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Electrical Engineering for Physicists How to get from the 208 VAC 3-phase power in the building to 110- and 220-VAC PowerPoint Presentation
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Electrical Engineering for Physicists How to get from the 208 VAC 3-phase power in the building to 110- and 220-VAC. Code issues All work must be done in conformance with the National Electrical Code and the Uniform Plumbing Code. NFPA 70: National Electrical Code

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slide1

Electrical Engineering for PhysicistsHow to get from the 208 VAC 3-phase power in the building to 110- and 220-VAC

Code issues

All work must be done in conformance with the National Electrical Code and the Uniform Plumbing Code.

NFPA 70: National Electrical Code

http://www.nfpa.org/aboutthecodes/AboutTheCodes.asp?DocNum=70

national electric code
National Electric Code
  • If you don’t know what Code requires
    • Learn What the Code requires
    • Hire a licensed electrician.
  • This applies to all utility issues
    • Plumbing (gas and water)
    • Electrical
    • Phone.
  • Violating the code is a very bad idea
    • Unsafe for fire (flood or contamination for plumbing violations)
    • Unsafe for proper equipment operation
    • Gets you in trouble with Plant Operations
good reference
Good reference
  • http://science.howstuffworks.com/power9.htm
  • Good description of how the electrical power distribution grid works
  • Source of photos on following slide
house wiring in the us
House Wiring in the US
  • House wiring
    • 220 V, 110 V and ground.
    • AC or DC? Why? How do you know?
center tapped transformer
Center Tapped Transformer

110 V

Input voltage

Typically 7200 V

220 V

110 V

slide10

For a sinusoidal AC voltage:

In the US, rad/sec

The power delivered to a simple resistive load is given by

slide11

Average power delivered over some number of cycles (like 1):

T = period =

Knowing the rms voltage allows you to calculate the power delivered to a load without having to do the integral every time!

110 v house wiring
“110 V” House wiring
  • “Hot” conductor
    • Always black
    • Potential is sinusoidal
    • 177 V zero-to-peak with respect to neutral
    • If you touch it with one foot in the bathtub, you will die!
  • “Neutral” conductor
    • Always white
    • Potential within a few volts of ground
    • Wire carries current “back from device”
  • “Ground” conductor
    • Green or bare
    • Common potential of interconnected water in earth
    • Ground conductor carries current only in emergency
house wiring

110 V

Input voltage

Typically 7200 V

220 V

110 V

House Wiring
  • “220 V”
    • From center-tapped transformer
house wiring14
House Wiring
  • “220 V”
    • From center-tapped transformer
    • Two hot leads, each 110 V from neutral.
    • 110 V leads are 180 degrees out of phase from each other
    • No neutral is required for a load that is only 220 V
    • Common uses
      • Stove elements
      • Water heaters
      • Air conditioners
      • Hot tubs
center tapped transformer15

110 V

Input voltage

Typically 7200 V

220 V

110 V

Center Tapped Transformer
expanded version of house wiring
Expanded Version of “House Wiring”
  • House wiring.pdf on website
fuses breakers and gfis
Fuses, Breakers and GFIs
  • Purpose of all: health and safety
  • Fuse
    • Contains a metal strip designed to quickly melt if the rated current is exceeded
    • Melted fuse breaks circuit
    • Single use
    • Fuse is ALWAYS inserted into the “hot wire” Why?
  • Breaker
    • Same function as fuse but can be reset
  • GFI (Ground fault interrupter)
    • Breaks circuit when net current in hot and neutral wires is non-zero
    • Activate when current flows to ground
    • Required in bathroom and outdoor applications
    • Two varieties: in breaker box and in outlet
advantages of 3 phase power
Advantages of 3-phase Power
  • 3-phase motors have higher starting torque, higher efficiency and lower current draw compared to the same size single phase motor.
  • Usually don’t need capacitors, switches or relays for start-up
  • Better for heavy duty applications
  • Stolen from McMaster-Carr catalog, p. 993!
slide22

All power towers like this have three wires for the three phases. Many towers, like the ones shown above, have extra wires running along the tops of the towers. These are ground wires and are there primarily in an attempt to attract lightning. http://science.howstuffworks.com/power4.htm

current capacities of wires from some website that looked authentic
Current Capacities of Wires(from some website that looked authentic)

Current

Wire gage

“Chassis” Free Air

These are just plain wrong and not by a small factor!

slide24

This really is Code!

“Young people must

be careful!

field trip
Field Trip!
  • Investigate a 3-phase breaker box
    • Measure voltages among phases
    • Measure voltage from each phase to neutral and ground
    • Measure voltage of neutral with respect to ground
    • What is phase difference between two phases?
  • Investigate properties of volt-, current- and resistance-meters
    • Digital
    • Analog
  • How many ways can you find to get the wrong answer?
  • Young people must be careful!