Transcript 346N_No17_Elect_Systems

Objectives
• Review electric circuits and electricity
pricing
- Introduce HW5
• Select conductor and conduit
• List electrical devices
Voltage and Current
I [A]
E or V [V]
Voltage (potential difference)
•Units?
•Symbols: V, E or U
•Current (electron flow)
•Units?
•Symbol I
R [Ω]
Transformers
Change
- Voltage [E] and
- Current [I]
Across a transformer
I1E1 = I2E2
Es/Ep = Ns/Np
Ref: Tao and Janis (2001)
Single-Phase vs. Three-Phase
For three phase:
P=√3 E I
Ref: Tao and Janis (2001)
Why Three-Phase?
• Larger loads
• Smaller wire sizes (because higher voltages)
• P = EI = I2R= E2/R (E = IR)
• More efficient use of neutral wire
• 3 needed for efficient operation of equipment
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.htm
Example: September cost of Electricity
for ECJ
• Assume ECJ is 120,000 ft2 and that it needs,
on average, 8 W/ft2 for 8 hours a day, 6 W/
W/ft2 for 4 hours a day, and 4 W/ft2 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
Solution
hours
8
4
12
Daily
kW
960
720
480
TOTAL
large primary service
$/kW
$/kWh
$ 12.60
$
0.015
kWh
7680
2880
5760
16320
demand
energy
$12,096.00
$ 7,344.00
Total
$19,440.00
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
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
Homework 5
Problem 1: Economical analyses of chilled water saving
Advantage of higher voltage
120/208
220/380
277/480
P  3EI
Neutral and 3-phase system
• If system is well
balanced
the current
through the
neutral wire is 0
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
Conductors
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Material
Form
Composition
Voltage class
Insulation
Covering
Temperature rating
AWG American Wire Gage
Wiring method –Raceways
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Electric tubes
Rigid conduits
Wire ways
Bus ducts
Underflow
• Different variation
www.hhrobertson.com/
Design issues with conductors
• Material (copper/aluminum)
• Size of conductor (pg. 365)
• Conduit requirements
• Location
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Residential or Commercial
With or without raceways
Compression fittings or set screws
Threaded connections
• NEC and local codes
Current-Carrying Capacity
Ref: Tao and Janis (2001)
Tubing size
Ref: Tao and Janis (2001)
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
Conductor Rules
• Explain each of the following:
• No more than 4 90 ° bends are allowed between
pull boxes
• In the same conduit:
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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
Receptacles and switches
• Receptacles (duplexes)
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Number
Shape
Voltage rating
Current Rating
Number of poles and
wires
• Switches
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Type (NEC rating)
Contact method
Speed of operation
Voltage rating
Number of poles
Method of operation
Enclosure
Duty
Other (dimming)
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
Why use fuses instead of circuit
breakers?
A.
B.
C.
D.
Fuses can be used multiple times
Fuses are more aesthetically pleasing
Fuses are safer
Fuses cause less damage to equipment
Ref: Tao and Janis (2001)
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
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
Reading Assignment
Tao and Janis:
Whole chapter 11