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# What will each wattmeter indicate?

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1.  Junior Member
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Jul 2017
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Reputation ## What will each wattmeter indicate?

Three coils with a resistance of 4 ohms and impedance of 13 ohms each are connected in wye to a 120/208 Volt panel. What will each wattmeter indicate if three single-phase wattmeters are used to measure the power?

The correct answer is 341W according to my answer sheet but I seem to keep getting 311W and I don't know what Im doing wrong. Any help would be great!  Reply With Quote

2. Originally Posted by Stephen30002 Three coils with a resistance of 4 ohms and impedance of 13 ohms each are connected in wye to a 120/208 Volt panel. What will each wattmeter indicate if three single-phase wattmeters are used to measure the power?

The correct answer is 341W according to my answer sheet but I seem to keep getting 311W and I don't know what Im doing wrong. Any help would be great!
how did you come up with 311W? i'm having trouble with this question, not sure where to start.  Reply With Quote

3.  Baldy
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Reputation ## I get same as you... Originally Posted by Stephen30002 Three coils with a resistance of 4 ohms and impedance of 13 ohms each are connected in wye to a 120/208 Volt panel. What will each wattmeter indicate if three single-phase wattmeters are used to measure the power?

The correct answer is 341W according to my answer sheet but I seem to keep getting 311W and I don't know what Im doing wrong. Any help would be great!
I get the same answer as you and verified it through calculator on this site: http://www.rapidtables.com/calc/elec...calculator.htm

Impedance Z = SQRT(4*4 + 13*13) = 8.823 Ohms
Angle Theta = ATAN (Impedance/Resistance) = ATAN (13/4) = 72.9 degrees
Total Power VA = V*V/Z = (120 * 120) / 8.823 = 1058.82 VA
Real Power Watts = Total Power * Cos Theta = 1058.82 * Cos(72.9) = 311 watts

Beats me how someone got 345 watts.  Reply With Quote

4.  Junior Member Pro Subscriber
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Reputation  Originally Posted by baldscreen I get the same answer as you and verified it through calculator on this site: http://www.rapidtables.com/calc/elec...calculator.htm

Impedance Z = SQRT(4*4 + 13*13) = 8.823 Ohms
Angle Theta = ATAN (Impedance/Resistance) = ATAN (13/4) = 72.9 degrees
Total Power VA = V*V/Z = (120 * 120) / 8.823 = 1058.82 VA
Real Power Watts = Total Power * Cos Theta = 1058.82 * Cos(72.9) = 311 watts

Beats me how someone got 345 watts.
Impedance should be 13 ohms instead of 8.8  Reply With Quote

5.  Baldy
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Reputation ## I stand corrected...more or less. Originally Posted by Allenxu Impedance should be 13 ohms instead of 8.8
You are correct on the impedance. I was thinking of it as reactance ohms.

So, I'll try it again:

COS(Theta) = (Resistance/Impedance) = 4/13 Total Power VA = V*V/Z = (120 * 120) / 13 = 1107.69 VA
Real Power Watts = Total Power * Cos Theta = 1107.69*4/13 = 340.8 watts

Closer, but not quite the 345 watts given as the answer.  Reply With Quote

6. ## either luck or ???

PF = R/Z = 4/13 = 0.3077

E = I*R , 120 = I*13 , I = 120/13 = 9.23A

P = I*E , P = 9.23*120 = 1107.7VA

0.3077(PF) = W/1107.7VA , W = 1107.7*0.3077 = 340.84

340.84(W)/9.23A = 36.93V

36.93V/4OHMS = 9.23A  Reply With Quote

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