that already...couldn't get what I needed...

4. ### t_n_k AAC Fanatic!

Mar 6, 2009
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So for either one or both questions did you have any thoughts at all about a method of solution?

For instance - in the case of Question 12. Imagine the rotor is rotating - what will the induced voltage waveforms across each winding pair [A-A' & B-B'] look like when compared on a common time scale?

In the case of Question 15. Start by asking why a wire termination would fail due to overheating. There may be more than one reason and you could try to nut out what the options might be.

5. ### ElecTechCan Thread Starter New Member

Jun 26, 2010
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I can't figure out #12 at all. I know the answer is 99 volts but I don't know why...I've typed in all kinds of numbers and formulas but never come out with 99.
I'm guessing that #15 answer has something to do with it being delta to wye. Could it be because the secondary is out of phase or working at a different frequency?

6. ### t_n_k AAC Fanatic!

Mar 6, 2009
5,448
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Well with respect to question 12, did you consider taking my advice about plotting the two time varying waves at A-A' and B-B' on a common time scale? If you did this you would be able to see there would be a phase shift between the two voltages and using that phase difference you could then calculate the required voltage. You can do it using a simple phasor (complex number based) addition.

Re question 15. Presumably the neutral wiring was sized for a worst case load imbalance &/or fault condition. So hopefully the neutral didn't fail due to over current at the terminal block. Here's one possibility - what if the electrical installer forgot to fully tighten the terminal connection? Might this cause a problem at some point in time?

7. ### ElecTechCan Thread Starter New Member

Jun 26, 2010
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I'm lost on how to calculate # 12. I don't know how to plot the waves on a common time scale and don't know which numbers I'd use to do a simple phasor addition.
For #12 It makes sense that the terminal connection could fail due to not being fully tightened. Would it simply wear down over time causing it to disconnect from overheating?

8. ### t_n_k AAC Fanatic!

Mar 6, 2009
5,448
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Re Q12: Perhaps you need some more study on AC generation. The voltages are 90° out of phase. This can then be solved by Pythagoras - two sides of a right angled triangle are equal to 70 [volts]. What is the length of the hypotenuse [volts]?

Re Q15: Yes - a poorly terminated conductor is liable to overheat and burn out if the current is high enough. What does this also suggest about the balancing of the three phases in this scenario? Is anything else likely to give rise to a similar problem? In other words is this the most likely explanation? Would it be wise to investigate the problem further if you were the engineer in charge of the building electrical services?

Ever see the movie "Towering Inferno"?

9. ### ElecTechCan Thread Starter New Member

Jun 26, 2010
6
0
Thanks very much TNK!!! I've got them figured out now, much appreciated!