Objectives Review electric circuits and electricity pricing - Introduce HW5 Select conductor and...

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Transcript of Objectives Review electric circuits and electricity pricing - Introduce HW5 Select conductor and...

  • Slide 1
  • Objectives Review electric circuits and electricity pricing - Introduce HW5 Select conductor and conduit List electrical devices
  • Slide 2
  • Voltage (potential difference) Units? Symbols: V, E or U Current (electron flow) Units? Symbol I Voltage and Current E or V [V]R [] I [A]
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  • Across a transformer I 1 E 1 = I 2 E 2 Change - Voltage [E] and - Current [I] Transformers Es/E p = N s /N p
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  • Ref: Tao and Janis (2001)
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  • Single-Phase vs. Three-Phase Ref: Tao and Janis (2001) For three phase: P=3 E I
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  • Why Three-Phase? Larger loads Smaller wire sizes (because higher voltages) P = EI = I 2 R= E 2 /R (E = IR) More efficient use of neutral wire 3 needed for efficient operation of equipment
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  • Electricity Billing Electrical Use (energy) Peak Demand (power) Power factor Which is largest portion of residential bill? What about for commercial buildings? http://www.austinenergy.com/About%20Us/R ates/Commercial/index.htmhttp://www.austinenergy.com/About%20Us/R ates/Commercial/index.htm
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  • Example: September cost of Electricity for ECJ Assume ECJ is 120,000 ft 2 and that it needs, on average, 8 W/ft 2 for 8 hours a day, 6 W/ W/ft 2 for 4 hours a day, and 4 W/ft 2 for 12 hours a day Use Austin Energy Large Primary summer service rate 1.5/kWh, 12.60 $/peak kW/month Assume no power factor charges
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  • Solution large primary service $/kW $/kWh Daily $ 12.60 $ 0.015 hourskWkWh 89607680 demand $12,096.00 47202880 energy $ 7,344.00 124805760 TOTAL16320 Total $19,440.00
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  • Other Pricing Strategies Time of use pricing Becoming more common for residential and commercial Electricity cost related to actual cost Requires meter Interruptible pricing Utility can shut off electricity for periods of time
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  • Other Pricing Strategies Time of use pricing Becoming more common for residential and commercial Electricity cost related to actual cost Requires meter Interruptible pricing Utility can shut off electricity for periods of time For Austin http://www.austinenergy.com/About%20Us/Rates/Commercial/index.htm
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  • Homework 5 Problem 1: Economical analyses of chilled water saving
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  • Advantage of higher voltage 120/208 220/380 277/480
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  • Neutral and 3-phase system If system is well balanced the current through the neutral wire is 0
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  • Grounding What is electrical ground? Why do we ground electrical devices/systems? Protect equipment Lightening strike Protection of people Sometimes need ungrounded power If the hot side touches ground, it can put out of service our circuit Isolate ungrounded systems
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  • Conductors Material Form Composition Voltage class Insulation Covering Temperature rating AWG American Wire Gage
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  • Wiring method Raceways Electric tubes Rigid conduits Wire ways Bus ducts Underflow Different variation www.hhrobertson.com/
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  • Design issues with conductors Material (copper/aluminum) Size of conductor (pg. 365) Conduit requirements Location Residential or Commercial With or without raceways Compression fittings or set screws Threaded connections NEC and local codes
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  • Current-Carrying Capacity Ref: Tao and Janis (2001)
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  • Tubing size
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  • Other Issues In general, no more than 40% of raceway can be filled with wiring Why? To prevent extensive heat up To prevent physical installation of wires
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  • Conductor Rules Explain each of the following: No more than 4 90 bends are allowed between pull boxes In the same conduit: No mixing of high- and low-voltage conductors No mixing control and power conductors No mixing phone and power conductors Do place all three phases in the same conduit
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  • Receptacles and switches Receptacles (duplexes) Number Shape Voltage rating Current Rating Number of poles and wires Switches Type (NEC rating) Contact method Speed of operation Voltage rating Number of poles Method of operation Enclosure Duty Other (dimming)
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  • Protective Devices What are we protecting from? Overcurrent Overvoltage Circuit breakers Switch that responds to thermal or short circuit loads Can be bimetal, magnetic, or electronic Reusable, remote control, compact, can be used as disconnect switch Fuses Melting metal Self destructive, larger
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  • Why use fuses instead of circuit breakers? A.Fuses can be used multiple times B.Fuses are more aesthetically pleasing C.Fuses are safer D.Fuses cause less damage to equipment
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  • Ref: Tao and Janis (2001)
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  • Motor starters ON-OFF switches are NOT for motors Motor circuit-rated switches Reduce the current during the start until the motor rich full speed Protect the motor form overload Emergency power systems Batteries Power generators
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  • Summary Describe role of electrical system components Calculate billing for an electrical system given the rate structure Size conduit and conductors given current requirements and conductor type
  • Slide 29
  • Reading Assignment Tao and Janis: Whole chapter 11