Download Electrical Engineering for Physicists

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
•http://www.nfpa.org/aboutthecodes/About
TheCodes.asp?DocNum=70
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
That having been said, simple
things you can fix, build or modify
yourself.
Here’s how!
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
– 220 V, 110 V and ground.
– AC or DC? Why? How do you know?
Center Tapped Transformer
110 V
Input voltage
Typically 7200 V
220 V
110 V
Single
Phase
Breaker
Box
Single Phase Feed
Electrical Potential ("Voltage")
AC Voltage as a Function of Time
1
177 V
0
-1
-600
-400
-200
0
200
400
Time
600
800
1000
1200
For a sinusoidal AC voltage:
In the US,
V (t )  V p sin t
2
  2f 
 377rad/sec
T
The power delivered to a simple resistive load is given by
2
V p2
V
P

sin 2 t
R
R
Average power delivered over some number of cycles (like 1):
2
1
Pav 
T

0 P(t )dt  2
T
2
T = period =



0
2
2
V

p
sin 2 wtdt 
R
2 R
u  t
dt 
u sin u cos u
t

 sin u  2 
2
2
du
 sin
0

2


0
2
V
1 p
2
u
du

du  dt
2
2
V p2

V p2
sin t cos t

2
0
2
rms
Vp
V
1
Pav 

 

 Vrms 
2
R
2 R
R
2
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
• “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
• “220 V”
– From center-tapped transformer
110 V
Input voltage
Typically 7200 V
220 V
110 V
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 Transformer
110 V
Input voltage
Typically 7200 V
220 V
110 V
Expanded Version of “House
Wiring”
• House wiring.pdf on website
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
GFI Outlet and Breaker
3-phase Power
3-phase Breaker Box
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!
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.howstuffw
orks.com/power4.htm
Current Capacities of Wires
(from some website that looked authentic)
Current
Wire gage
“Chassis” Free Air
12
13
14
0.0808 2.05232
0.072
1.8288
0.0641 1.62814
1.588 5.20864
2.003 6.56984
2.525
8.282
41
4150
9.3 Hz
35
5300
7.4 Hz
32
6700
5.9 Hz
These are just plain wrong and not by a small factor!
This really is Code!
“Young people must
be careful!
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 resistancemeters
– Digital
– Analog
• How many ways can you find to get the wrong
answer?
• Young people must be careful!