Single Phase AC Induction Squirrel Cage Motors - FE-1100

Single Phase AC Induction Squirrel Cage Motors - FE-1100
FAN ENGINEERING
FE-1100
Information and Recommendations for the Engineer
Single-Phase AC Induction
Squirrel Cage Motors
Introduction
Index
It is with the electric motor where a method of converting electrical energy into mechanical energy to perform
some physical task or work is possible. The electric
motor is by far the most common method for powering
a ventilating fan today. There are many classifications of
motor types available, but this paper will focus on single-phase AC induction squirrel cage motors which is
where the largest number of motors are utilized in ventilation.
On the following pages the basic operation and types
of AC single-phase induction motor types will be covered. This paper will focus on the electrical arrangement
inside the motor housing, discuss the principles governing motor design, and help the user understand what
can and cannot be done with the motor in terms of
power and electrical control.
Figure 1 is a listing of the basic motor classifications.
Introduction – Motor Classifications. . . . . . . . . . . . 1
Basic Principles. . . . . . . . . . . . . . . . . . . . . . . . 2
AC Motor Fundamentals. . . . . . . . . . . . . . . . . . . 2
Motor Stator. . . . . . . . . . . . . . . . . . . . . . . . . . 2
Motor Rotor. . . . . . . . . . . . . . . . . . . . . . . . . . 3
Magnetic Field for Single-Phase Motors . . . . . . . . . 3
Motor Starting for Single-Phase Motors. . . . . . . . . . 3
Motor Speed-Torque Curves . . . . . . . . . . . . . . . . 4
Motor Efficiency. . . . . . . . . . . . . . . . . . . . . . . . 4
Split-Phase Motors. . . . . . . . . . . . . . . . . . . . . . 5
Capacitor-Start Motors. . . . . . . . . . . . . . . . . . . . 6
Permanently Split Capacitor Motors. . . . . . . . . . . . 7
Capacitor Start-Capacitor Run Motors. . . . . . . . . . . 8
Shaded Pole Motors. . . . . . . . . . . . . . . . . . . . . 9
Summary. . . . . . . . . . . . . . . . . . . . . . . . . . . 10
Figure 1. Electric Motor Classifications by Electrical Type
DC Motors
Permanent Magnet
Series Wound
Shunt Wound
Compound Wound
Squirrel Cage
Split-Phase
Capacitor-Start
Permanent Split Capacitor
Capacitor Start/Run
Shaded Pole
Wound Rotor
Repulsion
Repulsion Start
Repulsion Induction
Induction
Single-Phase
AC Motors
Polyphase
Synchronous
Shaded Pole
Hysteresis
Reluctance
Permanent Magnet
Induction
Wound Rotor
Squirrel Cage
Synchronous
Universal Motors
©1999 Twin City Fan Companies, Ltd.
AC Induction Motor Fundamentals
Basic Principles
Stator
In order to understand how the single-phase AC induction motor works a basic understanding of the physical
principles and fundamentals governing motor design and
operation is required. We will start with the basic principle on which the induction motor is able to convert
electrical energy into mechanical energy.
The basic operation of an AC induction motor is
based on two electromagnetic principles:
Whenever current flows in a conductor, a magnetic field
is built up around it (Figure 3). If the conductor is
formed into a coil, the magnetic field created is similar
to that of a permanent bar magnet (Figure 4). If a bar
of magnetic material such as iron or steel is placed
within the coil, the magnetic field is strengthened
because these materials transmit magnetic flux much
more readily than air.
1. Current flow in a conductor will create a magnetic
field surrounding the conductor, and,
Figure 3. Magnetic Field Surrounding Conductor
2. If a conductor is moved through this magnetic field,
current is induced in the conductor and it will create
its own magnetic field.
The fundamental single-phase AC induction motor consists of two basic parts (see Figure 2).
Figure 2. Basic Components of a Single-Phase
AC Induction Motor
Figure 4. Magnetic Field Produced by a Simple Coil
with Permanent Magnet
1.Stator. The stator is constructed of a set of stacked
laminated discs which are surrounded by a stator
winding. This winding is connected to the proper
power supply (voltage, phase and frequency) and
produces a magnetic field that revolves around the
motor at a speed designated “synchronous.”
2.Rotor. The rotor is connected to the output shaft and
consists of a shorted aluminum winding which is cast
into slots and stacked and joined at both ends of the
stack with end rings. The rotor acts as a conductor
which when placed in the magnetic field of the stator
winding creates a magnetic field of its own and interacts with the magnetic field of the stator, producing
torque.
Figure 5 is a view of one half of a four pole stator.
The placement of the coils resembles the relative positioning of the coil and bar of Figure 4 and the resulting
magnetic fields are also similar. By reversing the direction in which one coil is wound in the stator relative to
the adjacent coil, the direction of current flow is
reversed, as illustrated in Figure 5. This reversal in the
direction of current flow also changes the magnetic
polarity, creating adjoining north and south poles in the
stator.
Figure 5. Flux Patterns Produced in Stator
The first principle applies to the magnetic field created by the stator and the second applies to the rotor
as it rotates within the stator field.
Before we cover the characteristics of the various
motor types, it may be useful to review in detail the
basic electromagnetic principles which enable the induction motor to convert electrical energy into a mechanical
output through the motor shaft. We will start by taking
a look into the two main components (stator and rotor)
of single-phase AC motors. In the following section we
will discuss their construction and the fundamental principles of their operation.
2
Fan Engineering FE-1100
In the completed stator, a magnetic field is created
having alternating north and south poles. The number of
magnetic poles in the stator, together with the AC line
frequency, determines the speed at which the motor will
operate.
Rotor
If a conductor is moved between the faces of a horseshoe magnet (Figure 6), a voltage is induced in that
conductor, and if the conductor forms part of a completed circuit, current will flow. The action which produces this voltage and current is the cutting of the
magnetic field (called lines of magnetic flux) by the conductor. This current flow within the conductor will in turn
produce a magnetic field, as seen in Figures 3 and 6.
The interaction of these two magnetic fields produces a
mechanical force on the wire which is the basis for the
production of torque.
Figure 6. Elements of Motor Action
S
Magnetic Field for Single-Phase Motors
In order for the rotor to move, the stator must produce
a rotating magnetic field. With a single source of AC
voltage connected to a single winding, this is not possible. A stationary flux field is created which pulsates in
strength as the AC voltage varies, but it does not rotate.
This pulsating field strength is what simulates a rotating
field and gives the rotor its rotation. If a stationary rotor
is placed in this single-phase stator field, it will not
rotate, but if the rotor is spun by hand, it will pick up
speed and run. The single-phase motor will run (in either
direction) if started by hand, but it will not develop any
starting torque as illustrated in Figure 8.
Direction
of Motion
Direction of
Motor Starting for Single-Phase Motors
Opposing
Force
N
In an induction motor, the single conductor of the
previous example is replaced by the rotor winding. In
small motors, this winding is normally of cast aluminum
and consists of multiple conductors, or rotor bars, cast
into the slots in the rotor core and joined at both ends
of the stack with end rings. The resulting rotor winding
circuit is illustrated in Figure 7, and as can be seen from
this figure, the name has been designated as “squirrel
cage rotors.”
Figure 7. Representation of Voltage-Current
Paths (and Strengths) in Squirrel Cage Rotor
What is needed is a second (start) winding, with currents
out of phase with the original (main) winding, to produce
a net rotating magnetic field. The various single-phase
motor designs differ in the type of secondary (start)
winding employed. These start windings, which together
with other components such as capacitors, relays and
centrifugal switches, make up the starting circuit, provide
varying effects on motor starting and running characteristics.
Figure 8. Speed-Torque Characteristics of Single-Phase,
Single Main Winding Only Motor
100
Percent Synchronous Speed
Vdc
is passing through at that time. Because the strength of
the stator field varies around its circumference, being
strongest at each pole, the voltages and currents
induced in the rotor bars will also vary around the rotor.
Figure 7 illustrates the directions and magnitudes of a
typical rotor bar current pattern. The magnetic field built
up around each bar reacts with the stator flux, exerting
a force on each bar in the same manner as previously
described in the example of the horseshoe magnet and
simple conductor. The sum total of all forces acting on
all rotor bars is the output torque of the motor.
80
60
40
20
0
0
100
400
200
300
Percent Full-Load Torque
500
Two basic mathematical models best reveal two
important facts about single-phase motors:
As a rotor turns, the rotor bars cut through the flux
lines of the stator magnetic field and voltages and currents are induced in each bar. The magnitude of the
voltage and current in a given bar will depend on the
magnetic density of the stator field which the rotor bar
3
1.The performance characteristics of single-phase
motors can approach, but will not exceed that of the
two-phase polyphase motors.
2.The torque produced at a given RPM is not constant,
but pulsates at twice the line frequency around a
median value. These torque pulsations are inherent to
Fan Engineering FE-1100
Motor Speed-Torque Curves
Determining what motor is necessary for the application
is done through examining motor speed-torque curves.
There is much information on a speed-torque curve to
tell the end user if the motor will operate satisfactorily
for the intended application. The speed-torque curve will
allow the user to determine if the motor has enough
starting torque to overcome friction, to accelerate the
load to full running speed, and if it can handle the
maximum overload expected. See Figure 9 for a typical
speed-torque curve.
70
60
Permanently Split Capacitor Motor
(Single Phase, 4 Pole, 3/4 HP)
50
40
RPM Slip at Full Load or
Percent Rated Slip
Full Load Operating Point
Approx. Max. Torque Recommended
For Short Time Overloads
Breakdown or Max. Torque
(Developed During Acceleration)
30
Full Load or Rated Torque
Speed at Which
Breakdown Torque
Cusp or
is Developed
Lowpoint in
Acceleration
Torque
}
Service Factor Load or Torque
Pull-up Torque or Min.
Accelerating Torque
Locked Rotor or Starting Torque
There are many torques that can be obtained from a
motor’s speed-torque curve:
• Locked Rotor (Starting) Torque — Motor torque at zero
speed.
• Pull-Up Torque — Lowest torque value between zero
and full load speed.
• Breakdown Torque — Maximum torque without motor
stalling.
• Full Load Torque — Torque produced by motor at full
load operating point.
As we further describe the five types of single-phase
AC induction motors, reference to the speed-torque
curves will be crucial to understanding each motor type
and its operating characteristics.
Motor Efficiency
Single-phase AC induction squirrel cage peak motor
efficiencies range from as low as 30% to as high as
65%, depending on the motor type and design. Motor
efficiencies also depend on the actual motor load versus
rated load. Refer to Figure 10.
The best motor for the job is often suggested by the
nature of the load. Motor efficiency usually is greatest
at the full load rating and falls off rapidly for under and
overloaded conditions as can be seen in Figure 10.
The misconception that a motor running well below
its maximum load rating will run cooler and more efficiently is not true. Oversizing AC motors reduces efficiency by a substantial amount, causing a larger part of
the input energy to be dissipated as heat. On the other
end of the scale, overloading of motors is a much better understood concept as many other signs indicate a
poor motor selection (reduced speed, high ampere draw,
tripped motor overloads). The amount of electrical power
0
25
50
75
100
125
150
175
200
Motor Load (% of Full Load)
Another design criteria affecting motor efficiency is
operating voltage. Motors are generally designed to
operate at a given rated voltage, with a plus and minus
tolerance (10% is typical). Within the tolerance level,
efficiency generally increases for higher voltages, but
decreases for lower voltages. The decrease is due to
greater I2R losses (because of the substantial length of
wire required in the winding, this results in I2R losses
because of the principle that the wire resistance increases with its length). Low operating voltage also reduces
torque, which decreases as the square of the voltage.
Refer to Figure 11.
Figure 11. Effect of Voltage on Motor Efficiency
20
15
Percent Change in Motor Efficiency (%)
Full Load or
Rated Motor Speed
Synchronous Speed
No Load Motor Speed
Dotted Portion of Speed
vs. Torque Curve Applicable
to Initial Acceleration
Torque
4
Figure 10. Motor Efficiency vs. Motor Load
}
Motor Speed in RPM or Percent Synchronous Speed
Figure 9. Typical Motor Speed-Torque Curve
wasted can be reduced by a more careful application of
a motor to the actual load. It is typically best to run an
AC single-phase squirrel cage motor at no less than
75% full load and no greater than 125% full load from
an efficiency standpoint. Again, motor efficiency is greatest near its full load rating.
Efficiency (%)
all single-phase motors and can cause noise and
vibration problems if not properly isolated.
10
5
Permanently Split Capacitor Motor
(Single Phase, 4 Pole, 3/4 HP)
0
-5
c ie
Effi
n cy
-10
-15
-30
-20
-15
-10
-5
0
5
Percent Voltage Variation (%)
10
15
20
Beyond these considerations, the motor design determines its efficiency. There are several ways to reduce
power losses in the motor. One is to reduce losses in
the core, either by adding more material to the magnetic core structure or by using a steel with improved
core-loss properties. Another method is to increase the
cross-sectional area of conductors to reduce resistance.
This means that additional winding material must be
added to the stator and rotor. Another alternative is to
shorten the air gap between the rotor and stator to
reduce the magnetizing current required. A final method
Fan Engineering FE-1100
Split-Phase Motors (SP)
used is to simply add more material to the magnetic
core which will reduce the amount of current required
to magnetize the core.
Figure 12 illustrates schematically the winding arrangement of a typical distributed winding arrangement of a
split-phase motor. A split-phase motor’s components are
a main winding, start winding and a centrifugal switch.
Figure 12. Winding Schematic for SP Motor
Main
Winding
Switch
VL
Figure 14.General Performance Characteristic
(SP Motor)
Start
Winding
IL
IS
Small
Conductor
Volume
The main (run) winding is designed for operation from
75% synchronous speed and above. The main winding
design is such that the current lags behind the line voltage because the coils embedded in the steel stator
naturally build up a strong magnetic field which slows
the buildup of current in the winding. This relationship
between the line voltage and line current is shown in
Figure 13.
The start winding is not wound identically to the
main, but contains fewer turns of much smaller diameter
wire than that of the main winding coils. This is required
to reduce the amount the start current lags the voltage.
This can also be seen in Figure 13.
When both windings are connected in parallel across
the line, the main and start winding currents will be out
of time phase by about 30 degrees. This forms a sort
of imitation of a weak rotating flux field which is sufficient to provide a moderate amount of torque at standstill and start the motor.
Figure 13. Phase Relationships (SP Motor)
V
IM
IS
100
Percent Synchronous Speed
IM
The total current that this motor draws while starting
is the vector sum of the main and start winding currents.
Because of the small angle between these two, the line
current during starting (inrush current) of split-phase
motors is quite high. Also the small diameter wire in the
start winding carries a high current density, so that it
heats up very rapidly. A centrifugal switch mechanism
(or relay) must be provided to disconnect the start winding from the circuit once the motor has reached an
adequate speed to allow running on the main winding
only. Figure 14 illustrates the speed-torque relationship
of a typical split phase motor on both the running and
starting connection. Table 1 summarizes the split-phase
motor characteristics.
80
Run
Winding
60
40
Start
Winding
20
0
0
CHARACTERISTIC
Peak Efficiency
Power Factor
Starting Torque
Noise & Vibration
Components
Other
Cost
100
200
400
300
Percent Full-Load Torque
500
NOTES
50 to 60%
60 to 70%
100% Full Load Torque
120 Hz Torque Pulsations
Contains Centrifugal Switch
High Inrush Starting Current
Moderate
V
IS
~
= 30°
IM
5
IL
Fan Engineering FE-1100
Capacitor-Start Motors (CSIR)
Table 1. Summary of Split-Phase Motor Characteristics
Figure 15 illustrates schematically the winding arrangement of a typical distributed winding arrangement of a
capacitor-start motor. It should be noted that the capacitor-start motor utilizes the same winding arrangement as
the split-phase motor, but adds a capacitor in series
with the start winding. The effect of this capacitor is
shown in Figure 16. The main (run) winding current
remains the same as in the split-phase case, but the
start winding current is very much different. With the
capacitor in the circuit, the starting current now leads
the line voltage, rather than lagging as does the main
winding. The start winding is also different, containing
slightly more turns in its coils than the main winding and
utilizing wire diameters only slightly smaller than those
of the main.
Figure 15. Winding Schematic for CSIR Motor
The new result is a time phase shift closer to 90
degrees than with the split-phase motor. A stronger
rotating field is therefore created and starting torque is
higher than with the split-phase design. Also the vector
sum of the main and start winding currents is lower,
resulting in a reduction in the inrush current as compared to the split-phase design. Refer to Figure 16.
The starting and running speed-torque characteristic
of a capacitor-start motor is illustrated in Figure 17.
Again, a centrifugal switch and mechanism (or relay)
must be used to protect the start winding and capacitor
from overheating. When the capacitor-start motor is running near full load RPM, its performance is identical to
that of the split-phase motor. Table 2 summarizes the
capacitor-start motor characteristics.
Figure 17.General Performance Characteristic
(CSIR Motor)
Main
Winding
Switch
VL
IM
Start
Winding
IL
Short-Time
Rated
Capacitor
IS
Figure 16. Phase Relationships (CSIR Motor)
Percent Synchronous Speed
100
80
60
Run
Winding
Start
Winding
40
20
0
0
400
100
300
200
Percent Full-Load Torque
500
VL
IM
IS
IS
VL
IM
6
IL
Table 2. Summary of Capacitor-Start Motor Characteristics
CHARACTERISTIC
Peak Efficiency
Power Factor
Starting Torque
Noise & Vibration
Components
Other
Cost
NOTES
50 to 60%
60 to 70%
Up to 300% Full Load Torque
120 Hz Torque Pulsations
Contains Centrifugal Switch & Capacitor
(Intermittent Duty)
Capacitor Controls Inrush Starting Current
(Lower Than Split-Phase Type)
Slightly Higher Than Split-Phase Type
Fan Engineering FE-1100
Permanently Split Capacitor Motors (PSC)
Figure 18 illustrates schematically the winding arrangement of a typical distributed winding arrangement of a
permanently split capacitor motor. The windings of the
PSC motor are arranged like those of the split-phase
and capacitor-start designs, but a capacitor capable of
running continuously replaces the intermittent duty
capacitor of the capacitor-start motor and the centrifugal
switch of both the split-phase and capacitor-start
motors. The main winding remains similar to the previous designs, current lags the line voltage (refer to Figure
19).
Figure 18. Winding Schematic for PSC Motor
Main
Winding
IM
Continuous
Rated
Capacitor
Start
Winding
IL
IS
Figure 20.General Performance Characteristic
(PSC Motor)
The start winding of a PSC motor is somewhat different than in the capacitor-start design. Because the
capacitor for a PSC motor usually has a small rating, it
is necessary to boost the capacitor voltage by adding
considerably more turns to its coils than are in the main
winding coils. Start winding wire size remains somewhat
smaller than that of the main winding. The smaller
microfarad rating of the capacitor produces more of a
leading phase shift and less total start winding current,
so starting torques will be considerably lower than with
the capacitor-start design. Refer again to Figure 19.
Figure 19. Phase Relationships (PSC Motor)
VL
100
Percent Synchronous Speed
VL
However, the real strength of the permanently split
capacitor design is derived from the fact that the start
winding and capacitor remain in the circuit at all times
and produce an approximation of two-phase operation
at the rated load point. This results in better efficiency,
better power factor, and lower 120 Hz torque pulsations
than in equivalent capacitor-start and split-phase
designs.
Figure 20 illustrates a typical speed torque curve for
a permanently split capacitor motor. Different starting
and running characteristics can be achieved by varying
the rotor resistance. In addition, by adding extra main
windings in series with the original main windings, PSC
motors can be designed to operate at different speeds
depending on the number of extra main windings energized. It should also be noted that for a given full load
torque, less breakdown torque and therefore a smaller
motor is required with a permanently split capacitor
design than with the other previously discussed designs.
Table 3 summarizes the characteristics of the permanently split capacitor design.
Low
Resistance
Rotor
80
60
High
Resistance
Rotor
40
20
0
0
100
200
300
400
Percent Full-Load Torque
500
IM
IS
Table 3.Summary Permanently Split Capacitor
Motor Characteristics
IS
V
IM
7
IL
CHARACTERISTIC
Peak Efficiency
Power Factor
Starting Torque
Noise & Vibration
Components
Other
Cost
NOTES
55 to 65%
80 to 100%
50 to 80% Full Load Torque
120 Hz Torque Pulsations Reduced
Contains Capacitor (Continuous Duty)
Can be used with speed control devices (not
possible with SP & CSIR types)
Smallest motor for given output
Fan Engineering FE-1100
Capacitor Start-Capacitor Run Motors (CSCR)
Figure 21. Winding Schematic for CSCR Motor
Auxiliary
Winding
Main
Winding
Start
Capacitor
Rotor
Run
Capacitor
Figure 22 illustrates a typical speed torque curve for
a capacitor start-capacitor run motor. With the advantage of combining both the PSC and capacitor-start
designs these motors have been extended up to ratings
as high as 20 HP, far beyond what the other singlephase motor types are capable.
Table 4 summarizes the characteristics of the capacitor start-capacitor run motor.
Figure 22.General Performance Characteristic
(CSCR Motor)
100
Percent Synchronous Speed
Figure 21 illustrates schematically the winding arrangement of a typical distributed winding arrangement of a
capacitor start-capacitor run motor. These motors have
a run capacitor and an auxiliary winding permanently
connected in parallel with the main winding. In addition,
a starting capacitor and a centrifugal switch are also in
parallel with the run capacitor. The switch disconnects
as the motor accelerates. It should be noted that the
capacitor start-capacitor run motor utilizes the same
winding arrangement as the permanently split capacitor
motor when running a full load speed and the same
winding arrangement as a capacitor-start motor during
startup.
80
60
8
Start
Winding
40
20
0
The advantage of the capacitor start-capacitor run
design is derived from the fact that the start winding
and capacitor remain in the circuit at all times (similar
to PSC type motor) and produce an approximation of
two-phase operation at the rated load point, plus with
an additional capacitor in series with the start winding
circuit (similar to the capacitor-start type motor), the
starting current now leads the line voltage, rather than
lagging as does the main winding, dramatically increasing starting torque. Capacitor start-capacitor run motors
feature a low running current due to an improved power
factor caused by the run capacitor.
This results in better efficiency, better power factor,
increased starting torque and lower 120 Hz torque pulsations than in equivalent capacitor-start and split-phase
designs. The capacitor start-capacitor run motor is basically a combination of the capacitor-start and PSC
motor types and is the best of the single-phase
motors.
Run
Winding
0
100
200
300
400
Percent Full-Load Torque
500
Table 4.Summary Capacitor Start-Capacitor Run Motor
Characteristics
CHARACTERISTIC
Peak Efficiency
Power Factor
Starting Torque
Noise & Vibration
Components
Other
Cost
NOTES
55 to 65%
80 to 100%
Up to 300% Full Load Torque
120 Hz Torque Pulsations Reduced
Contains Centrifugal Switch & Capacitor
(Intermittent Duty).
Contains 2nd Capacitor (Continuous Duty).
Capacitor controls inrush starting current & run
capacitor simulates 2-phase operation.
The best of the single-phase motor types.
Exceptionally quiet.
Most expensive motor design type.
Fan Engineering FE-1100
Shaded Pole Motors
The shaded pole motor differs widely from the other
single-phase motors which have been discussed. All of
the other designs contain a main and start winding, differing only in details of the starting method and corresponding starting circuitry.
The shaded pole motor is the most simply constructed and therefore the least expensive of the single-phase
designs. It consists of a run winding only plus shading
coils which take the place of the conventional start
winding. Figure 23 illustrates the construction of a typical shaded pole motor. The stator is of salient pole
construction, having one large coil per pole wound
directly in a single large slot. The shading coils are short
circuited copper straps which are wrapped around one
pole tip of each pole.
point “b”. As slight as this shift is, it is sufficient to
generate torque and start the motor.
The single coil winding is the crudest possible
approximation of a rotating magnetic field. Therefore, the
shaded pole motor efficiency suffers greatly due to the
presence of winding harmonic content, particularly the
third harmonic which produces a dip in the speed torque
curve at approximately 1/3 synchronous speed (refer to
Figure 24). In addition there are losses in the shading
coils. These factors combine to make the shaded pole
the least efficient and noisiest of the single-phase
designs. It is used mostly in air moving applications
where its low starting torque and the third harmonic dip
can be tolerated. Extra main windings can be added to
provide additional speeds in a manner similar to that
used on permanently split capacitor motors. Table 5
summarizes the characteristics of a shaded pole
motor.
Figure 24.General Performance Characteristic
(Shaded Pole Motor)
Figure 23. Shaded Pole Motor Construction
The shaded pole motor produces a very crude approximation of a rotating stator field through magnetic coupling which occurs between the shading coils and the
stator winding. The placement and resistance of the
shading coil is chosen so that, as the stator magnetic
field increases from zero at the beginning of the AC
cycle to some positive value, current is induced in the
shading coil. As previously noted, this current will create
its own magnetic field which opposes the original field.
The net effect is that the shaded portion of the pole is
weakened and the magnetic center of the entire pole is
located at point “a”. As the flux magnitude becomes
nearly constant across the entire pole tip at the top of
the positive half cycle, the effect of the shading pole is
negligible and the magnetic center of the pole shifts to
9
Percent Synchronous Speed
100
80
60
40
Third
Harmonic
Dip
20
0
0
100
300
400
200
Percent Full-Load Torque
500
Table 5. Summary Shaded Pole Motor Characteristics
CHARACTERISTIC
Peak Efficiency
Power Factor
Starting Torque
Noise & Vibration
Components
Other
Cost
NOTES
20 to 40%
50 to 60%
40 to 50% full load torque plus
third harmonic dip
120 Hz torque pulsations plus
winding harmonics
No additional components needed
Can be used with speed control devices (not
possible with SP & CSIR types)
Cheapest of all single-phase motors
Fan Engineering FE-1100
Summary
Single-phase AC motors are not all equal. There are five
basic types, all with different operating characteristics
and capabilities. As it has been shown, the differences
between each motor type are great enough that it is
important for the user to understand each motor type,
where it makes sense to apply them and how to apply
them.
What does knowing the previous technical information
on these five types of motors have to do with fans?
The answer is simply this: almost every commercial fan
is powered by an electric motor and other than the fan
wheel it is by far the most important component of a
fan. The motor is the most likely component to fail in a
fan and a large percentage of fan motor failures can be
attributed to poor selection and application of the
motor.
Speed control capability is also becoming more commonplace today (especially on direct drive fans) and
applying today’s speed controllers to any single-phase
AC motor can prove disastrous. It is important to understand that there are only two types of single-phase AC
squirrel cage motor types that are suited for speed
control applications.
Understanding the starting characteristics of each
motor type is another area of importance for the motor
application to a particular fan. All five motor types have
distinct starting characteristics that need to be properly
matched with the fan starting characteristics.
The table below can be used as a quick reference
to the five motor types along with typical (acceptable)
fan applications for each motor type.
Table 6. Summary of Five Single-Phase Motor Types
MOTOR TYPE
SPLIT-PHASE
CAPACITOR-START
PERMANENTLY SPLIT
CAPACITOR
CAPACITOR STARTCAPACITOR RUN
Single main winding with shading
coils for providing
starting torque.
Start winding connected in parallel
with main winding,
connection controlled
by centrifugal switch
or relay.
Identical to the splitphase design except
includes the addition
of a capacitor in
series with the start
winding circuit.
Start winding permanently connected in
parallel to main winding with a continuous
duty capacitor in the
circuit at all times.
Combination of
capacitor-start and
PSC type motor. Start
winding permanently
connected in parallel
to main winding with
a continuous duty
capacitor in the circuit
at all times and
capacitor in series
with the start winding
circuit.
1/6 to 1
1/4 to 2
1/100 to 1
3/4 to 20
1/1000 to 1/4
860, 1140, 1725,
3450
860, 1140, 1725,
3450
1050, 1625, 3250
1725, 3450
1050, 1550, 3100
EFFICIENCY
RANGE
50 to 60%
50 to 60%
55 to 65%
55 to 65%
20 to 40%
POWER FACTOR
60 to 70%
60 to 70%
80 to 100%
80 to 100%
50 to 60%
100%
Up to 300%
50 to 80%
Up to 300%
40 to 50%
Suitable for frequent
starting of fans in
both direct and belt
driven units.
All-purpose motor for
high starting torque,
low starting current
used in both direct
and belt driven units.
Intended for direct
drive models and
applications requiring
speed control.
All-purpose motor for
high starting torque,
low starting current
used mainly in larger
belt driven units.
Suitable for direct
drive low power
fans and multispeed applications.
a.High starting
torque.
b.Lower starting
current than splitphase design.
c.Available in larger
HP sizes than
capacitor-start or
PSC motor types.
d.High running
efficiency.
a.Inexpensive to
manufacture.
b.Multi-speed
operation.
c.Compact.
a.Most expensive
single-phase motor
type.
b.Not applicable for
speed control.
a.Low efficiency.
b.Low starting
torque.
DESCRIPTION
HP RANGE
TYPICAL RATED
STARTING
TORQUE
TYPICAL
ADVANTAGES
a. Good starting
torque.
b.Medium efficiency.
DISADVANTAGES
10
SHADED POLE
a.Not suited for
high starting
torque loads.
b.Not applicable for
speed control.
c.High starting current
a. High starting
torque.
b. Lower starting
current than split
phase design.
a.More expensive
than split-phase
design.
b.Not applicable for
speed control.
a.High running
efficiency.
b.Capable of multispeed operation.
c.Can be used with
speed control
devices (i.e., triacs).
d.Quietest of all
small induction
motors.
a.Low starting
torque.
b.Speed varies more
under load.
Fan Engineering FE-1100
11
Fan Engineering FE-1100
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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