By Norm Christopherson
These motor calculations are contained here as they apply to the changing
conditions of the blower motor when pulley adjustments are made.
However, these calculations are useful wherever electric motors are used.
What is learned here can be used to solve motor installation and
troubleshooting problems in a large variety of applications.
These motor formulas and related information is also basic to
understanding energy consumption, power consumption by motor driven devices and
the motor side of performing energy audits.
Fundamental to these useful calculations is some understanding of the
information contained on a motor data plate.
Every motor has a metal nameplate or data plate listing the manufacturer,
motor type and electrical requirements for the motor.
The voltage listed on the data plate
is the design voltage the motor was made for.
The motor may be operated at any voltage within 10% of the design
voltage.
Full
Load Amps (FLA)
This amperage is the current the
motor will draw when the motor is loaded up to its rated horsepower.
The motor will draw less than the listed FLA if the motor is operating at
less than the rated horsepower. The
motor will draw more than the rated FLA when it attempts to operate at more than
the rated horsepower. Measuring the
actual motor amperage and comparing it to the FLA is a good way to quickly tell
if a motor is overloaded.
Locked
Rotor Amps (LRA)
This amperage is the current the
motor will draw when the motor is started and when the motor is attempting to
start and run but is unable to do so for some reason such as mechanically stuck
bearings. A rotor is said to be
locked anytime the rotor is not rotating. This is normal on a motor that is off and ready to start.
If a motor has bad bearings and cannot start, the proper terminology for
that condition is to say the motor has stuck or frozen bearings.
It would not be correct to say the motor has a locked rotor.
All motors when off have locked rotors.
The LRA rating of a motor is the most amperage the motor can draw under
any condition.
%Efficiency
The rated percent of efficiency for
the motor is listed on most motors but not all.
Motors that fail to state their efficiency probably have poor efficiency
ratings. If the efficiency is not
given it can still be determined using basic math.
This formula is included in this book later in this chapter.
Generally higher efficiency motors are those that use more metal in their
construction and the metal is laminated and insulated between laminations to
reduce eddy currents, which create heat. Heat
in a motor is lost efficiency. The
efficiency rating may be given as a percent such as 86%, or the same percentage
may be listed as a decimal fraction like .86.
When using the percent efficiency in a math formula, it must be used as a
decimal fraction.
The power factor is also given as a
decimal fraction and may be any number less than one.
Common power factor ratings range from .70 to .98.
The higher power factor is always more desirable.
The
power factor is a number, which tells to what extent the motor voltage and
current are out of phase from one another.
Unlike pure resistive circuits like electric heaters and incandescent
lights, motors operate with strong magnetic fields present.
The magnetic fields add a new element of magnetic resistance to the motor
circuit, which throws the voltage and current out of phase from each other.
When the voltage and current are not in phase ohm's law will not work
unless the power factor is used to correct for this phase difference.
The power factor listed on a motor is very useful to the technician when
making motor horsepower and current calculations.
These handy calculations are included in this chapter.
The service factor of a motor is a
number, which indicates how much more work a given motor, can do beyond the
rated horsepower. This is a safety
factor and is not to be considered as a part of the motors normal useful
horsepower. A motor may have no
service factor whatsoever and thus has no safety factor in the event the motor
becomes overloaded. A common
service factor on motors is a SF of 1.15. This
number multiplied times the rated horsepower gives the actual horsepower the
motor could operate at in an emergency. For
example; a 10 HP motor with a SF of 1.15 could actually provide service for a
short time up to 11.5 HP. A motor
with a high service factor is used on applications where the load may vary and
may occasionally be confronted with an unexpected overload in horsepower.
Air conditioning systems often use motors with the SF rating of 1.15.
The
service factor can also be multiplied times the FLA of the motor to give the
absolute highest operating amperage the motor should be allowed to operate with.
This use of the service factor is not recommended, as it is not
completely reliable as it assumes the voltage the motor is getting is exactly
correct. This is also a poor
service practice because it encourages technicians to allow loading motors up
into the safety zone the service factor provides.
Some motors such as air conditioning and refrigeration compressors do not
state their motor horsepower. A
rule of thumb for air conditioning compressor motors is that there is one
horsepower per ton. This rule will
not work for applications other than comfort cooling air conditioning
applications. The number of
horsepower required to provide each ton of cooling varies with the suction
pressure and head pressure. The
higher the head pressure or the lower the suction pressures, the more motor
horsepower required to achieve a ton of cooling.
Another way of stating it is that as the pressure difference between the
suction and head pressures increase, the system tonnage decreases.
This is why it is so important to keep condensers clean, evaporator air
filters replaced and airflow on both condensers and evaporators up to normal.
Anything, which increases head pressure or decreases suction pressure
will decrease system capacity and at the same time increase operating costs.
The
ability to determine actual motor operating horsepower on any motor, compressor
motor, evaporator blower motor or tower motor, is useful in troubleshooting
problems or performing energy audits. Understanding
the elements and process of basic motor horsepower calculations also increases
awareness of energy consumption, motor operation and potential problem areas.
The formula given above is used to determine the actual operating
horsepower of any single-phase motor. If
a motor such as a compressor does not state the motor horsepower, then this
formula will also work to determine the rated or actual operating horsepower.
If the motor name/data plate information such as the rated voltage, rated
fla, rated efficiency and listed power factor are placed in the formula, the
resulting answer will be the rated horsepower.
Of course there is not reason to use the formula in that way unless the
motor is a compressor and does not state the motor horsepower.
The common use for this calculation is to determine the actual operating
horsepower for a motor as it is presently being used.
If a blower motor for example, is going to get a drive pulley change this
formula will give the motors actual operating horsepower prior to the change so
the technician can tell if the motor has enough horsepower capacity available or
if the motor must be changed. If
the motor must be changed this calculation also gives added information to help
determine what new size motor will be necessary when the pulley is changed.
Example:
A single-phase blower motor has the following data plate information.
5 HP
230 Volts
FLA 20.72
%Eff .86
PF .91
SF 1.15
A technician measures the actual running amperage and voltage and finds
the motor is getting 230 volts and is drawing 16 amps.
The amperage is less than full load so the motor is not working at the 5
HP it is rated for. But, what
amount of work is it actually doing? Inserting
actual measured values of voltage and amperage in the formula gives the answer.
The calculation shows the motor to be operating about one horsepower
under the 5HP rating.
A careful look at the single-phase horsepower formula reveals some useful
information. The power law states
that the voltage (E) times the amperage (I) gives the wattage or power
consumption of a circuit. Therefore
the horsepower formula includes both voltage and amperage.
There are 746 watts in one horsepower, so every time a motor develops or
uses 746 watts it has done one horsepower of work.
Both wattage and horsepower are two ways of stating the same thing.
This is why the 746 watts per horsepower is divided into the watts on the
top of the formula.
But,
notice that the voltage times the amperage in the top of the formula not only
gives the wattage but the wattage is then multiplied times the % efficiency and
the power factor. The wattage
gotten by multiplying the motor voltage times the motor amperage assumes that
the motor is 100% efficient. Nothing
is 100% efficient and motors are no exception.
The efficiency of this motor is 86 % so; the wattage is reduced to 86 %
of the calculated wattage by multiplying by .86.
The lost efficiency was lost in the form of heat and never produced the
work in the form of rotational force which motors are made to do.
Obviously
the higher the efficiency the better and also the higher the initial cost to
purchase. However, the energy
savings from the higher efficiency may offset the higher initial cost.
Remember, the power the power bill reflects is all the power the motor
uses, not just what the motor converted to useful rotational work but even the
power lost to heat. A higher
efficiency motor wastes less power to heat loss and may run less to get the same
amount of work accomplished than a lower efficiency motor. Generally, it pays to purchase higher efficiency motors to
begin with unless the motor is a small one.
Small motors usually do not consume enough power to pay back the cost of
the higher efficiency.
After correcting the motor wattage for the efficiency of the motor, the
result is then corrected for the power factor of the motor by multiplying by the
power factor rating listed on the motor data plate.
The power factor was defined earlier in the chapter as the degree that
the voltage and current are out of phase from one another.
A power factor less than one indicates that the two are not perfectly in
phase so the actual power the motor is working at is not what it could be if the
voltage and amperage were perfectly in phase.
Knowing and using the power factor and efficiency in motor calculations
is vitally important if solid useful numbers are to be gotten.
The calculation shows the motor to be operating about one horsepower
under the 5HP rating.
A careful look at the single-phase horsepower formula reveals some useful
information. The power law states
that the voltage (E) times the amperage (I) gives the wattage or power
consumption of a circuit. Therefore
the horsepower formula includes both voltage and amperage.
There are 746 watts in one horsepower, so every time a motor develops or
uses 746 watts it has done one horsepower of work.
Both wattage and horsepower are two ways of stating the same thing.
This is why the 746 watts per horsepower is divided into the watts on the
top of the formula.
But,
notice that the voltage times the amperage in the top of the formula not only
gives the wattage but the wattage is then multiplied times the % efficiency and
the power factor. The wattage
gotten by multiplying the motor voltage times the motor amperage assumes that
the motor is 100% efficient. Nothing
is 100% efficient and motors are no exception.
The efficiency of this motor is 86 % so; the wattage is reduced to 86 %
of the calculated wattage by multiplying by .86.
The lost efficiency was lost in the form of heat and never produced the
work in the form of rotational force which motors are made to do.
Obviously
the higher the efficiency the better and also the higher the initial cost to
purchase. However, the energy
savings from the higher efficiency may offset the higher initial cost.
Remember, the power the power bill reflects is all the power the motor
uses, not just what the motor converted to useful rotational work but even the
power lost to heat. A higher
efficiency motor wastes less power to heat loss and may run less to get the same
amount of work accomplished than a lower efficiency motor. Generally, it pays to purchase higher efficiency motors to
begin with unless the motor is a small one.
Small motors usually do not consume enough power to pay back the cost of
the higher efficiency.
After
correcting the motor wattage for the efficiency of the motor, the result is then
corrected for the power factor of the motor by multiplying by the power factor
rating listed on the motor data plate. The
power factor was defined earlier in the chapter as the degree that the voltage
and current are out of phase from one another.
A power factor less than one indicates that the two are not perfectly in
phase so the actual power the motor is working at is not what it could be if the
voltage and amperage were perfectly in phase.
Knowing and using the power factor and efficiency in motor calculations
is vitally important if solid useful numbers are desired.
Three phase motors use the same calculation as was used on single-phase
motors with one addition to the formula. Three
phase motors have three separate voltages each 120 degrees out of phase from on
another. This is what gives the
three phase motor its superior starting and running power and eliminates the
need for start capacitors and start relays to remove a starting winding as is
often necessary on single phase motors.
The three-phase motor is 73% more powerful than an equivalent motor using
single phase. The number 1.73 is
added to the wattage side of the calculation to reflect this increase for 3
phases.
Here is the three-phase formula for determining motor horsepower
including the addition of the 1.73 in its proper place.
The 1.73 is the square root of the number 3 for the 3 phases.
In
order to determine missing values of efficiency and/or power factor the formula
is the same as for single phase except the 1.73 is included in the calculation.
Use this formula just like the single-phase version.
The technician who works with blowers and blower motors and makes
adjustments to correct airflow quantities must understand the relationships
between the various elements involved. A
change in any one of the operating conditions will have an important effect on
several others. Making adjustments
without considering the effects of the adjustments may cause a blower motor
burnout or may result in an inefficient system operation.
The purpose of making adjustments in the first place is to increase
efficiency and provide for increased comfort at lowered operating costs as well
as to avoid causing expensive system failures.
Relationship
of Rpm to Cfm.
To increase cfm the blower rpm must be increased.
This is a directly proportional change.
The percent of cfm change is proportional to the percent of rpm change.
Since the rpm and cfm are directly proportional, the two are interchangeable in
formulas. Where one is used the
other may be substituted.
Relationship
of Rpm and Cfm to the Motor Pulley Size.
To increase the rpm and cfm of the blower the motor pulley size must be
increased. This relationship is
easy to remember as the motor pulley is always smaller than the blower pulley
and if the motor pulley were made the same size as the larger blower pulley they
would both go the same speed, which is the speed of the motor.
Except for direct drive, no blower operates at the same speed as the
motor.
The relationship of the rpm, cfm, and motor pulley is a directly
proportional one. The rpm and cfm
change by the same percentage change of the motor pulley change.
This paper was taken from a larger work by the same author. The full text of the larger work goes into greater depth, detail and provides additional material.
Norm is a technical writer, seminar speaker and test proctor for EPA, 410A and ESCO & NATE certifications.
He can be contacted at nchristo@juno.com