Environmental Engineering Reference
In-Depth Information
■
Example 2.106
Problem
: A 60-hp motor operates at full load 12 hr/day, 7 days a week. How many
kilowatts of energy does it consume per day?
Solution
:
Kilowatt-hours/day = 60 hp × 0.746 kW/hp × 12 hr/day = 537 kWh/day
Given the cost per kilowatt-hour, the operator (or anyone else) can calculate the
cost of power for any given period of operation.
Cost = Power required/day × kWh/day × Days/period × Cost/kWh
(2.24)
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Example 2.107
Problem
: A 60-hp motor requires 458 kWh/day. The pump is in operation every day.
The current cost of electricity is $0.0328/kWh. What is the yearly electrical cost for
this pump?
Solution
:
Cost = 458 kWh/day × 365 days/year × $0.0328/kWh = $5,483.18
p
oWer
f
aCtor
Power factor is important because customers whose loads have low power factor
require greater generation capacity than what is actually metered. This imposes a
cost on the electric utility that is not otherwise recorded by the energy and demand
charges. Two types of power make up the total or
apparent power
supplied by the
electric utility. The first is
active
(also called
true
)
power
. Measured in kilowatts, it
is the power used by the equipment to produce work. The second is
reactive
power
.
This is the power used to create the magnetic field necessary for induction devices to
operate. It is measured in units of kilovolt-ampere reactive (kVAR).
•
Active power (true power) (P)
—Power that performs work, measured in
watts (W)
•
Reactive power
—Power that does not perform work (sometimes called
wattless power
), measured in volt-ampere reactive (VAR)
•
Complex power
—The vector sum of the true and reactive power, measured
in volt-ampere (VA)
•
Apparent power
—The magnitude of the complex power, measured in volt-
a m p e r e ( VA)
The vector sum of the active and reactive power is complex power. Power factor is
the ratio of the active power to the apparent power. The power factor of fully loaded
induction motors ranges from 80 to 90%, depending on the type of motor and the
speed of the motor. Power factor deteriorates as the load on the motor decreases. Other
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