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Sdhabik

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About Sdhabik

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    Project Engineer

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  • Engineering Field
    Electrical & Instrumentation
  • License
    EIT
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    Electrical

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  1. This question is for finding the condition where system stability is maintained after the fault. From equal angle criteria, for stability of system: area C = area B. Since network restores at 80deg and the power equations are not given we assume this as critical clearing angle. "Critical clearing angle states that If the fault is cleared before critical clearing angle (here 80deg), stability is maintained." So for fault clearing angle less that 80 deg; area C will be less than area B and still stability is maintained. Hence the condition is : area c <= area B; conversely area B>=area C. If this may help you.
  2. can you try with wildi method as well plz.
  3. From Wildi equation 15.8: For max power/torque; R2/S=IZ1I where R2= 0.3 Z1= r1+jx = 0.2+j(0.25+0.35) = 0.2+j0.6 = 0.632<71.6 deg Therfore, S=R2/IZ1I = 0.3/0.632 = 0.475
  4. You can find hand written pdf solution 3rd edition somewhere in the internet.
  5. @LyceeFruit @tpkjr2006 Thank you for your kind suggestion. I do have the 2017 version as well.
  6. I am 32 Passed FE 1st try Jan 2018 ( from JPN) Failed PE 1st try Oct 2018 PE 2nd try target on Oct 2019
  7. Still waiting for someone to response on this.
  8. Thank you Zach for the attachment, and happy to say that is what i followed. I believe Open wye must be the same approach like open delta that you have mentioned. I wonder if some one agree with my above answers so that i can know i am in right track .
  9. My understanding may be incorrect but i see the 460.9 statement as following: "When a capacitor is connected on the load side of the motor overload device, line current will be reduced due to an improved power factor, which must be taken into account when selecting the rating of a motor overload device. But this effect of capacitor mentioned above shall not be considered while determining the motor circuit conductor rating. This is for determining motor overload device only. "
  10. If no other adjustments or correction factors are given use 75degC. You can see in NEC'17_Annex D Example D7 doing the same .
  11. I tried to solve Graffeo Ex38 by Wilid method but i got different answers than one in the Graffeo. I think i am missing something with my calculation. Please let me know who have done this problem with Wildi method. For those who don't know the question: "A 120V, 6OHz, 6 pole, delta connected, three phase induction motor has the following characteristics. At standstill: Z stator = 0.2+j 0.25 ohm; Z rotor = 0.3+j 0.35 ohm. Find Pmax. The slip at which Pmax occurs and the Torque N- m at that point." Book ans: Pmax= 16.88 KW; Slip=0.278; Torque=186.05 N-m My ans: Pmax=8.687 KW; Slip=0.476; Torque=69 N-m
  12. Sorry for the late response. Thank you @ellen3720 for your kind suggestion. I do have Wildi book. I just want to know if Graffeo Pmax came from same logic as mentioned in Wildi. Yes definitely i will follow Wildi.
  13. While doing VD problems, my instinct will say to use (5+15j) * 30 < -36.86 = 474.34volts which i think is the most accurate one. And choose the most near one option. I know the difference is too much but no other way. If got extra time in exam to check then will apply VD = I (RCosφ + XSinφ) at the end. I wish in actual exam we don't have to see both answers as option choice and hope the answer is near to our calculation. What i conclude to do is if the question asks for NEC. use NEC: VD = I (RCosφ + XSinφ). If not then use Ohms law.
  14. It depends on which phase to phase fault occurred. IA=0 if there is Phase B to Phase C fault. Here in this problem phase A to phase B fault occurs so condition will be IC=0.
  15. I appreciate your views on this. Here is mine. First i thought it should be 72A because of the statement in NEC 460.8 "The ampacity of conductors that connect a capacitor to the terminals of a motor or to motor circuit conductors shall not be less than one-third the ampacity of the motor circuit conductors and in no case less than 135 percent of the rated current of the capacitor." So back to question thinking that only conductor from capacitor to motor can be 1.35*60 = 81A rated, but the question is asking for feeder ckt conductor, it will be 72A as it is. But when i peek at NEC 2011handbook i saw this additional statement that change my mind to 81 A. It states that "The capacitor circuit conductors and disconnecting means must have an ampacity not less than 135 percent of the rated current of the capacitor. The reason is that all capacitors are manufactured with a tolerance of zero percent to 15 percent, so a 100-kVAr capacitor may actually draw a current equivalent to that of a 115-kVAr capacitor. In addition, the current drawn by a capacitor varies directly with the line voltage, and any variation in the line voltage from a pure sine wave form causes the capacitor to draw an increased current. Considering these several factors, the increased current can amount to 135 percent of the rated current of the capacitor." So then i thought the capacitor mentioned in the question may draw 81A from the source, 72A feeder may not be sufficient. Thats why my answer changed to 81A. I may not be correct. These are just my views.
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