CHAPTER 17
SINGLE-PHASE MOTORS
INTRODUCTION
Single-phase AC motors are the most common motors built. Every
home, workshop, and vessel has them. Since there is such a wide
variety of these motors, it is impossible to describe all of
them. This chapter will describe the most common types found on
Army watercraft. Figure 17-1 shows the
basic schematic diagrams for the single-phase motors.
The basic diagram (view A) shows a
circle with two leads labeled T1 and T2. Just as in the
three-phase motor diagram, the motor shows the power supply lines
as being identified with the T. For most shore facility
applications, this is the case. In many cases, the single-phase
motors on board a ship will be wired into the lighting
distribution panels. The lighting distribution panels are the
source for single-phase power supply. The power distribution
panels are the source of the three-phase power supply. For this
reason, the single-phase motors are commonly connected to L1 and
L2, as shown in Figure 17-2.
Figure 17-1 shows four single-phase
motor diagrams. Diagram A shows the motor
as it will be seen on blueprints and general layouts. It is
concerned only with the overall operation of the electrical
distribution system. Diagram B and C show a more involved internal wiring system
indicating two inductors and three terminals. These diagrams are
necessary to understand the exact nature and function of the
single-phase motor. Refrigeration and manufacturer's wiring
schematics also use diagrams B and C to ensure a positive troubleshooting
application.
Figure 17-3 shows a very basic one-line
diagram of the single-phase motor. Refer back to this diagram as
the operational requirements of the single-phase motor are
discussed.
The single-phase induction motor is much the
same in construction as the three-phase motor. Some single-phase
induction motors are also called squirrel cage motors because of
the rotor's similarity to a circular animal exercise wheel. As
discussed in Chapter 16, the squirrel
cage comprises the bars and shorting-rings that make up the rotor
windings. The squirrel cage is also considered the secondary
windings of the motor (Figure 17-4).
Despite the fact that the three-phase motor
has more phases than the single-phase motor, the single-phase
motor is a much more complex machine. Several additional
components are necessary to operate the single-phase motor.
Single-phase motors have only two power
source supply lines connected. The single-phase motor can operate
off either the A-B, B-C, C-A, A-N, B-N, or C-N power source
phases. The two-wire power supply can provide only a single-phase
alternating source (Figure 17-5). The
individual single-phase current arriving in the stator winding of
the single-phase motor does not have the same
"revolving" effect that the three individual phases of
the three-phase power supply provides. The magnetic field
developed by the single-phase current is created in the stator
windings and then is gone. An entire cycle must be completed
before current is again available at the single-phase motor
stator. This prevents the development of the revolving field so
easily obtained with the three-phase power supply. The problem
with the single-phase motor is its inability to develop a
revolving field of its own accord. Without a revolving field,
torque cannot be developed, and the rotor will never turn. With
only one stator winding, the single-phase motor can only produce
an oscillating magnetic field.
Figure 17-6 shows a main winding
separated into two coils. Each winding is wound in a different
direction. The importance of the two different coil winding
directions is to emphasize the application of the left-hand rule
for coils as expressed in previous chapters. By winding the wire
in a different direction, the polarity of the coil face closest
to the rotor can be changed. By using one wire wrapped in two
different directions, the polarity of every other coil can be
changed.
When current flows in the main winding, the
magnetic field is established throughout the windings (Figure 17-6). Soon the current flow stops and
changes direction (Figure 17-7). With this
change in current direction comes a change in all the coil
polarities.
The magnetic field of the rotor is developed
through induction in the same manner as described for the
three-phase induction motor rotor. The rotor bars and the
shorting rings have an induced EMF created in them, and a current
flow develops. This current flow establishes a magnetic field of
an opposite polarity of the stator coil directly across from it.
Unfortunately, there are no overlapping 120-degree individual
stator windings in this single-phase motor.
Whenever current changes direction and a new
magnetic field is established in the stator, the induced rotor
magnetic field changes to the opposite polarity of the stator
coil directly across from it. All the rotor can do is oscillate.
Without some force to twist or turn the rotor, no torque can be
developed.
A person examining this motor will hear a
distinct hum. This is called an AC hum. It is often heard coming
from transformers or single-phase motors that are not turning. If
the soldier physically turned the rotor shaft (not recommended)
in either direction, the rotor would start to move. The speed
would continue to increase until it reached its normal operating
speed.
NOTE: Although certain motors,
such as fans, can be found to be started physically by
turning the rotor shaft, this action is not recommended.
Whenever a motor does not start of its own accord, it is
because something is wrong. If the motor has an electrical
malfunction, it is not wise to touch the electrical
components when current is applied.
As long as the rotor's magnetic field is
slightly displaced from the magnetic field in the stator, a
torque can be developed. Slip will keep the rotor's field
slightly behind the stator's field. The difference in speed
(relative motion) is necessary to maintain the torque. Relative
motion is necessary to induce the EMF into the rotor to maintain
the rotor's magnetic field. If the soldier disconnects power and
allows the rotor to stop, he again must provide the initial
movement to start the rotor. This is not an acceptable condition
for a motor.
Without the use of a three-phase alternating
current, an artificial phase displacement must be established. If
the stator could only develop another current, slightly out of
phase from the original cur-rent, a revolving field could be
assimilated. This is the problem encountered by single-phase
induction motors. It is also the area of greatest component
failure and maintenance requirements. In fact, the specific names
for induction motors represent the means in which the revolving
field is developed from a single-phase power source.
There are a multitude of single-phase motor
combinations. This text will discuss only five basic designs:
Single-Phase Motor Starting
In addition to the run or main winding, all
induction single-phase motors are equipped with an auxiliary or
start winding in the stator. The auxiliary or start winding
overlaps the main or run winding. This provides the revolving
field necessary to turn the rotor. The terms are used in sets.
The frost group is the run and start set. The second group is the
main and auxiliary winding set. Each group has a common terminal
connection.
Run and Start Winding Set. The term
"run winding" is used to designate a winding that
receives current all the time the motor is in operation. It is
the outermost winding, located next to the motor housing. The
term "run" is used only when the other winding is a
start winding.
A start winding is in parallel with the run
winding. The start winding receives current only during the
initial starting period. Then it becomes disconnected from the
power source. The start winding is the set of coils located
nearest to the rotor (Figure 17-8).
Main and Auxiliary Winding Set. The term
"main winding" is used to designate a winding that
receives current all the time the motor is operating. The main
winding is located next to the motor housing. The term
"main" is used only when the other winding is an
auxiliary winding.
An auxiliary winding receives current all
the time the motor is operating. It is always in parallel with
the main winding. The auxiliary coils are located closest to the
rotor. By creating a winding with better insulating properties
and a motor housing with better heat dissipation qualities, the
auxiliary winding can remain in the circuit as long as the main
winding. This then increases the motor's running load
capabilities.
Common Connection. The auxiliary or start
winding is connected to the main or run winding through a
connection called the common. The auxiliary or start winding is
in parallel with the main or run winding (Figure
17-9). Both the windings in the motor use the same
single-phase power source. The common connection between the set
of windings is necessary to complete the parallel circuit.
Figure 17-10 is a basic one-line
diagram of the split-phase motor. It shows the run and start
winding of the stator as well as the centrifugal switch (CS).
The run and start stator windings are
connected in parallel. If you apply current to both windings and
establish a magnetic field simultaneously, the rotor could do
nothing more than oscillate. Unless two or more slightly out of
phase currents arrive in different windings, torque cannot be
achieved. Every time current changed directions, the magnetic
polarities of the stator coils would switch as well. The induced
rotor EMF and its resulting magnetic field would also switch. No
torque can be produced. Something must be done so that a given
magnetic field in one winding can happen at a slightly different
time than in the other winding, thus producing a pulling or
pushing effect on the established magnetic polarity in the rotor.
The would create motion.
Figure 17-11 illustrates the run
winding (view A) and the start winding (view B) as separate coils of wire. In view C, the two coils are connected at a
common terminal. This is how the two windings are placed in the
circuit in parallel.
Figure 17-12 shows how the start and
run windings are in parallel with the same voltage source
available to each.
Current entering a node must divide between
the two windings (Figure 17-13).
Magnetism is a property of current. Forcing current to arrive at
one winding before it arrives at the other winding would create
the phase difference necessary to create a torque.
The split-phase motor takes advantage of an
increased resistance in the start winding. This is done by merely
making the start winding wire a smaller diameter. Contrary to
popular beliefs, the higher resistance in the start winding lets
the current develop a magnetic field in the start winding before
the run winding.
More current goes into the run winding
because there is less resistance in the wire. The greater current
in the run winding generates a greater CEMF than can be developed
in the start winding. This forces the run current to lag voltage
by about 50 degrees.
The smaller current entering the start
winding generates less CEMF. Power supply EMF quickly overcomes
the start winding CEMF. Start winding current lags voltage by
about 20 degrees. This puts the magnetic field in the start
winding ahead of the run winding by about 30 degrees (Figure 17-14).
In Figure 17-15,
the start winding current precedes the current arriving in the
run winding. The magnetic field develops in the start winding
first. A moment later, the start winding current starts to
diminish, and its magnetic field decreases. As this happens, the
current and the magnetic field in the run winding is increasing.
The induced rotor EMF, resulting current
flow, and magnetic polarity remain the same. The magnetic
polarities of the rotor winding were first developed under the
start winding. Now the increasing magnetic pull of the run
winding, which is displaced physically, attracts the rotor. This
is the phase displacement necessary for torque. The direction of
rotation will always be from the start winding to the adjacent
run winding of the same polarity.
At about 75 percent of the rotor rated
speed, the centrifugal switch disconnects the start winding from
the power supply. Once motion is established, the motor will
continue to run efficiently on the run winding alone (Figure 17-16).
Centrifugal Switch
Many single-phase motors are not designed
to operate continuously on both windings. At about 75 percent of
the rated rotor speed, the centrifugal switch opens its contacts.
It only takes a few moments for the motor to obtain this speed.
An audible click can be heard when the centrifugal switch opens
or closes.
The centrifugal switch operates on the same
principle as the diesel governor flyballs. Weights attached to
the outside periphery of the switch rotate with the rotor shaft (Figures 17-17 and 17-18).
As the rotor shaft speed increases, centrifugal force moves the
weights outward. This action physically opens a set of contacts
in series with the start winding.
Once the start winding is disconnected from
the circuit, the momentum of the rotor and the oscillating stator
field will continue rotor rotation. If, however, the motor is
again stopped, the start winding is reconnected through the
normally closed and spring-loaded centrifugal switch. The motor
can only develop starting torque with both start and run windings
in the circuit.
Reversal of Direction of Rotation
The rotor will always turn from the start
winding to the adjacent run winding of the same polarity.
Therefore, the relationship between the start and run windings
must be changed. To change the relationship and the direction of
rotation, the polarity of only one of the fields must be
reversed. In this manner, only one field polarity will change,
and the rotor will still move toward the run winding of the same
polarity as the start winding. The current entering the run
winding or the current entering the start winding must be
reversed, but not both. Figure 17-19
shows a schematic of the reversal of the start winding.
If the main power supply lines, L1 and L2,
are switched, then the polarity of all the windings will be
reversed. This, however, will not change the direction of
rotation because the polarity of both the start winding and the
run winding reverses. The relationship between the start winding
and the run winding has not changed. The rotor will still turn in
the direction from the start winding to the run winding of the
same polarity (Figure 17-20).
Split-Phase Motor Applications
Split-phase motors are generally limited to
the l/3 horsepower size. They are simple to manufacture and
inexpensive. The starting torque is very low and can be used for
starting small loads only.
Capacitor-start motors are the most widely
used single-phase motors in the marine engineering field. They
are found on small refrigeration units and portable pumps. They
come in a variety of sizes up to 7.5 horsepower. The
characteristic hump on the motor frame houses the capacitor (Figure 17-21).
The capacitor-start motor is derived from
the basic design of the split-phase motor. The split-phase motor
had a current displacement, between the start and run winding, of
30 degrees with wire resistance alone. To increase this angle and
increase motor torque, a capacitor can be added. The product of
capacitance can be used to increase the current angles, or in
other words, to increase the time between current arrival in the
start and current arrival in the run windings. In capacitance,
current leads voltage.
The capacitor, unlike a resistor, does not
consume power but stores it so it can be returned to the circuit.
The combining of the inductive run (current lagging) winding and
the capacitive start (current leading) winding would create a
greater current displacement. This would increase the torque.
Capacitor Application
The capacitor is placed in series with the
start winding. Figure 17-22 shows a
line diagram of its position. Optimum torque can be delivered if
the current entering the run and the start winding is displaced
by 90 degrees. With this in mind, and knowing an inductive run
winding current can lag voltage by 50 degrees, an appropriated
capacitor can be selected. A capacitor that can effectively
produce a current lead of 40 degrees would give the optimum
90-degree displacement angle (Figure 17-23).
Once the motor has attained 75 percent of
its rated speed, the start capacitor and start winding can be
eliminated by the centrifugal switch. It is not necessary for
this motor to operate on both windings continuously.
The capacitor of the capacitor-start motor
improves the power factor of the electrical system only on
starting. Letting a capacitor remain in the circuit will improve
the electrical power factor that was modified initially by the
use of a motor. The permanent capacitor is placed in series with
one of the windings. The two windings are now called the main and
auxiliary (sometimes called the phase) windings. They are
constructed exactly alike. Both are left in the circuit during
the operation of the motor. A centrifugal switch is no longer
needed. Another switch will let the capacitor be connected to
either the main or auxiliary winding. The advantage of this is
the comparative ease in which the capacitor can be connected to
the main or auxiliary winding to reverse direction of rotation.
The capacitance forces the current to lead the voltage in the
winding it is connected to. This means that the magnetic field is
developed in the capacitor winding first.
Certain disadvantages become apparent. The
permanent-capacitor motor is very voltage-dependent. How much
current delivered to the winding depends on the capacity of the
capacitor and the system voltage. Any fluctuation in line voltage
affects the speed of the motor. The motor speed may be reduced as
low as 50 percent by small fluctuations. Speed changes from no
load to full load are extreme. No other induction motor undergoes
such severe speed fluctuations.
When additional torque is required to start
and keep a motor operating, additional capacitors can be added.
An excellent example is the refrigeration compressor. A lot of
torque is required to start the motor when the compressor it
turns may be under refrigerant gas pressure. Also, the compressor
may become more heavily loaded during operation, as the
refrigeration system requires it. In this case, the high starting
torque of the start capacitor motor and an increased phase angle
while the motor is running are needed to handle additional torque
requirements.
Figure 17-24 shows the two-capacitor
motor. It is commonly referred to as the
capacitor-start/capacitor-run motor. Notice that the start
capacitor is in series with the auxiliary winding. The
centrifugal switch is used to control the start capacitor in the
same manner as it did in the capacitor-start motor. This
capacitor is used only to develop enough torque to start the
motor turning.
The run capacitor is connected in parallel
with the start capacitor. In this manner, both capacitor
capacitances add together to increase the total phase angle
displacement when the motor is started. Also, the run capacitor
is connected in series with the auxiliary winding. With the run
capacitor connected in series with the auxiliary winding, the
motor always has the auxiliary winding operating, and increased
torque is available.
At about 75 percent of the rated motor
speed, the centrifugal switch opens and removes the start
capacitor from the auxiliary winding. The run capacitor is now
the only capacitor in the motor circuit.
The capacitor is the heart of most
single-phase revolving field motors. If the single-phase motor
fails to operate, always check the source voltage first. Then
check the fuses or circuit breakers. If these areas are operable,
check the capacitor. Visually inspect the capacitor for cracks,
leakage, or bumps. If any of these conditions exist, discard the
capacitor immediately.
Always discharge a capacitor before
testing, removing, or servicing the single-phase motor. This
is done by providing a conductive path between the two
terminals.
Never connect a capacitor to a
voltage source greater than the rated voltage of the
capacitor. Capacitors will explode violently due to excessive
voltage.
Capacitor Operation
A capacitor is not a conductor. Current
does not pass through the device as it would a resistor or motor
winding (Figure 17-25). Instead, the
capacitor must depend on its internal capacity to shift
electrons.
The power supply voltage establishes a
magnetic polarity at each plate. Remember, even AC generators
establish a freed polarity (or difference in potential)
throughout the distribution system. However, the polarity changes
120 times a second. The capacitor plates change polarity from
negative potential and positive potential rapidly, depending on
the frequency of the generated voltage (Figure
17-26).
Between the two capacitor plates is an
insulator called a dielectric. The dielectric can store energy in
an electrostatic field, known commonly as static electricity.
This is done in the following manner: The electrons in the
dielectric of the capacitor are tightly bound in their orbits
around the nucleus of their atom. A positive polarity is
established in one capacitor plate by virtue of the connection to
the positive ion terminal of the generator. A negative polarity
is established in the other plate of the capacitor by virtue of
the negatively charged electrons from the other generator
terminal.
The positive polarity at the capacitor
plate pulls the negative electrons in the dielectric. The
negative polarity at the other plate pushes the dielectric
electrons away. The distorted electron orbit has energy much like
that found in a stretched out spring. When the spring is no
longer forcibly held in the extended position, it pulls itself
back together (Figure 17-27).
The greater the circuit voltage, the
greater the difference in potential at the capacitor plates. The
stronger the magnetic effects at the capacitor plates, the
greater the effect on the electrons in the dielectric.
When the voltage in the AC system is
reduced, before changing its direction, the magnetic field
decays, and the dielectric electrons are pulled back into their
original orbits by their nucleus. This movement of dielectric
electrons offsets all the other electrons throughout the
capacitor circuit (Figure 17-28).
This generates the electron flow (current) that is required to
produce the desired magnetic effects in motors. Current flows
through the circuit in the opposite direction as would have been
originally intended by the generator. Because of this action,
current now arrives before the voltage of the next comparable
voltage direction.
Capacitor Inspection
The internal condition of a capacitor maybe
checked with an ohmmeter (Figure 17-29).
Always consult the manufacturer's manuals or appropriate
technical manuals for specific information on the capacitor being
inspected. Remove the capacitor from the motor and disconnect it.
Always short the capacitor terminals before making a test. If a
spark occurs when you short the capacitor terminals, this is a
good indication that the capacitor is serviceable and maintaining
its charge.
The capacitor starting tool should
have an insulated handle. The actual shorting bar should be
high-resistance (15k to 20k ohms).
Consult the meter manual to determine the
correct range for testing capacitors with the ohmmeter. This is
usually a range that provides the highest internal battery
voltage from the ohmmeter.
Connect the meter leads to the terminals.
Notice the meter display. A good capacitor will indicate charging
by an increase in the display's numerical value. This indicates
that the capacitor is accepting the difference in potential from
the ohmmeter's battery. Once the display stops charging, remove
the meter leads and discharge the capacitor (short the
terminals).
Reconnect the ohmmeter again, but this time
remove one of the meter leads just before the meter display would
have indicated the capacitor has stopped charging. Remember the
display reading. Wait 30 seconds and reconnect the ohmmeter leads
to the same capacitor terminals. The meter's display should start
off with the value displayed before removing one ohmmeter lead.
If the meter returns to zero, this indicates that the capacitor
is unable to hold its charge and must be replaced.
NOTE: Digital meters require
some familiarity before this test can be done with a degree
of confidence. It may take a moment for the digital meter to
display the correct reading upon reconnection. Practice with
known good capacitors.
Shorted and Open Capacitors
Capacitors that are shorted or open will
not display a charge on the ohmmeter. These meters will show
either continuity or infinity.
A shorted capacitor means that the plates
of the capacitor have made contact with each other and pass
current readily. This will be indicated by a very low and steady
resistance reading on the ohmmeter. A shorted capacitor must be
replaced.
An open capacitor means that the distance
between the plates of the capacitor is too far apart. The
magnetic fields are not close enough to properly distort the
electrons and their nucleus in the dielectric. The ohmmeter will
not show a charging condition. For example, when the terminals of
the capacitor have become disconnected from the capacitor plates,
there will bean indication of infinite or maximum meter
resistance. The capacitor must be replaced.
Types of AC Motor Capacitors
There are two capacitors commonly found on
single-phase motors: the start capacitor, which has a plastic
housing, and the run capacitor, which has a metal housing.
The start or electrolytic capacitors are
encased in plastic and have as much as 20 times the capacitance
of the run capacitor. One of the plates consists of an
electrolyte of thick chemical paste. The other plate is made of
aluminum. The dielectric is an aluminum oxide film formed on the
aluminum plate surface. These capacitors cannot be operated
continuously.
Run or paper capacitors are generally used
for the motor-running circuit in the single-phase motor. These
capacitors are encased in metal and made durable for continuous
operation. The internal construction is made of two or more
layers of paper rolled between two layers of aluminum foil (Figure 17-30).
AC Capacitors
The start winding of a single-phase motor
can be damaged if the run capacitor is shorted to ground. This
type of damage can be easily avoided if care is taken when
installing replacement capacitors.
Manufacturers mark the capacitor terminal
connected to the outermost foil. General Electric uses a red dot.
Cornell Dubilier indents a "dash." Sprague points an
arrow to the problem terminal. When the outer foil fails and
comes in contact with the capacitor housing, a short to ground
completes a circuit which bypasses the normal circuit protection.
When this happens, the start winding can be destroyed. To prevent
this casualty from developing, connect the marked terminal to the
"R" or power supply line. Never connect the
marked terminal to the "S" (start) terminal.
DC Capacitors
The discussion on capacitors has been
directed toward the AC capacitor. Our field technology, however,
spans decades of marine engineering. For this reason, a few
cautions are in order for installing DC capacitors.
The DC capacitor is designed differently
from the AC capacitor. The DC capacitor must be placed in the DC
circuit in one position only. Always connect the positive
terminal of the capacitor to the positive conductor in the DC
circuit. Connect the negative terminal in a like manner to the
negative conductor. Always observe the polarity of the capacitor.
The terminals will be marked positive(+) and negative (-). If the
capacitor terminals are incorrectly connected in the circuit, the
capacitor will be ruined.
Never connect the DC capacitor in an
AC circuit. If this is done, the DC capacitor can explode.
Capacitor Rating
Capacitors are rated by the amount of
current that results from the changing frequency of the generated
voltage. Every time voltage changes polarity, current is
displaced through the capacitor circuit. This action is a
measurement of farads (F). A capacitor has a capacity (to
displace electrons) of 1 farad when a current of 1 ampere (6.242
x 10 to the 18th electrons per second) is produced by a rate of
change of 1 volt per second.
The farad is an extremely large value for
our motor applications. Most common motor capacitor ratings will
be found in the microfarad range.
The capacitance of a capacitor is
determined by its construction. The area of the capacitor plates
as well as the dielectric material and thickness determine the
capacity. Always select a capacitor by the capacitance desired
(farad rating) and the voltage rating of the system.
Capacitor Characteristics
When two capacitors are connected in
series, the magnetic effects that distort the electron's orbit
are further apart. Remember that distance determines the
influence that can be exerted by a magnetic field. The capacitor
is not a conductor so that only the outermost capacitor plates
have a magnetic polarity when they are connected in series (Figure 17-31).
The total capacitance of capacitors
connected in series can be derived by using the product-over-sum
method (as used for determining resistance in a parallel
circuit). Notice that the total capacitance is now less than the
smallest capacitor.
Capacitors connected in parallel are like
adding extra storage batteries in parallel (Figure 17-32). The voltage does not change,
but the current, or ability to move electrons, increases. To
determine the total capacitance of the circuit, add all the
capacitors in parallel.
Voltage is constant in a parallel circuit.
This provides an equal positive potential at every capacitor
plate connected by a node. A negative potential is also available
at the other plates of the other capacitors. In this manner, the
magnetic effects available from a difference in potential
(voltage) can be most effectively used to displace electrons in
the dielectric.
The shaded-pole motor does not use two
windings to develop the torque necessary to turn the rotor.
Instead, the stator pole piece is divided into two sections. One
section has a copper ring encircling the tip (Figure 17-33).
Alternating current enters the stator
winding field coil surrounding the stator pole. A magnetic field
is readily developed in the stator pole portion without the
copper ring.
This expanding magnetic field develops an
EMF and resulting magnetic field in the squirrel cage rotor of
the opposite polarity of the stator field that induced it. In
other words, the stator pole might have been a north polarity,
but by virtue of the property of induction, the polarity in the
squirrel cage rotor winding directly beneath the stator north
polarity would become a rotor pole of south polarity.
While this is happening, the copper ring
has impeded the developing magnetic field in the shaded-pole
section of the stator pole piece. First, the growing magnetic
field expands across the copper ring. The copper ring is
short-circuited, like the winding in an induction motor rotor,
and an EMF is induced in the ring. An EMF is induced into the
copper ring (shaded pole) by the impeded, yet expanding magnetic
field. Since the copper ring is short-circuited a current ensues.
With this shaded pole current, a magnetic field is established.
All of this takes time and inhibits the magnetic field from
developing, or decaying, during the same time as the remaining
field winding.
By the time the magnetic field finally
becomes established in the shaded-pole section of the pole piece,
the current flow through the field coil encompassing the entire
pole piece has stopped. The shaded-pole section has developed a
strong north pole. The unshaded portion weakens rapidly because
of the elimination of current in the field coil.
The shaded-pole section retains its
magnetic field longer because it takes longer for the field to
collapse. The magnetic field developed in the copper ring
collapses first. This relative motion of the collapsing field
helps induce and sustain an EMF. The resulting current flow and
magnetic field are momentarily maintained in the pole piece
surrounded by the copper ring.
The property of induction states that
induction opposes a change in current. This reluctance to stop
current flow maintains the magnetic field longer.
The south polarity developed in the rotor
winding directly under the unshaded portion of the pole piece is
now attracted to the stronger magnetic field of the shaded-pole
section. This is how torque is developed.
Figure 17-34 shows the magnetic field
developed in the unshaded portion of the stator pole, the field
developed in the shaded stator pole section, and finally the
field developed in the copper ring. All these things happen very
rapidly, but at different periods in time.
Shaded-pole motors are low cost but are not capable of
developing enough torque to turn large equipment. Shaded-pole
motors usually range from 1/500 to 1/4 horsepower.
HOMEPAGE