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ELECTRIC MOTOR SYSTEMS
In industrial and commercial facilities,
motor driven systems are responsible for as much as 70 percent of
a building's electricity consumption. For some industries,
such as pulp and paper or textiles, motor system electricity consumption
can reach 90 percent of the industry's total use.
Motors can consume excess amounts of energy if
they are improperly mounted, if they are not connected to their
load, or in the case of three phase motors, if the voltages
of the opposing leads are different. Manufacturers of three phase
motors recommend that they should not be operated when this imbalance
(of the opposing leads) is greater than 1%. Bearing wear can
also contribute a motor's reduction in efficiency.
Energy efficient motor systems can decrease energy
costs while improving operating costs and reducing equipment maintenance.
The following recommendations provide an insight to the potential
energy and monetary savings that can be achieved.
General Rules of Thumb:
- The average cost of electricity is $0.05/kWh
($15/MMBtu)
- There are 2000 hours per year per shift
(based on the assumption that one shift is 8 hours per day,
5 days per week, 50 weeks per year)
- Upgrading to an energy efficient motor can
result in savings of about 5% over the operating costs of a
standard motor.
- A typical standard motor has an efficiency
of 90 %.(See table below for horsepower ratings and nominal
efficiencies)
- The cost of operating a typical heavy industrial
motor is: $62/hp/shift/year
Notes:
Before choosing the following targeted recommendations READ THE
FOLLOWING:
Pay back estimates for the following recommendations will use the
equation below. They will vary depending on the, application,
type of installation, and purchase quantity of material and labor
associated with each recommendation. It will be up to the
person doing the analyses to use the URL references below each
equation to help estimate an implementation cost.
The data correlating to
the variables below each equation will be prompted for in order
to execute a calculation. Frequently the fuel cost (FC)
associated with the specific recommendation will be prompted for
in order to calculate the annual cost savings (ACS). Unless otherwise
specific to a particular recommendation the ACS will be calculated
as follows:
HORSEPOWER vs. EFFICIENCY
| ------------------ |
--------------Base |
Speed------------- |
--------------------- |
| HP |
3600 RPM |
1800 RPM |
1200 RPM |
| 1 |
75.5 |
82.5 |
80.0 |
| 1.5 |
82.5 |
84.0 |
85.5 |
| 2 |
84.0 |
84.0 |
86.5 |
| 3 |
85.5 |
87.5 |
87.5 |
| 5 |
87.5 |
87.5 |
87.5 |
| 7.5 |
88.5 |
89.5 |
89.5 |
| 10 |
89.5 |
89.5 |
89.5 |
| 15 |
90.2 |
91.0 |
90.2 |
| 20 |
90.2 |
91.0 |
90.2 |
| 25 |
91.0 |
92.4 |
91.7 |
| 30 |
91.0 |
92.4 |
91.7 |
| 40 |
91.7 |
93.0 |
93.0 |
| 50 |
92.4 |
93.0 |
93.0 |
| 60 |
93.0 |
93.6 |
93.6 |
| 75 |
93.0 |
94.1 |
93.6 |
| 100 |
93.6 |
94.5 |
94.1 |
| 125 |
94.5 |
94.5 |
94.1 |
| 150 |
94.5 |
95.0 |
95.0 |
| 200 |
95.0 |
95.0 |
95.0 |
| AVE. 36 HP |
89.6 |
91.0 |
90.0 |
- Optimize motor
size with load; size motors for peak operating efficiency.
- Use multiple speed
motors or variable speed drives for variable pump, blower and
compressor loads.
- Replace existing
motors with energy efficient motors.
- Improve lubrication
practices for motor driven equipment.
- Install Energy
Efficient V-Belts on motor driven equipment.
1. Optimize motor size with load; size
motors for peak operating efficiency.
All motors run more efficiently at full load than at part load;
most motors operate near peak efficiency from 75% to 110% of their
rated load. To obtain optimal power use motors should
be operated within this range. The following equation illustrates
the savings obtained when a motor or motor driven equipment is retrofitted
for peak efficiency.
Power
Engineering Books
Thomas Register
HP = average horse power of motor(s)
CON = conversion factor, 0.7465 KW/hp
LF = load factor (i.e. the approximate percentage of time
the motor is fully loaded)
HY = operating hours per year
hc = efficiency of current motor*
hp = efficiency of proposed motor*
*Note, when considering multiple motors,
the efficiencies should be the same to obtain an accurate estimation
of the savings that can be achieved
Motor efficiency table
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2. Use multiple speed motors or variable
speed drives (VSD) for variable pump, blower and compressor loads.
A variable or adjustable speed drive (ASD or VSD) will reduce the
speed of a motor by adjusting the frequency, voltage, or current
of the motor input so that the motor performance just matches the
present load. The speed of AC motors is proportional to the
frequency of the power supply. AC adjustable drives convert
3 phase 60 Hz to an adjustable frequency and voltage source for
controlling the speed of AC squirrel cage induction motors.
Moreover, VSD's or ASD's control motor speed by synthesizing the
voltage and frequency of power supplied to the motor so that it
runs only as fast as necessary to do the work required at a given
time during operation. VSD's or ASD's can control speed over
a wide range, from 0 to 300% of the rated speed. There are
four basic types: 1. Inverter based, 2.
Cyclo-inverters,3. Wound-rotor slip recovery,
and 4. Voltage-level controls. ASD's and
VSD's can provide accurate process control, and match the speed
of a motor driven device to varying load requirements.
The following equation (for one motor at a time) can be used to
determine the savings that can be achieved when a VSD or ASD is
implemented.
Power
Engineering Books
NEMA
Thomas Register
HP1 = current motor horsepower (if multiple
motors are considered, the efficiencies and loads must be the
same, otherwise the analysis must be done on an individual basis)
HP2 = anticipated effective motor horsepower
CON = conversion factor, 0.7465 KW/hp
HY = operating hours per year
LF = load factor of equipment using motor
h = motor efficiency
Motor efficiency table
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For machines which have a
free discharge the following relation should be used:
Q1 is current volume of flow, CFM
Q2 is anticipated volume of flow, CFM
NOTE: Both Q1 and Q2
must have the same units
If a static head is present, precise knowledge
of the pump and the system curves is required to calculate the
reduced horsepower.
Motor efficiency table
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3. Replace existing motors with energy
efficient motors.
Motor
losses occur from five major areas: core losses, stator
losses, rotor losses, stray load losses, winding losses, and friction.
High efficiency motors are designed to reduce these losses by
about 2 to 10 percent. For the motor size range of (7.5 to 125
hp), the losses are likely to be in the range of about of 2 to
7 percent. In addition to having lower losses, high efficiency
motors also have higher power factors during operation. Cost premiums
for high efficiency motors range from 10 to 25 percent, since
an average motor uses 75 times its initial cost in electrical
energy over its lifetime, there is great savings potential which
is illustrated in the following equation.
Power
Engineering Books
NEMA
Thomas Register
Note: the above equation is used for
an individual motor.
HP = required motor horsepower
LF = load factor (i.e. the approximate fraction of time
the motor is fully loaded)
CON = conversion factor, 0.7465 KW/hp
HY = operating hours per year
h1 = current motor efficiency
h2 = anticipated motor efficiency
Motor efficiency table
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4. Improve lubrication practices for
motor driven equipment.
Replace oil in
gear motors with synthetic oil to reduce friction losses. One
manufacturer of synthetic oil claims a 5-10% decrease in average
power consumption of gear motors when their product is used;
this is due to the high viscosity of the oil, resulting in less
power loss in the gears. The following equation shows the
savings potential that can be obtained.
Power
Engineering Books
NEMA
Thomas Register
HP = horse power of motor driven equipment
(for individual motors systems or multiple systems with identical
efficiencies and loads)
PS = fractional savings due to synthetic oil (enter as
a decimal value)
LF = load factor (i.e. the approximate fraction of time
the motor is fully loaded)
CON = conversion factor, 0.7465 KW/hp
HY = operating hours per year
h = efficiency of motor(s)
Motor efficiency table
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5. Install Energy Efficient V-Belts
Replace old
power transmission belts with energy efficient ones to reduce power
transmission losses. The cogged V-belt is a direct replacement
for a conventional V-belt. Cogged V-belts depend on friction
to transmit power and thus use the same pulleys as the standard
V. They have slots perpendicular to the belt length that reduce
the belt's bending resistance, cogged belts bend more easily and
exhibit reduced bending losses. Typically, cogged belt efficiency
is about 2% higher than that of standard belts. Cogged belts
also run cooler because of their lower energy absorption, and last
20-30 percent longer than standard V-belts. The following
equation shows the savings that can be obtained with a V-belt retrofit.
Power
Engineering Books
NEMA
Thomas Register
HP = total belt driven horsepower (for
individual motors systems or multiple systems with identical efficiencies
and loads)
PS = power savings (manufacturer estimate 0.02) due to
installation of cogged V-belts
LF = load factor (i.e. the average fraction of time the
motor is fully loaded)
CON = conversion factor, 0.7465 KW/hp
HY = operating hours per year
h = efficiency
Motor efficiency table
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