generators

Discussion in 'Homework Help' started by muni, Sep 16, 2008.

1. muni Thread Starter Active Member

Jul 29, 2008
45
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sir in this attachment please find the question 12 and 13. i'm unable to understand how the answer has been derived. if any one can help me, i'm thanking them in advance

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2. vetterick Active Member

Aug 11, 2008
35
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Yes, phase shift is indeed the answer, you can only directly add AC voltages that are in phase with each other.

Question #12 looks like a stepper motor, I don't see any real practical use for it as a generator, but it works to show how much voltage is lost with the configuration.

Question #13 is a standard 3 phase generator, the generator actually generates 1.73 (square root of 3) times the output voltage, seems a little bit non productive, untill you find the motor connected to it uses 1.73 times less amperage than a single phase one.

3. scubasteve_911 Senior Member

Dec 27, 2007
1,202
1
12) Is a two-phase generator.. You should know that the phase will be 90 degrees out of phase, so you plot both 70Vrms signals as vectors. One is at 0degrees, the other at 90degrees. Then, you draw them head to tail, then draw a new vector from the origin. Do the trig, it works out to be 90Vrms.

13) Same idea as 12, except the angle is now smaller.

Steve

Jul 29, 2008
45
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Apr 14, 2005
7,050
657

6. scubasteve_911 Senior Member

Dec 27, 2007
1,202
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He just rounded up to 90Vrms. If you input RMS into the trig, then you will return RMS.

Steve

7. Ron H AAC Fanatic!

Apr 14, 2005
7,050
657
√9800≈99, not 90. 99 is the answer in his problem sheet.

8. scubasteve_911 Senior Member

Dec 27, 2007
1,202
1
Am I bad? That was a strange thing, I saw 98.994 and rounded down to 90

Steve

9. Ron H AAC Fanatic!

Apr 14, 2005
7,050
657
I'm glad you said that. I thought I was going nuts.

10. muni Thread Starter Active Member

Jul 29, 2008
45
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sir thank you. i too have not noticed answer before asking you reply. i thank ron H sir also for his valuable suggestion.
now my doubt is in Q12 it was only 2 vectors. but in Q13 there are 3 vectors. how should i do it ? first shall i calculate the third vector with any one of them. and should i go to final calculation?

11. scubasteve_911 Senior Member

Dec 27, 2007
1,202
1
Question 13 asks "how much voltage would be measured between any two open wires?"

So, there are 3 pole pairs, thus 360/(3*2) spacing between phases, or 60 degrees. Use trig. to solve for the resultant vector.

Steve

12. muni Thread Starter Active Member

Jul 29, 2008
45
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sir thank you, with this i got the funda of this question. sir i think if there are more no. of pole pairs, then the spacing between the phases will be by the formula 360/(n*2). if any thing wrong, please diect me
thank you sir
muni

13. scubasteve_911 Senior Member

Dec 27, 2007
1,202
1
muni,

A little advice, try not to rationalize with formulae, try to be able to reason to a formula. This is because, a lot of times, a variable is changed and your handy formula sheet is no longer valid. For example, what if one were to make the windings asymmetrical or have more poles on the rotor?

Steve

14. DrNick Active Member

Dec 13, 2006
110
2
Both of these questions are intended to test knowledge of polyphase systems. Both are essentially asking:

"what is the line voltage of a N phase system, given that the phase voltage."

question 12 is a N=2 phase system so

Vline=Vphase*sqrt(2)=70*sqrt(2)=98.99

question 13 is a N=3 phase Wye so:

Vline = Vphase*sqrt(3)=70*sqrt(3)=121.24

Jul 29, 2008
45
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