Download Electrical Systems, Motors and Drives

Electrical Systems
Facility Electrical Systems and
Understanding Electric Power
Basic Electrical Systems in Our
Buildings
• Electricity is the flow of electrical energy
through some conductive material, such
as our power cords or distribution wiring,
and powers equipment such as lights and
air conditioners to do useful work in our
building or facility.
• Electricity has two current types: AC
(alternating current) and DC (direct
current).
– AC is the power we get from the standard
outlets in our office building or other facility,
and
– DC is the power we get from batteries, or from
power supplies inside our computers, copiers
and FAX machines.
• Solar cells, fuel cells, and most wind
generators produce DC as their initial
output.
• In AC systems, the voltage changes
directions 50 times per second,
moving first positive and then
negative. This is called 50 Hertz, and
is the basic frequency of most
countries power systems. In the US
and North America they use 60 Hz
electric power. The AC voltage
changes as shown in Figure 1.
• AC power exists because it has
advantages for the power company since
they can step up the voltage to transmit
and distribute it to our buildings and
facilities; and this reduces the transmission
and distribution losses from the resistance
in the power lines.
• AC also makes it easier to build large, high
efficiency motors, and to have high
efficiency lighting such as fluorescent
lights. Using transformers, the AC can also
easily be changed to different voltages for
different uses in our homes and facilities.
Phases and Voltages in
AC Power Systems
• The number of phases in an AC power
system is given by the number of different
sine waves that make up the power.
– A single phase system has only one sine wave,
whereas a three phase system has three sine
waves.
• Typical voltages in the European area are:
220 volts single phase (240 volts for Cyprus)
380 volts three phase (415 volts for Cyprus)
Single Phase AC Electrical
Systems
• This is a power system having only one
sine wave, and the frequency of that sine
wave is 50 Hz.
Fig. 1: Single phase voltage
• For example, in most homes or small
buildings where only single phase AC
systems supply the electric power, the
voltage is 220 volts. It is brought to the
building with 3 wires, one being a
neutral or ground, and the other two
having a voltage difference of 220 volts.
In England and Cyprus this voltage
difference is 240 volts.
Three-Phase AC Electrical
Systems
• This is a power system having three sine waves, and
the frequency of each sine wave is 50 Hz. The three
sine waves are separated in time with a phase angle of
120 degrees. See Figure 2.
Figure 2: Three-phase
voltage
• For another explanation see:
http://www.howstuffworks.com/power1.htm
Grounded Wye Three Phase
Systems
• In new and recent installations, the most
popular systems are called 4 wire
grounded wye systems.
– For most larger buildings and facilities it is a
220/380 volt system.
– The low voltage part is 220 volts single
phase.
– The high voltage part is 380 volts 3 phase.
– In a 4 wire grounded wye system the high
voltage is 1.73 (the square root of 3) times
higher than the low voltage.
Grounded Wye Three Phase
Systems (cont)
• This system is flexible since it has the
ability to handle single phase plug loads
or lighting circuits that operate at 220
volts, from the same system that feeds
the 3 phase circuits for motors,
equipment for heating, air conditioning,
elevators, and industrial machinery.
380 volt, four wire wye
system
A
B
Neutral
Ground
N
G
C
VL L = 380 V
VL N = 380 / 3 = 220 V
IN = 0 in a balanced 3Φ system
3 Φ (phase)
Y system
Power in Simple AC Circuits
and Systems
• In DC circuits, and in AC circuits with only
resistors in them, if we substitute the voltage
from Ohm’s Law (V=IR) into the expression for
power, the result is:
P = VI = I × R × I = I2 R
• Example - Find the power dissipated by a 100
ohm resistor with a current of 1.2 amps through
it.
• Solution P = I2R = (1.2)2 x 100 = 1.44 x 100
= 144 watts
Power in General AC Circuits
and Systems
• Most of our AC power is in circuits and systems
that have other physical effects in them besides
the effect of a resistor.
• One of these effects is from an inductor – a
device often made with wire wrapped around an
iron core.
• This effect shows up in transformers, induction
motors, and magnetic fluorescent ballasts, for
example.
• Power in these AC systems is much more
complicated.
Power Triangle
• We sometimes show this power relationship in
the form of a triangle, called the Power Triangle,
to show this more complicated relationship.
kVA
kVAR
kW
• In the power triangle, the horizontal leg is
the real power in kW.
– Real power does real work, as in an
induction motor with its shaft work.
• The vertical leg of the triangle is the
reactive power in kVAR – kilovolt
amperes reactive.
– The reactive power is physically present, but
it does not do real work.
– In the case of an induction motor, the
reactive power is the power that magnetizes
the motor windings, and helps the motor
start and develop running torque.
• The hypotenuse of the triangle is the total
or apparent power in kVA. All electric
systems have capacities that are rated in
kVA.
• Commonly we hear someone talk about
a 500 kW distribution panel, but this is
not correct. It is a 500 kVA distribution
panel.
• The power triangle is a right triangle, and
the relationship between the kW, the
kVAR, and the kVA is given by the
Pythagorean Theorem:
kW2 + kVAR2 = kVA2
• Also, in this triangle, the ratio of kW to
kVA is given the name Power Factor.
PF =kW/kVA
• For a DC power system there is no
reactive power, and thus no power
triangle.
• In addition, for DC power systems the
power factor is always 1.0, or 100%.
• In AC power systems, pure resistive
loads such as incandescent lights,
resistance space heaters, resistance
water heaters and electric ovens all have
power factors of 1.0 or 100%.
Example
An AC induction motor has the following
power triangle. Verify the relationship
between kW, kVAR and kVA. What is the
power factor of the motor?
50 kVA
40 kW
30 kVAR
Solution
kW2 + kVAR2 = kVA2
(40)2 + (30)2 = (50)2
1600 + 900 = 2500
2500 = 2500
Yes, the relationship is verified.
The power factor of the motor is found from
the ratio of kW to kVA:
PF = kW/kVA = 40/50 = 80%
Power in Single Phase Systems
• Most residential and small commercial
buildings only have single phase AC 220
volt power available.
– Devices like full house air conditioners,
electric hot water heaters, and electric
clothes dryers are almost always operated
on 220 volts AC, single phase.
Power in Single Phase Systems
• For single phase AC systems, the
equation for the electric power used is:
P = V × I × PF
Where P = real power in watts
V = voltage in volts
I = current in amperes
PF = power factor
Example
Find the real power drawn by a 220 volt AC,
single phase electric resistance water heater,
if it is drawing 20 amps of current.
Solution:
Since the water heater is an electric
resistance device, its power factor is 100%.
Thus, the real power drawn is:
P = V × I × 1.0 = (220)(20)(1.0)
= 4400 watts
= 4.4 kW
Power in Three Phase Systems
• Almost all electric motors over 2 kW, as well
as all large pieces of electrical equipment in a
facility, run on three phase power.
• The equation for the power drawn by a
general three phase load is:
P = √3 × V × I × PF watts
Example
A three phase, 380 volt electric motor draws
100 amperes, and has a power factor of 87%.
How much real power is the motor using?
Solution
P = √3 × V × I × PF watts
= 1.732 x 380 x 100 x 0.87 watts
= 57,259.9 watts
= 57.3 kW
References for Basic Electrical
Systems
•
Tom Igoe, Interactive Telecommunications Program, New
York University, http://www.itp.nyu.edu/tigoe, Brooklyn, NY,
2003.
•
Motors and Voltages, Cowern Papers, Motorsanddrives.com.
•
A+ Certification: The Basics of Electronics
Adapted From: A+ Certification For Dummies, 2nd Edition,
Volts, Amps, and Watts: What are they?
•
Energy Education: Concepts and Practices, Wisconsin Energy
Education Program, KEEP.
•
John Fetters, An Introduction to Electricity for Energy
Managers, Energy User News, August 2002
•
Guide to Energy Management, 4th Edition, Capehart, Turner
and Kennedy, Fairmont Press, Lilburn, GA, 2003.