Transcript Agricultural Buildings (AT 3084) Basic Electrical Theory

Electrical Motors Wiring
S. Christian Mariger Ph.D.
Biological Systems Engineering
Virginia Tech
Examples using Ohm’s Law
• Determine the resistance of a soldering
iron that draws 9.5 amps. The iron is
plugged into a 120v outlet.
• Volts = amps x resistance or E = IR
• R = E/I = 115volts / 9.5 amps = 12.1 Ohms
Examples using Ohm’s Law
• Determine the resistance of a 100 Watt
light bulb on a 120 volt AC circuit.
• Watts = volts x amps
Examples using Ohm’s Law
• Determine the resistance of a 100 Watt
light bulb on a 120 volt AC circuit.
• Watts = volts x amps
• Watts / volts = amps = 100/120 = 0.83
Examples using Ohm’s Law
• Determine the resistance of a 100 Watt
light bulb on a 120 volt AC circuit.
• Watts = volts x amps
• Watts / volts = amps = 100/120 = 0.83
• Resistance (Ohms) = volts/amps
Examples using Ohm’s Law
• Determine the resistance of a 100 Watt
light bulb on a 120 volt AC circuit.
• Watts = volts x amps
• Watts / volts = amps = 100/120 = 0.83
• Resistance (Ohms) = volts/amps
• R = 120/0.83 = 145 ohms
Electrical Generation & Delivery
• Most electricity is generated a great distance
from where it is used.
• Large AC generators at power plants use
induction to convert mechanical energy in to vast
quantities of electricity (Three phase AC @
25,000 volts)
• This electrical power is run through a step-up
transformer at the power plant to raise the
voltage to 765,000 volts for transmission across
the electric grid.
Power Transmission
The “Electrical/Power
Grid” is made up of
thousands of
interconnected high
tension towers like this
one. These towers carry
the high voltage current
from the power plant
over long distances to
special substations
called receiving stations.
Electrical Power Delivery
• The receiving substations house stepdown transformers to reduce the voltage
to 34,000 Volts for branch distribution
• Branch substation transformers further
reduce the voltage to 12,500 Volts
• Finally the transformer on the pole outside
your house steps the voltage down to 240
or 120 Volts.
Power Delivery
Receiving Station
Electric Service Drop
Branch Substation
Pole Transformer
(Electrical Motors)
Electrical Motors
• Though lighting and heating are very
important, in terms of agricultural
structures electrical motors are the most
significant application of electrical power.
– Ventilation fans
– Pumps
– Material handling
– Etc.
Advantages of Electric Motors
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Efficiency (50 to over 90%)
Low initial cost
Relatively inexpensive to operate
Easy to start
Can be started with a reasonable load
Can be remotely/automatically controlled
Can withstand temporary overloads
Advantages of Electric Motors
(Continued)
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Long service life (35,000 hours)
Compact
Simple to operate
Low noise level
No exhaust fumes
Minimize safety hazards
Single Phase AC Electric Motors
• Split Phase (fans and pumps)
• Capacitor Start (compressors, augers, pumps and
elevators)
• Two Valve Capacitor (conveyors, elevators and silo
unloaders)
• Permanent Split Capacitor (fans and blowers)
• Shaded Pole (small blowers and appliances)
• Wound Rotor (conveyors, mills/grinders, hoists and
deep well pumps)
• Universal/Series (portable power tools)
• Synchronous (clocks and timers)
• Soft Start (crop driers, forage blowers, irrigation pumps,
manure agitators)
AC Electric Motor Power
(P) in Watts
• Single Phase Motors:
– P = E x I x PF
• Three Phase Motors:
– P = 1.73 x E x I x PF
– Where:
•
•
•
•
P = Power in (Watts)
E = Electromotive force in (Volts)
I = Current flow in (Amps)
PF = Power Factor
Power Factor (PF)
• The power factor of an AC electric power
system is defined as the ratio of the active
(true or real) power to the apparent power.
– Where:
• Active (Real or True) Power is measured in watts
(W) and is the power drawn by the electrical
resistance of a system that does useful work.
• Apparent Power is measured in volt-amperes (VA)
and is the voltage on an AC system multiplied by
all the current that flows in it. It is the vector sum of
the true and the reactive power.
Power Factor (PF)
• For purely resistive loads like incandescent
lighting and heating elements the power factor is
1.0 so in these special cases:
– Power (P) = E x I = Watts
• The power factor for inductive or capacitive
loads such as electric motors varies by motor
type, hp rating, and the mechanical load on the
motor. Power factor can vary from 0.3 for
unloaded fractional hp motors to nearly 1.0 for
large capacitor run motors.
Electric Motor Efficiency
• Efficiency The power output divided by the total
power consumed =
– Useful power output / Total power input.
• For single phase motors (%) =
– ((Rated hp x 746 W/hp) / (E x I x PF)) x 100
• For three phase motors (%) =
– ((Rated hp x 746 W/hp) / (1.73 x E x I x PF)) x 100
Electric Motor Data Plate
Gives critical data such as:
Voltage, Amps, Phase, Hertz, RPM, Power Factor
and Duty Rating
25 hp Electric Motor Example
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Actual power consumed
(P) Watts (3 phase) = 1.73 x E x I x PF
P = 1.73 x 230 x 59.4 x 0.843 = 19,925 W
Theoretical power output
25hp x 746 W/hp = 18,650 W
Efficiency (%)
(18,650 / 19,925) x 100 = 93.6%
Electric Motor Data Plate
Notice how the calculated efficiency is exactly the
same as the nominal efficiency on the data
plate!
Starting Current (Amps)
• Refer to the electric motor characteristics
table in the handout!
• Notice that an electric motor can draw
between 1.5 and 8 times the full-load
current at start up.
• When sizing conductors and overload
protection devices for electric motors
multiply the full-load amperage by 1.25 or
125% to find the minimum ampacity.
Ampacity: the current (amps) a conductor can carry without exceeding it’s
temperature rating.
(Electrical Wiring)
AC Phases
• Residential electrical service is exclusively single
phase.
• However some agricultural applications e.g.
some large electric pumps, require three phase
power.
• Farms and shops will often have high voltage
three phase electrical service.
• Three phase is more complicated to wire and
should be handled by a qualified electrician
Distribution Panel
• A circuit may have
many branches, but in
each branch, the
delivery wire and the
return wire are
attached to the
corresponding wire on
the main circuit.
• The main circuit and
branch circuits meet at
a breaker/fuse box or
distribution panel.
Types of Branch Circuits
• General purpose: 120 V 20-Amps or less.
– Permanent lighting and appliances.
– Convenience outlets (<1,500 Watts)
• Individual: 120 or 240 V
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All motors ½ hp or larger
Appliances over 1,500 Watts
All 240 V applications
Continuous service equipment (incubators, milk
coolers, brooders, etc.)
• The minimum recommended AWG for both
types of branch circuits in agricultural
applications is 12 gauge (copper).
Protection of Branch Circuits
• Each branch circuit is protected by it’s own
fuse or circuit breaker.
• The over-current protective device is
selected in accordance maximum current
(Amps) that can be carried by the gauge
wire used in the circuit.
Over-current Protective Devices
Circuit Breakers automatically
open the circuit when current
flow exceeds the breaker rating
Fuses have a circuit opening
“fusible member” that is
directly heated and destroyed
by the passage of too much
current
Fuses and circuit breakers protect the wires and equipment not people!
Ground Fault Circuit Interrupter
(GFCI)
• Fuses and circuit breakers are designed to
protect equipment and wiring from current
overloads. (These are fairly slow to react)
• A GFCI is designed to protect people from
stray current and shorts.
• The GFCI compares the current on the hot
wire to the current on the neutral wire, if
the amperage is not equal the GFCI cuts
off the current immediately!
Designing a Single Phase Branch
Circuit
1.
2.
3.
4.
Determine the electrical load
Measure the length of the run (one-way)
Select the appropriate wire gauge
Select the appropriate wire insulation
type
5. Select the appropriate over-current
protective device
For all convenience outlets use a GFCI
Electrical Load for General Purpose
Branch Circuits
A. Count the number of lamps or appliances
that will be used on the circuit and note
the wattage of each.
B. Add up the number of watts used on the
circuit.
C. For a 120 V AC circuit divide the watts by
120 to determine the amp load
D. Recall that Watts = Volts X Amps
Electrical Loads for Individual
Branch Circuits
A. Find the full-load current (Amps) for the
motor.
B. To adjust for starting current multiply the
full-load current (Amps) by 1.25 or 125%
Measure the length of the run
• Measure the distance from the main circuit
box to the end of the branch circuit to be
wired.
• Recall that long runs will cause a voltage
drop on the circuit ( No more than a 2%
Voltage Drop should be allowed)
• For example a five amp load needs only
12 gauge wire to go 50 feet, but will
require a 10 gauge wire at 100 feet.
Voltage Drop
• Is the reduction in voltage between the
power supply and the electrical load. This
loss occurs any time electricity flows on a
conductor such as a wire. Voltage drop is
equal to the product of the current (A) and
the resistance (Ω) of the conductors in the
circuit.
• Voltage Drop (%) = ((I x R) / E) x 100
Wire Gauge for (120 Volt) AC
Single Phase (Lighting & Outlets)
Amp
Load
5
amp
10
amp
15
amp
20
amp
30’
100’ 200’
300’ 400’ 500’
Run
Run Run
Run Run Run
12 ga 12 ga 10 ga 8 ga 6 ga 6 ga
12 ga 10 ga
6 ga
4 ga
4 ga
3 ga
12 ga
8 ga
4 ga
4 ga
2 ga
1 ga
12 ga
6 ga
4 ga
2 ga
1 ga
0 ga
Copper Wire Insulation Types
Wire Type
T
TW
THHN
THW
XHHW
UF
Application
Dry locations
Dry or wet locations
Dry locations with high temps
Wet locations with high temps
High moisture heat resistance
Direct burial in soil (not concrete)
Livestock and poultry structures are wet and corrosive
environments, the use of UF cable, corrosion resistant
boxes and fittings are strongly recommended!
Making connections
• There are a number of ways to make
electrical connections:
– Screw terminals
– Electrical solder
– Solder-less connections
– Wire nuts
– Splices
– Etc.
Screw terminals
• Commonly found on
switches, outlets,
and lamp holders.
• Note the clockwise
direction of the
connection
Soldering connections
• Solder is an excellent way to make
electrical connections.
• Connections are sure and permanent.
Solder-less connections
• These connections are fast and secure,
but must be crimped properly in-order to
give good results
Wire nuts
• Wire nuts are used extensively in wiring
homes and agricultural structures.
Wire splices
• Wire splices are
often used as a last
resort when one of
the preceding
methods will not
work.
Splicing multiple wires
• When more than one wire is to be spliced
the splices should be staggered to prevent
a short circuit.
Wire color codes (120 V)
Wire color
Wire Function
Black
Positive (hot wire)
Red
Positive (hot wire)
Blue
Positive (hot wire)
White
Neutral (current back to the source)
Green
Ground (to metal box)
Bare
Ground (to metal box)
Wiring
• Switches
• Lamp sockets
• Outlets
Switches
• Single pole –
• Three way –
Single pole switch
Three-way switch
Three-way circuit
Three-way circuit
Lamp sockets
Convenience outlet
Convenience outlet
Convenience outlet
GFCI outlet
GFCI plug
GFCI protection can also be built into
appliances & extension cords!
Circuit Failures
• Short
– A short circuit occurs when two or more wires come in
contact with each other that should not.
– May result from worn insulation or a wire becoming
unhooked.
– Causes a by-pass to be created in the original circuit,
reducing the resistance of the circuit.
– When the resistance is reduced, current flow or
amperage increases producing excessive heat.
– Heat can melt insulation, cause circuit breakers to
trip, fuses to melt or even damage to electrical
equipment.
Circuit Failures
• Open Circuit
– An interruption or break in the flow of
electricity.
– This can occur when a conductor is
accidentally cut, comes loose from its
connection or corrosion has created too much
resistance in the circuit.
– Construction workers digging into
underground cables is a common occurrence
for the utility companies.