Integral AC Motor Selection and Application Guide for Fans - FE-800

Integral AC Motor Selection and Application Guide for Fans - FE-800
FAN ENGINEERING
Information and Recommendations for the Engineer
FE-800
Integral AC Motor Selection
and Application Guide For Fans
Introduction
This discussion will focus on 3-phase asynchronous
induction motor selection. Because of the simplicity of
operation, ruggedness of construction and low maintenance, these are the most commonly used motors for
industrial fan applications having power requirements of
one horsepower or greater.
AC Motor Construction
Shown in Figure 1 is a cutaway view of a typical AC
induction motor with the components labeled and the
overall assembly indicating the simplicity of construction.
Note that there are only two wearing parts – the two
bearings. There are no other sliding surfaces such as
commutators, brushes or collector rings.
Figure 1. Cutaway View of a Typical AC Induction Motor
includes the rotor conductors, end rings and blower fins.
A machined shaft is pressed into the rotor assembly,
placed into a lathe and the shaft ends, and rotor diameter machined to their final dimension. Finally the complete assembly is balanced and bearings are pressed on
each end of the shaft.
Frame and Assembly
The stator assembly is pressed into a steel or cast iron
frame, the rotor assembly is placed inside the stator and
the end brackets added. The motor is completed with
the addition of the conduit box, painting and nameplate
installation.
Fundamentals
There is nothing mysterious about the 3-phase, asynchronous induction motor. All that is required to understand it is common sense and some knowledge of the
fundamentals.
NEMA Standards
All manufacturers of integral electric motors support and
comply with NEMA (National Electrical Manufacturers
Association). NEMA’s standards govern certain important
items common to all motors such as:
● HP (horsepower ratings)
● RPM (speed)
● Frame sizes and dimensions
● Standard voltages and frequencies
● Service factors
● Torques
● Starting current (amps) and KVA
● Enclosures
● Balance level
There is a broad range of types and sizes of motors
manufactured and naturally not all are manufactured the
same way, but they all incorporate the following similar
basic components.
Stator Assembly
Stator laminations are punched out of magnetic steel
and stacked to a predetermined depth, compressed and
welded together, or bound together by a series of locking bars. The stator windings and insulation components
are then placed in the stator core, secured and the
complete stator assembly is thoroughly impregnated with
multiple dips of varnish.
Rotor Assembly
A series of rotor laminations (usually made from the
center of the stator laminations) is placed in a diecasting machine where molten aluminum is forced into
the die. The resulting homogeneous assembly now
What this provides is the assurance that a motor purchased from any manufacturer will comply with the user’s
requirements without modification. One word of caution,
however: NEMA does not control the motor stack length
(overall axial length from end of shaft to end of motor).
Although most motor manufacturers’ overall lengths are
reasonably close there is the reality that in some cases
this length can vary as much as 2 to 3 inches.
Where space is critical, particularly in replacement
situations, procuring a specific manufacturer’s motor may
be required.
Voltage and Frequency
The nominal 3-phase, 60 cycle (Hz) power system voltages commonly available in the United States for industrial plants are 208, 240, 480 and 600 volts. The utilization (motor nameplate) voltage is set at a slightly lower
level to allow for voltage drop in the system between
the transformer and motor leads. Typically the motors
are nameplated for 200, 230, 460 and 575 volts.
©1999 Twin City Fan Companies, Ltd.
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Motor speed = 5/6 nameplated rated speed
Service factor = 1.0
+10
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Allowable voltage variation at derated HP = ±5%
Select motor overload protection for 60 Hz amps and 1.0 service factor.
PERCENT CHANGES IN MOTOR PERFORMANCE
Rated HP at 50 Hz = Nameplate HP x Derate Factor
P
0.75
SL I
0.80
PS
0.85
AM
50 Hz
OPTIONAL VOLTAGE RATINGS (±5%)
190
200
210
380
400
420
440
500
525
+15
AD
60 Hz
VOLTAGE
230
460
575
DERATE
FACTOR
+20
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Figure 2. Percent Voltage Variation
LL
Table 1. Derating Values for 60 Hz Motors
4.At full load an increase of 10% in voltage will result
in a 17% reduction of slip. A decrease of 10% in
voltage will increase slip approximately 23%.
FU
While rarely required in the United States and Canada,
50 Hz is the prevailing frequency in Europe and Asia.
The nominal 3-phase, 50 Hz power system voltages vary
from country to country. Therefore, motor voltage should
be selected for the country in which it will operate. The
preferred 50 Hz motor voltages are 190, 380 and 440
volts and they cover most nominal system voltages.
Although motors built for 50 Hz are becoming more
readily available in the U.S., consideration should be
given to the accepted practice of derating 60 Hz motors.
A 60 Hz motor may be successfully operated at 50 Hz
at reduced horsepower and voltage as shown in the
following table.
–20
These motors should be ordered as 60 Hz motors
with no reference to 50 Hz.
NEMA standards state that motors must be able to
carry their rated horsepower at nameplate voltage ±10%
although not necessarily at rated temperature. Occasionally
we see a motor rated at 208-230/460 volts. Using this
motor on a 208 volt system means that the network
must have very good regulation. For instance, a 230 volt
motor applied at 208 volts (90% of rated) loses any
service factor indicated on the nameplate and could run
hotter than at rated voltage. A motor used on a 208
volt system should be ordered as a 200 volt motor with
winding and nameplate so designed and stamped.
The following conditions may occur with variations in
frequency (see Figure 3).
Voltage and Frequency Variation
2.Conversely, a decrease in frequency will usually lower
the power factor and speed while increasing lockedrotor maximum torque and current.
+5
+10
+15
1.Frequency greater than rated frequency normally
improves the power factor but decreases lockedmotor and maximum torque. This condition also
increases speed and therefore friction and windage
losses.
Figure 3. Frequency Variation
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PERCENT CHANGE IN MOTOR PERFORMANCE
+8
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3.Locked-rotor and breakdown torque will be proportional to the square of the voltage. Therefore, a
decrease in voltage will result in a decrease in available torque.
0
NG
2.An increase in voltage will usually result in a noticeable decrease in power factor. Conversely, a decrease
in voltage will result in an increase in power factor.
–5
TI
1.An increase or decrease in voltage may result in
increased heating at rated horsepower load. Under
extended operation this may accelerate insulation
deterioration and shorten motor insulation life.
–10
PERCENT VOLTAGE VARIATION
AR
ST
Motors are designed to operate successfully with limited
voltage and frequency variations. Voltage variation with
rated frequency must be limited to ±10% and frequency
variations with rated voltage must be limited to ±5%.
The combined variation of voltage and frequency must
be limited to the arithmetic sum of 10%. Variations are
expressed as deviation from motor nameplate values,
not necessarily system nominal values. The allowable
±10% voltage variation is based on assumptions that
horsepower will not exceed nameplate rating and that
motor temperature may increase.
The following conditions may occur with variations in
voltage (see Figure 2).
–15
–8
–10
–5
–4
–3
–2
–1
0
+1
+2
+3
+4
+5
PERCENT CHANGE IN FREQUENCY
Page 2
Engineering Data 800
Power Factor
Power factor is an important consideration when selecting a motor for a particular application since low power
factor may result in power factor penalty charges from
the utility company. Since the power company must
supply KVA but normally meters kilowatts used, low
motor power factors require additional KVA with low
return on kW utilized; hence, power factor penalties.
Unbalanced Voltage
AC 3-phase motors will operate successfully under running conditions at rated load when the voltage unbalance at the motor terminals does not exceed 1%.
Performance will not necessarily be the same as when
the motor is operating with a balanced voltage at the
motor terminals.
When the line voltages applied to a 3-phase induction
motor are not equal, unbalanced currents in the stator
windings will result. A small percentage voltage unbalance will result in a much larger percentage current
unbalance. Consequently, the temperature rise of the
motor operating at a particular load and percentage voltage unbalance will be greater than for the motor operating under the same conditions with balanced voltages.
Should voltages be unbalanced, the rated horsepower of the motor should be multiplied by the factor
shown in Figure 4 to reduce the possibility of damage
to the motor. Operation of the motor above a 5% voltage unbalance condition is not recommended.
Figure 4. Integral HP Motor Derate Factor
DERATING FACTOR
Efficiency — A marked reduction of motor efficiency will
exist because of increased current.
Speed and Slip
The speed of a 3-phase induction motor depends on
the frequency and number of poles for which the motor
is wound. The higher the frequency, the faster the
speed. The more poles the motor has, the slower the
speed. The smallest number of poles ever used is two.
A two-pole, 60 cycle motor will run at approximately
3600 RPM (unloaded).
To find the approximate speed of any induction motor
we can use the formula for synchronous speed which
is the speed of the rotating magnetic field:
Synchronous Speed (Ns) = (60 x 2f) ÷ p
Where f = frequency of the power supply in Hz
p =# of poles for which the machine is wound
Three-phase induction motors are wound for the following synchronous speeds:
Table 2. Induction Motor Speed
NO. OF
60 HZ
50 HZ
POLES
SYNC. SPEED
SYNC. SPEED
2
3600
3000
4
1800
1500
6
1200
1000
8 900 750
10 720 600
12 600 500
Almost all standard commercial motors (143T to 445T
frame sizes) are wound with a maximum of 8 poles.
The actual full load speed of an induction motor is
somewhat less than its synchronous speed. This difference between synchronous speed and full load speed is
called “slip.” Percent slip is defined as follows:
1.0
0.9
Percent Slip =
0.8
Sync. Speed – Nameplate Speed
x 100
Sync. Speed
A typical 5 HP, 4-pole motor has a full load speed
of 1750 RPM. Therefore it would have a slip of
0.7
0
1
2
3
4
PERCENT VOLTAGE UNBALANCE
5
Percent Unbalance =
100 x Max. Voltage Deviation from Avg. Voltage
Avg. Voltage
Example: With voltages of 220, 215 and 210, the average
is 215 and the maximum deviation from average is 5.
Therefore, percent unbalance = (100 x 5) ÷ 215 = 2.3%
From Figure 4 the motor derating factor is 0.93.
Consequently, if we have a 20 HP, 3-phase motor its
output should be derated to approximately 18.6 HP to
reduce the possibility of damage to the motor.
Unbalanced voltage will produce the following effects
on performance characteristics:
Torques — Unbalanced voltage results in reduced
locked-rotor and breakdown torques for the application.
Full Load Speed — Unbalanced voltage results in a
slight reduction of full load speed.
Current — Locked-rotor current will be unbalanced to
the same degree that voltages are unbalanced but
locked rotor KVA will increase only slightly. Full load
current will be unbalanced in the order of six to ten
times the voltage unbalance.
Page 3
Temperature Rise — A 3.5% voltage unbalance will
cause approximately 25% increase in temperature rise.
1800 – 1750
x 100 = 2.8%
1800
The exact slip percentage varies from one motor size
and type to another. Slip is also somewhat dependent
upon load. A partially loaded motor will run slightly
faster than a fully loaded motor. For convenience the
following nominal speed table has been established to
be used to determine fan performance.
Table 3. Nominal Speeds for 60 Hz Motors
NO. OF SYNCHRONOUS
POLES
SPEED (RPM)
2 POLE:
3600
THRU 1 HP
1
1 ⁄2 THRU 25 HP
30 HP & UP
4-POLE:
1800
THRU 3/4 HP
1 THRU 20 HP
25 HP & UP
6-POLE:
1200
THRU 3 HP
5 HP & UP
8-POLE: 900
THRU 1/8 HP
1/2 HP & UP
NOMINAL
SPEED (RPM)
3450
3500
3550
1725
1750
1770
1150
1175
850
875
NOTE: 50 Hz motor speeds can be determined by multiplying the above
ratings by 0.833 (50/60).
Engineering Data 800
Multi-Speed Motors
Special multi-speed motors are available in two, three or
four speeds. Industrial applications usually only deal with
the more common two-speed motors.
Two-Speed, Single Winding Motor — These motors are
called “consequent pole” motors. The low speed on a
single winding motor is always one-half of the higher
speed. If requirements dictate speeds of any other ratio,
a two-winding motor must be used.
Two-Speed, Two Winding Motor — These motors must
be used when the desired speeds are not in the ratio
of 2:1. A separate winding is installed in the motor for
each desired speed (e.g., 1750/1160 RPM). Speeds with
a 2:1 ratio can be delivered by two-winding motors as
well as by single-winding motors.
The choice between one- and two-winding motors is
affected by the speeds desired, the motor price, the
control price, wiring complexity and physical size. Onewinding motors have lower prices than two-winding
motors but usually require a higher price control.
Inverter Duty Motor — AC induction motors powered by
adjustable-frequency drives are an efficient way to vary
fan speed. While almost any induction motor will operate
with adjustable-frequency drives, it’s a good idea to
specify that the motor selected be suitable for use with
VFDs. This will help minimize future motor problems. A
baseline specification for inverter duty motors is NEMA
MG1, Part 31 for insulation capability. However, it’s best
to select a motor that exceeds this specification because
the minimum 1600V peak voltage limitation covered in
MG1, Part 31 is often exceeded in real life.
Briefly, the most common speed controls use PWM
(pulse width modulated) inverter drives. High carrier frequency (fast switching) is preferred because it generates
more pulses resulting in a smoother sine wave which
more closely simulates line power. Additionally, motors
connected to AC drives produce an audible noise at a
tone close to the carrier frequency. Frequencies above
3 kHz make this noise less discernible to humans and
above 10 kHz it can’t be heard at all. The downside is
that the high carrier frequencies tend to result in a
buildup of voltage which far exceeds the motor nominal
operating voltage. These voltage spikes not only break
down the motor insulation, they also create a corona
discharge across any small voids in the system thus
generating ozone which is an aggressive oxidizing agent
that causes further degradation of the insulation.
These higher voltage spikes also cause the stator to
induce an electrical charge to the rotor. This electrical
charge passes to the motor shaft and into the support
bearings. At this point it discharges across the greasefilled gap between the balls and outer race of the bearing, often producing pitting and fluting in the race.
Inverters For Underspeed Considerations — Typically
inverters are used on direct drive fan applications and
are sized for the full load (HP) condition. Where a
higher speed motor, when compared to fan running
speed, is specified care must be exercised in selecting
the correct size motor and drive.
For example, a direct driven fan selected for 600
RPM requires 12.8 BHP to meet a specified condition
of flow and pressure. An 1160 RPM motor has been
specified for this application to be used in conjunction
with a customer supplied inverter drive. AC motors running on an AC line operate with a constant flux (ɸ)
because the voltage/frequency ratio is constant. Motors
operated with constant flux are said to have constant
torque. An AC drive is capable of operating a motor
Page 4
with constant flux from approximately 0 Hz to the motor
nameplate frequency (typically 60 Hz). This is the constant torque range. Therefore, because HP = (Torque x
RPM) ÷ 5250, holding torque constant, then HP varies
directly as the speed. So from our example:
600 x HP = 12.8
1160
1160
or the required motor HP = 12.8 x
= 24.75
600
A 25 HP, 1160 RPM motor is required.
This example refers to direct drive applications only.
For a V-belt driven fan, a 15 HP motor (1800 RPM or
1200 RPM) will suffice.
Inverters For Overspeed Considerations — Some applications require the motor to operate above its full load
speed. An adjustable speed drive can operate at frequencies well over 60 Hz, permitting the higher RPMs.
However, there is some risk involved. First of all, excessive speeds can damage the rotor, fan or bearings and
the bigger the motor the greater the risk. Second, consideration must be given in sizing the motor.
Unfortunately, voltage cannot be higher than motor
nameplate voltage. Therefore, if the voltage remains
constant flux will decrease because the voltage/frequency ratio decreases. This then will result in a decrease in
torque. Above 60 Hz the motor horsepower remains
constant as speed increases and torque decreases in
proportion. So above 60 Hz:
HP (constant) = Torque (decrease) x RPM (increase)
5250
The motor must be sized for the anticipated speed
increase. For example, a direct drive fan operating at
1160 RPM and 60 Hz requires 10 BHP. If the anticipated fan speed increase is 1334 RPM the corresponding frequency would be 69 Hz. Therefore, the motor
must be sized for (69/60)3 x 10 = 15.2 BHP.
Rotation — The direction of a 3-phase induction motor
depends on the motor connection to the power lines.
Rotation can be readily reversed by interchanging any
two input leads to match the fan rotation.
Service Factor — The service factor is a percentage
multiplier applied to the nameplate horsepower to obtain
continuous overload capacity of the motor.
Thus a standard drip-proof 10 HP motor rated with
a 1.15 service factor at 40°C ambient could carry a
continuous load of 1.15 SF x 10 HP or 11.5 HP.
Integral open drip-proof and totally enclosed fan
cooled motors usually have a service factor of 1.15 while
explosion proof motors usually have a 1.0 service factor.
It is important to remember that the value obtained by
applying a service factor is valid only if usual service
conditions, rated voltage, 40°C ambient, and rated frequency are maintained.
Insulation — NEMA has established insulation classes to
meet motor temperature requirements found in different
operating environments. The four insulation classes are
A, B, F and H. Class F is the most commonly used
while Class A is seldom used. Before a motor is started
its windings are at the temperature of the surrounding
air. This is known as “ambient temperature.” NEMA has
standardized on an ambient of 40°C or 104°F for all
motor classes. The most common insulation classes are
shown below.
Engineering Data 800
Table 4. Common Insulation Classes
Insulation
Class
B
F
H
Ambient Temperature
Range
Up to 40°C (104°F)
41°C (105°F) to 65°C (149°F)
66°C (150°F) to 90°C (194°F)
Hot Spot
Temperature
130°C (266°F)
155°C (311°F)
180°C (356°F)
Not all parts of the motor windings operate at the
same temperature. The temperature at the center of the
coil is the hottest and is referred to as the “hot spot
temperature.” The hot spot temperature is the basis for
establishing the insulation class rating.
Ambient Temperature — It’s best to select a motor with
the appropriate insulation for the specific ambient conditions. For example, a TEFC motor with Class F insulation
is suitable for ambient temperatures of 40°C (104°F) with
a service factor of 1.15. That same motor can be operated to 65°C (149°F) with a service factor of 1.0.
Operating this motor above these limits will reduce its
life expectancy. A 10°C increase in the ambient temperature to 75°C (167°F) can decrease the motor’s life
expectancy as much as 50%.
explosion of gas or vapor within it, and to prevent ignition of dust, gas, or vapor surrounding the machine by
sparks, flashes or explosions which may occur within the
motor casing.
The National Electric Code categorizes common hazardous atmospheres and locations. Since the type and
degree of hazard varies widely according to the materials encountered and their presence in hazardous quantities, the following methods of identification are used:
●
CLASS – Hazardous materials are assigned to three
broad categories: gases, dusts, and fibers.
●
Class I – A hazardous location in which flammable
gases or vapors are present in sufficient quantities to
produce an explosive mixture.
●
Class II – A hazardous location in which flammable
dusts are present in sufficient quantities to produce
an explosive mixture.
●
Class III – A hazardous location in which ignitable
fibers or combustible flyings are present in sufficient
quantities to produce an explosive mixture. There is
no group designation for this class. An example
would include cotton and rayon in textile mills.
●
Group – Hazardous materials (gases, dusts, fibers)
grouped according to their relative degree of hazard.
●
Class I, Group A – Atmospheres containing acetylene.
Class I, Group B – Atmospheres containing hydrogen,
or gases or vapors of equivalent hazards such as
manufactured gas.
Motor Enclosures
For proper application, the selection of the type of
enclosure to employ is quite important in order to meet
the particular atmospheric conditions.
There are two broad enclosure classifications: Open
and Totally Enclosed. Each is divided into a number of
specific types. The primary types are as follows:
Open
Totally Enclosed
Drip-proof
Non-ventilated
Splash proof
Fan-cooled
Weather protected
Explosion proof
Air over
Waterproof
Heat exchanger
Only those most frequently used are covered in this
guide.
Drip-proof (ODP) — This motor has ventilation openings
which permit the passage of ambient air over the rotor
and windings. The openings are constructed so as to
prevent drops of liquids or solid particles falling at any
angle from 0 to 15 degrees downward from vertical from
entering the motor.
These motors are typically used in relatively clean,
dry, mild humidity indoor applications. They are not usually suited for wet, outdoor duty unless protected.
Totally Enclosed — Totally enclosed motors are designed
to prevent the free exchange of air between the inside
and outside of the enclosure, but not sufficiently
enclosed to be airtight. The ones most commonly used
are:
TEFC – Totally Enclosed Fan Cooled: This type
includes an external fan mounted on the motor shaft.
The fan is enclosed in a fan casing which both protects
the fan and directs the air over the motor frame for
cooling. The TEFC motor is used in indoor or outdoor
duty applications where dust, dirt, mild corrosives, and
water are present in modest amounts.
TEAO – Totally Enclosed Air Over: This type is similar to the TEFC design except that the cooling fan and
casing are not provided. This motor is not self-cooling.
It should only be used in applications where the fan
itself provides sufficient airflow over the motor surface
for cooling.
FCXP – Fan Cooled Explosion Proof: This is a totally enclosed motor designed and built to withstand an
Page 5
●
Explosion proof equipment is generally not available
for Class I, Groups A and B, and it is necessary to
isolate motors from the hazardous area.
●
Class I, Group C – Atmospheres containing ethyl,
ethylene or cyclopropane vapors.
●
Class I, Group D – Atmospheres containing gasolene,
hexane, naptha, benzene, butane, alcohol, acetone,
benzol, lacquer solvent vapors or natural gas.
●
Class II, Group E – Atmospheres containing metal
dust.
●
Class II, Group F – Atmospheres containing carbon
black, coal or coke dust.
●
Class II, Group G – Atmospheres containing flour,
starch or grain dust.
●
DIVISION – Separates hazardous locations into two
categories based on whether the hazardous material
(gas, dust, fiber) is present under normal operating
conditions or only under extraordinary circumstances.
●
Division I – Hazard exists under normal operating
conditions. (Motor must be explosion proof.)
●
Division II – Hazard exists only during abnormal circumstances. (Article 501 of the National Electric Code
states that in Division II locations, “The installation of
open or non-explosion proof enclosed motors such
as squirrel cage induction motors without brushes,
switching mechanisms, or similar arc-producing devices shall be permitted.”)
Caution: The responsibility of specifying the proper
Class, Group, and Division of a hazardous location
resides with the ultimate user and the involved regulatory agency.
Engineering Data 800
Motor Starting Methods
Full Voltage Starting – This is the most common and
least expensive starting method for induction motors and
does not require any special construction. All standard
motors are designed for full voltages (across-the-line)
starting. The down side is that the high inrush current
(six to nine times the running current) required to come
up to speed sometimes causes premature tripping of
overload breakers, particularly when accelerating high
inertia loads which are common in fans.
Auto Transformer Starting – Employs auto transformers
to directly reduce voltage and current on start-up. After
a preset time interval the motor is connected directly
across the line. The most commonly furnished taps on
auto transformers are 50, 65 or 80 percent of full voltage. This is the most expensive method but no special
motor winding is required.
Wye-Delta Starting – Sometimes call “Star Delta,” this
method impresses the voltage across the Y connection
to reduce the current on the first step, and after a preset time interval the motor is connected in delta permitting full current. This method requires a motor winding
capable of wye-delta connection. It may fail to accelerate high inertia loads.
Part Winding Starting – Requires a special motor with
two separate winding circuits. Only one winding circuit
is energized on startup. After a preset time interval the
full winding of the motor is put directly across the line.
This method may fail to accelerate high inertia loads. A
standard dual voltage motor (230/460) can be used but
only if the lower voltage is used.
These last three starting methods are commonly
referred to as reduced voltage starting. They all require
special starters designed for the particular method and
are controlled between the start and run functions by
an adjustable timer.
Figure 5. Conduit Box Mounting Locations
FLOOR MOUNTINGS
ASS’Y. F-1
ASS’Y. F-2
WALL MOUNTINGS
ASS’Y. W-1
ASS’Y. W-5
ASS’Y. W-2
ASS’Y. W-6
ASS’Y. W-3
ASS’Y. W-7
ASS’Y. W-4
ASS’Y. W-8
CEILING MOUNTINGS
ASS’Y. C-1
ASS’Y. C-2
F-1 is most commonly used on the majority of fan
applications, regardless of fan arrangement, mostly
because of availability. Unless otherwise specified, NEMA
assembly F-1 will be provided.
Bracket Mountings – Motors are also available with special machined end shields: Type C-face and Type
D-flange. These are shown below.
Figure 6. Type C-face
Types of Mountings
The Frame – Includes the mounting feet and forms the
foundation for the complete assembly. This is commonly referred to as a “T” frame motor and it can be
positioned in any plane – top, bottom, or side.
The Conduit Box – Can be located on either side of the
frame. The standard location of this box is on the right
side of the motor when viewed opposite the shaft end,
and is referred to as an “F-1” motor. The opposite location is referred to as “F-2”.
NEMA lists twelve standardized assembly combinations and has assigned code designations as shown
below.
Figure 7. Type D-flange
The Type C-face provides a male rabbet and tapped
holes for mounting bolts while the Type D-flange provides a male rabbet and flange holes for through-bolts.
When a face or flange mounted motor is required for
a fan application, Type C-face is the motor of choice
and it is standardly provided with feet. The Type
D-flange motor, when used, is standardly provided without feet.
Page 6
Engineering Data 800
To avoid confusion, however, either motor when
required should be specified with or without feet.
Load Inertia, WK
2
It is not always enough to select a motor based on the
horsepower requirements of the fan. The motor not only
must develop sufficient torque to overcome the fan
loads, but is also must have enough excess torque to
overcome the inertia of the fan and accelerate it to
speed within a required amount of time. Generally
speaking, if motors built in frame 447T and smaller can
come up to speed in less than 20 seconds, they should
be acceptable for fan duty.
Figure 8 represents a fan load curve superimposed
on a typical NEMA Design B induction motor speed
torque curve.
Starting Frequency
Generally speaking, for the majority of fan applications,
the duty is continuous over long time periods. There are,
however, situations requiring repetitive starts with a given
fan load and inertia. Motor selection must take into
account the excess heating caused by these conditions.
A motor draws approximately six to nine times the
full load current on startup, so resistance losses (current2) during this time are up to 36 to 81 times the
losses at full load. If the impeller inertia is low, acceleration will be rapid, which could allow for multiple starts
per minute without overheating. On the other hand, if
the impeller inertia is high, requiring maximum time to
attain speed, starts may be limited to one per hour.
Motors used on these high inertia loads should be thermally protected to avoid costly burnouts.
Figure 8. Fan Load and Motor Speed Torque Curves
S P EED , R P M
OPERATING
POI NT
Effect of “EPACT”
MOTOR
With the passage of the Energy Act of 1992, the government mandated that after October, 1997, all general
purpose motors must meet nominal full load efficiency
levels. EPACT specifically targeted motors with the following characteristics:
● General purpose
● Continuous rated
● Foot mounted
● 60 Hertz
● Design A & B
● 2, 4, or 6 pole
● Open and enclosed design● Single speed
● 3-digit NEMA frame
● Squirrel-cage induction
● T-frame
● 230/460 volt
● Polyphase
● 1 to 200 HP
FAN
LOAD
ACCELERATING
TORQUE
TORQUE, LB-FT.
The shaded area represents the accelerating torque
available at any speed. Fan speed torque curves are
available from the fan manufacturer.
Failure of the motor to attain full speed, or to attain
full speed within the required time frame, results in
excessive motor temperatures, which lead to premature
motor failure.
The fan impeller inertia is referred to as WR2.
Typically, motor manufacturers refer to the motor inertia
as WK2. What we’re actually looking at is a value of
WK2 (WR2) which reflects the motor’s capability of accelerating a fan type load.
If the motor is direct connected, the impeller inertia
(WR2) may be used as stated in the catalog. If the motor
is used to drive the fan through a sheave combination,
at either higher or lower speeds than the motor, then it
is necessary to calculate the inertia “referred to the
motor shaft”; that is, an equivalent inertia based on the
speed of the motor.
WK2ms = WR2fs
( NN )
f
2
m
Where: WK2ms= inertia of fan load referred to motor
speed (lb-ft2)
WR2fs = inertia of fan load and inertia drives
and
shaft (lb-ft2)
Nf
= Speed of fan (RPM)
N m
= Speed of motor (RPM)
Page 7
This covers all motors sold in the United States,
whether domestic or imported. Explosion proof motors
are being added to this list effective October, 1999.
While complying with the EPACT standards has the
most impact on the motor manufacturers, it also creates
some design considerations for applying these motors to
fans.
Higher efficiency motors come at an increased cost
to the fan manufacturer. A premium efficiency motor can
cost 30 to 40 percent more than a general purpose
motor. Premium efficient motor design requires larger
diameter copper wires which occupy more space. This
means larger slots, which reduces the amount of active
steel in the laminations. This loss of steel is compensated for by adding additional laminations which increases the length of the rotor and stator coil. Rotor designs
are improved, air gap between rotor and stator is
reduced, oversized bearings are used, and the cooling
fan is redesigned. All these things improve efficiency but
add costs.
The higher efficiencies required by EPACT lie somewhere between general purpose and premium duty
motors and also come as a somewhat higher cost: 10
to 15 percent.
Associated with these higher efficient motors are
some issues which impact the application with both the
fan manufacturer and the end user:
1.High efficiency motors draw higher amps during startup. This may require circuit breakers or starters with
higher trip ratings.
2.High efficient motors tend to have less slip, which
increases full load speed (RPM). This can cause
higher amp draw on direct drive installations due to
higher horsepower requirements. Speed increase on
belt drive units can affect drive selections.
Engineering Data 800
3.The increase in motor length can cause fit-up problems on some fan designs. Some may require larger
diameter frames.
When replacing high efficient motors for older designs
the issues mentioned above should be considered.
4.Acceleration capabilities of fan type loads may be
compromised by lower allowable WK2 values.
Conclusion
5.High efficient motors tend to run cooler and more
efficiently, but to deliver these economies they must
be operated between 75% and 100% of rated load
operation. Operation below these values may require
more electricity than standard motors for the same
service.
The information included in this document is intended to
provide the designer/agent/user with basic information to
aid in the selection and application of integral 3-phase
asynchronous induction motors. There are many other
considerations for proper motor selection. Additional
information may be obtained directly from the motor
manufacturer.
6.High efficient motors tend to weigh more than standard designs. While this may not have significant
impact on our fan designs, it certainly will affect
freight charges.
Miscellaneous Electrical Formulas
Volts = Amperes x Ohms
Efficiency = 746 x Output Horsepower
Input Watts
3-Phase Horsepower = Volts x Amperes x Efficiency x Power Factor x 1.732
746
3-Phase Kilowatts = Volts x Amperes x Power Factor x 1.732
1000
746 x Horsepower
3-Phase Amperes =
1.732 x Volts x Efficiency x Power Factor
746 x Horsepower
3-Phase Efficiency =
Volts x Amperes x Power Factor x 1.732
Input Watts
3-Phase Power Factor =
Volts x Amperes x 1.732
Horsepower x 5250
RPM
Torque (ft. lbs.) =
Kilowatts = 0.746 x Horsepower
Horsepower = 1.341 x Kilowatts
Twin city fan & blower | www.tcf.com
5959 Trenton Lane N | Minneapolis, MN 55442 | Phone: 763-551-7600 | Fax: 763-551-7601
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