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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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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


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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 l.jpg
Good reference modify yourself.

  • http://science.howstuffworks.com/power9.htm

  • Good description of how the electrical power distribution grid works

  • Source of photos on following slide


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House Wiring in the US modify yourself.

  • House wiring

    • 220 V, 110 V and ground.

    • AC or DC? Why? How do you know?


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Center Tapped Transformer modify yourself.

110 V

Input voltage

Typically 7200 V

220 V

110 V


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Single modify yourself.Phase Breaker Box


Single phase feed l.jpg
Single Phase Feed modify yourself.


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AC Voltage as a Function of Time modify yourself.

177 V


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For a sinusoidal AC voltage: modify yourself.

In the US, rad/sec

The power delivered to a simple resistive load is given by


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

T = period =

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


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“110 V” House wiring modify yourself.

  • “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 l.jpg

110 V modify yourself.

Input voltage

Typically 7200 V

220 V

110 V

House Wiring

  • “220 V”

    • From center-tapped transformer


House wiring14 l.jpg
House Wiring modify yourself.

  • “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


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110 V modify yourself.

Input voltage

Typically 7200 V

220 V

110 V

Center Tapped Transformer


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Expanded Version of “House Wiring” modify yourself.

  • House wiring.pdf on website


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Fuses, Breakers and GFIs modify yourself.

  • 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


Gfi outlet and breaker l.jpg
GFI Outlet and Breaker modify yourself.


3 phase power l.jpg
3-phase Power modify yourself.


3 phase breaker box l.jpg
3-phase Breaker Box modify yourself.


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Advantages of 3-phase Power modify yourself.

  • 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!


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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


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Current Capacities of Wires 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 l(from some website that looked authentic)

Current

Wire gage

“Chassis” Free Air

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


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This really is Code! 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 l

“Young people must

be careful!


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Field Trip! 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 l

  • 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!


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