INTRODUCTION
There
seems to be a great deal of confusion among the users of electric
motors regarding the relative importance of power factor, efficiency
and amperage, as related to operating cost. The following information
should help to put these terms into proper perspective.
At
the risk of treating these items in reverse order, it might be helpful
to understand that in an electric bill, commercial, industrial or
residential, the basic unit of measurement is the kilowatt hour. This
is a measure of the amount of energy that is delivered. In many
respects, the kilowatt hour could be compared to a ton of coal, a cubic
foot of natural gas, or a gallon of gasoline, in that it is a basic
energy unit. The kilowatt hour is not directly related to amperes, and
at no place on an electric bill will you find any reference to the
amperes that have been utilized. It is vitally important to note this
distinction. You are billed for kilowatt hours: you do not necessarily
pay for amperes.
POWER FACTOR
Perhaps
the greatest confusion arises due to the fact that early in our science
educations, we were told that the formula for watts was amps times
volts. This formula, watts = amps x volts, is perfectly true for direct
current circuits. It also works on some AC loads such as incandescent
light bulbs, quartz heaters, electric range heating elements, and other
equipment of this general nature. However, when the loads involve a
characteristic called inductance, the formula has to be altered to
include a new term called power factor. Thus, the new formula for
single phase loads becomes, watts are equal to amps x volts x power
factor. The new term, power factor, is always involved in applications
where AC power is used and inductive magnetic elements exist in the
circuit. Inductive elements are magnetic devices such as solenoid
coils, motor windings, transformer windings, fluorescent lamp ballasts,
and similar equipment that have magnetic components as part of their
design.
Looking
at the electrical flow into this type of device, we would find that
there are, in essence, two components. One portion is absorbed and
utilized to do useful work. This portion is called the real power. The
second portion is literally borrowed from the power company and used to
magnetize the magnetic portion of the circuit. Due to the reversing
nature of AC power, this borrowed power is subsequently returned to the
power system when the AC cycle reverses. This borrowing and returning
occurs on a continuous basis. Power factor then becomes a measurement
of the amount of real power that is used, divided by the total amount
of power, both borrowed and used. Values for power factor will range
from zero to 1.0. If all the power is borrowed and returned with none
being used, the power factor would be zero. If on the other hand, all
of the power drawn from the power line is utilized and none is
returned, the power factor becomes 1.0. In the case of electric heating
elements, incandescent light bulbs, etc., the power factor is 1.0. In
the case of electric motors, the power factor is variable and changes
with the amount of load that is applied to the motor. Thus, a motor
running on a work bench, with no load applied to the shaft, will have a
low power factor (perhaps .1 or 10%), and a motor running at full load,
connected to a pump or a fan might have a relatively high power factor
(perhaps .88 or 88%). Between the no load point and the full load
point, the power factor increases steadily with the horsepower loading
that is applied to the motor. These trends can be seen on the typical
motor performance data plots which are shown in figure 1.
EFFICIENCY
Now,
let’s consider one of the most critical elements involved in motor
operating cost. This is efficiency. Efficiency is the measure of how
well the electric motor converts the power that is purchased into
useful work. For example, an electric heater such as the element in an
electric stove, converts 100% of the power delivered into heat. In
other devices such as motors, not all of the purchased energy is
converted into usable energy. A certain portion is lost and is not
recoverable because it is expended in the losses associated with
operating the device. In an electric motor, these typical losses are
the copper losses, the iron losses, and the so-called friction and
windage losses associated with spinning the rotor and the bearings and
moving the cooling air through the motor.
In
an energy efficient motor, the losses are reduced by using designs that
employ better grades of material, more material and better designs, to
minimize the various items that contribute to the losses in the motor.
For
example, on a 10 HP motor, a Super E energy efficient design might have
a full load efficiency of 91.7%, meaning that, at full load (10 HP), it
converts 91.7% of the energy it receives into useful work. A less
efficient motor might have an efficiency of 82%, which would indicate
that it only converts 82% of the power into useful work.
In general, the efficiency of motors will be relatively constant from 50% to 100% of rated load. |
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AMPERES
Now,
let’s discuss amperes. Amperes are an indication of the flow of
electric current into the motor. This flow includes both the borrowed
as well as the used power. At low load levels, the borrowed power is a
high percentage of the total power. As the load increases on the motor,
the borrowed power becomes less and less of a factor and the used power
becomes greater. Thus, there is an increase in the power factor as the
load on the motor increases. As the load continues to increase beyond
50% of the rating of the motor, the amperage starts to increase in a
nearly straight line relationship. This can be seen in figure 1.
SUMMARY
Figure
1 shows significant items that have been discussed as plots of
efficiency, power factor and watts, as they relate to horsepower. The
most significant factor of all these is the watts requirement of the
motor for the various load levels because it is the watts that will
determine the operating cost of the motor, not the amperage.
The
customer that has an extremely low power factor in the total plant
electrical system, may be penalized by his utility company because he
is effectively borrowing a great deal of power without paying for it.
When this type of charge is levied on the customer, it is generally
called a power factor penalty. In general, power factor penalties are
levied only on large industrial customers and rarely on smaller
customers regardless of their power factor. In addition, there are a
great many types of power customers such as commercial establishments,
hospitals, and some industrial plants that inherently run at very high
power factors. Thus, the power factor of individual small motors that
are added to the system, will not have any significant effect on the
total plant power factor.
It
is for this reasons that the blanket statement can be made, that
increasing motor efficiency will reduce the kilowatt hour consumption
and the power cost for all classes of power users, regardless of their
particular rate structure or power factor situation. This same type of
statement cannot be made relative to power factor.
The
following basic equations are useful in understanding and calculating
the factors that determine the operating costs of motors and other
types of electrical equipment.
OPERATING COST CALCULATIONS
MOTORS
Kilowatt Hours = (HP** x .746 x Hours of Operation)/Motor Efficiency
** Average Load HP (May be lower than Motor Nameplate HP)
General Formula All Loads
Kilowatt Hours = (Watts x Hours of Operation)/1000
Approximate Operating Cost* = Kilowatt hours x Average Cost per Kilowatt Hour
* Does not include power factor penalty or demand charges which may be applicable in some areas.
Useful Constants
Average Hours per Month = 730
Average Hours per Year = 8760
Average Hours of Darkness per Year = 4000
Approximate Average Hours per Month(Single Shift Operation) = 200 |