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

• 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

Input voltageTypically 7200 V

110 V

110 V

220 V

Single Phase

Breaker Box

Single Phase Feed

AC Voltage as a Function of Time

Time

-600 -400 -200 0 200 400 600 800 1000 1200

Ele

ctric

al P

oten

tial (

"Vol

tage

")

-1

0

1

177 V

For a sinusoidal AC voltage:

In the US, rad/sec

The power delivered to a simple resistive load is given by

3772

2 T

f

tR

V

R

VP p 2

22

sin

tVtV p sin)(

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

T

ppav

duu

R

Vwtdt

R

VdttP

TP

0

2

0

2

0

22

22

sin2

sin2

)(1

22

cossin

2sin 2

tuuuu

2

22

1

2

1 222p

rmsrmspp

av

VV

R

V

R

V

R

VP

T = period =

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

du

dt tu dtdu

2

2

cossin tt

2

0 0

“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

Input voltageTypically 7200 V

110 V

110 V

220 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

Input voltageTypically 7200 V

110 V

110 V

220 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.howstuffworks.com/power4.htm

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

12 0.0808 2.05232 1.588 5.20864 41 9.34150 Hz

13 0.072 1.8288 2.003 6.56984 35 7.45300 Hz

14 0.0641 1.62814 2.525 8.282 32 5.96700 Hz

Wire gageCurrent

“Chassis” Free Air

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 resistance-meters– Digital– Analog

• How many ways can you find to get the wrong answer?

• Young people must be careful!