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BACKGROUND
Single phase alternating current electric motors conventionally are
provided with two windings on a stator core. These windings are
inductively coupled to the rotor of the motor. Such motors are widely used
for various purposes and range in size from very small fractional
horsepower motors on up to multiple horsepower sizes. Single phase motors
are particularly popular, since most home and business alternating current
supplies are in the form of single phase power.
Single phase electric motors include a stator core, which is wound with a
start winding and a run winding connected to the source of operating
power. These windings surround and are inductively coupled to a rotor
which rotates a shaft to produce the motor output. Rotors are made in a
number of different configurations, such as squirrel cage rotors, high
resistance rotors, low resistance rotors, wound rotors, or multiple
winding high and low resistance rotors. All of these configurations, along
with various stator winding arrangements, are well known in the electric
motor industry.
Typically, the start winding is made of relatively small diameter wire; and
the run winding is made of equal or relatively large diameter wire,
compared to the diameter of the start winding. These windings are
physically and electrically angularly displaced from one another on the
stator.
In conventional capacitor-start and capacitor-start/capacitor-run motors, a
starting capacitor is connected in series with the starting winding and a
switch. At motor start-up, the switch is closed and the capacitor, in
conjunction with the relatively small diameter starting winding, produces
a leading current in the starting winding which is approximately equal to,
and approximately 90.degree. displaced in phase from, the lagging current
in the main or run winding of the motor. Such arrangements produce high
values of starting torque.
Usually, a conventional capacitor-start motor has a centrifugal or thermal
switch connected in series with the capacitor and start winding across the
input terminals. The run winding is connected in parallel with this
series-connected starting circuit. The starting condition is such that the
instantaneous locked rotor current is high; and the motor starting current
demand factor also is high. As a consequence, such motors undergo
relatively high operating temperatures. Because the starting winding of
such motors generally is a relatively small diameter wire, overheating can
and frequently does occur. Such overheating results in a relatively
limited life of the starting winding due to burnout, particularly under
overload conditions of operation of the motor.
Applicant has developed capacitor-start/capacitor-run motors which do not
use small diameter starting windings, but instead utilize two
series-connected windings (of substantially the same diameter heavy wire)
electrically phase displaced 90.degree. from one another on the stator
core.
Applicant's U.S. Pat. Nos. 4,734,601 and 4,772,814 disclose motors in which
one of the windings has a capacitor connected in parallel with it to form
a parallel resonant circuit at the operating frequency of the motor. These
motors are high efficiency motors which overcome most of the disadvantages
of the prior art capacitor-start/capacitor-run motors. The motors
disclosed in both of these patents, however, have relatively low starting
torque. Consequently, such motors primarily are suitable for use in
situations which do not require very high starting torques, such as pumps,
blowers, machine tools and many commercial and domestic appliances.
For situations where higher starting torques are required, a variation of
the parallel resonant configuration has been developed. This is disclosed
in applicant's U.S. Pat. No. 4,675,565. The motor disclosed in this patent
also use a parallel resonant circuit at the operating frequency of the
motor. In addition, however, a second capacitor is connected in series
with a switch in parallel with the first capacitor. This switch is closed
during start-up of the motor, and is opened during normal load conditions
of operation of the motor. This permits a substantial increase in the
starting torque of the motor. During normal operating or running
conditions of the motor when the switch is opened, the parallel resonant
circuit functions in the same manner as disclosed in the motors of U.S.
Pat. Nos. 4,734,601 and 4,772,814.
Applicant also has developed a motor with improved starting torque which
utilizes a series resonant circuit formed by the run winding and a
capacitor having a high capacitance. The start winding is connected in
parallel with the series connected run winding and capacitor. This system
is disclosed in applicant's U.S. Pat. No. 4,794,288. During full load and
no load running conditions of operation of the motor, the major portion of
the current passes through the run winding and capacitor, with lower
current (approximately 25% to 50%) flowing through the start winding.
All of the motors disclosed in the above-identified patents are designed
for single speed operation. Multiple-speed single phase induction motors
typically include internally connected motor switches in the form of
centrifugal switches, relays, or the like. Such internally located
switches frequently produce arching and additionally require space for
accommodating such switches. The location of internally connected motor
switches also subjects the switches and the mechanism for operating them
to increased temperatures from the motor windings, particularly when the
motor is operating under heavy load and start conditions.
Two prior art multiple-speed single phase induction motors, of the type
mentioned above, are disclosed in the patents to Schaefer U.S. Pat No.
1,961,793 and Michelsen U.S. Pat. No. 2,068,559. Both of these patents
disclose the use of centrifugal switches for disconnecting a relatively
small-sized start winding from the operating circuit for the motor after
the motor attains a pre-established running speed. In the motor disclosed
in the Schaefer patent, the motor always is first started in its
high-speed mode. After start-up has been effected and the centrifugal
switch has removed the starting winding, an external selector switch
which, can be operated to select either medium or low speed windings for
the running operation of the motor. It is always necessary to start the
motor in its high-speed mode.
In the motor disclosed in the Michelsen patent, the motor always must be
started in its low-speed mode. After a minimum running speed has been
attained, the centrifugal switch removes the starting winding from the
circuit. The speed selection switch then can be moved to either an
intermediate or high-speed winding for running the motor. In both Schaefer
and Michelsen, the centrifugal switch is operated to remove the starting
winding from the operating circuit of the motor once the operating speed
of the motor has been attained.
Accordingly, it is desirable to provide a multiple speed induction motor of
the general types described in the above-mentioned prior art patents which
overcomes the disadvantages of such patents and which utilizes switching
controls located externally of the stator rotor and the associated
operating windings of the motor.
SUMMARY OF THE INVENTION
Accordingly, it is an object of this invention to provide an improved
alternating current motor.
It is another object of this invention to provide an improved
multiple-speed alternating current motor.
It is an additional object of this invention to provide an improved
single-phase multiple-speed capacitor induction motor.
It is a further object of this invention to provide a multiple-speed
alternating current motor in which the switches for controlling the motor
speed are located externally of the stator and operating windings of the
motor.
In accordance with a preferred embodiment of the invention, a
multiple-speed single-phase alternating current motor includes a stator
core and a rotor. First and second windings are electrically angularly
displaced from one another substantially 90.degree. on the stator core and
they are inductively coupled to the rotor. A third winding also is wound
on the stator core and inductively coupled to the rotor. A switch is
provided with at least first and second states of operation and is coupled
between a source of alternating current power and the windings. In the
first state of operation, the switch interconnects the source of power to
the first and second windings to operate the motor at a first speed. In
the second state of operation the switch additionally interconnects the
source of power to the third winding, or interconnects the second and
third windings in a different configuration to operate the motor at a
different speed.
Other variations of the multiple speed motor operate such that the switch
varies the impedance or the capacitance of the motor between the two
states of operation to cause different speeds of operation of the motor to
occur.
BRIEF DESCRIPTION OF THE DRAWINGS
FIGS. 1-4 are schematic circuit diagrams of preferred embodiments of four
motors constructed according to the teachings of the present invention.
FIG. 4A is a vector diagram of the voltages occurring across the various
ones of the components shown in FIG. 4 at the start-up condition of
operation.
FIGS. 5-10 are schematic circuit diagrams of other preferred embodiments of
six motors constructed according to the teachings of the present invention
.
DETAILED DESCRIPTION
Reference now should be made to the drawings, where the same reference
numbers are used in the different figures to designate the same or similar
components.
FIG. 1 is a schematic diagram of a two speed motor operating as a
capacitor-run motor in the high speed mode of operation thereof. In the
circuit of FIG. 1, single phase alternating current power is supplied from
a suitable source 9 through a double-pole single-throw switch 10/12. A run
winding 13 is wound on the stator core (not shown) and is connected in
series with a run capacitor 17 to the switch 10/12 through contacts 20, 23
of a three position switch in the high speed mode of operation of the
motor. A start winding 14 is connected in parallel with the winding 13 and
capacitor 17. The stator on which the windings 13 and 14 are placed
surrounds a rotor 15, so that inductive coupling between the rotor 15 and
the windings takes place to cause rotation of the rotor 15 when
alternating current power is applied to the windings 13 and 14.
In the lowermost position of the three position switch 20, 22, 23, 24,
shown in FIG. 1, the moveable contact 20 engages the contact 22 of the
switch. This is the "off" position of the switch, and no power is applied
to any of the windings of the motor in this position. The switch 20 is
capable of movement from the position 22 counterclockwise to the positions
23 and 24, and then back to the position 22, in sequence, as illustrated.
When the switch 20 is moved to the contact point 23, the motor is
interconnected for its high speed mode of operation as described above.
The wire size of the winding 13 is large compared to that of a second
"run" winding 14. Typically, the wire 13 is wound with No. 16 or No. 17
wire, while the wire size of the winding 14 is No. 18 or No. 20 wire. The
capacitace of the capacitor 17, is approximately 80 microfarads. The motor
with the switch interconnecting the moveable contact 20 with the contact
23, is configured as series resonant capacitor motor having the
characteristics of the motors disclosed in U.S. Pat. No. 4,794,288.
During the operation of the motor from start through no-load and/or
full-load conditions, the capacitor 17 in series with the winding 13 forms
a series resonant circuit, the resonance of which is selected to be at or
near the 60 Hz frequency of the power supply 9. Obviously, if power
supplies of different frequencies, such as 50 Hz or 120 Hz are used, the
resonance of the series resonant circuit consisting of the winding section
13 and the capacitor 17 is selected to match the frequency of the
particular alternating frequency source 9. The capacitor 17 is an
alternating current non-polarized capacitor and may be an electrolytic
capacitor, a metallized foil capacitor, or a metallized polypropylene
capacitor.
At start-up, the motor of FIG. 1 causes a substantial portion of the total
current to flow through the winding 14. This current, however, rapidly
drops to a low current, with essentially all of the operating current
flowing through the run winding section 13. This shift of the current flow
occurs auto matically as a result of the characteristics of the series
resonant circuit, so that the winding 14 may be made of relatively small
diameter wire. This winding obviously functions as a start winding as a
result.
There is little or no danger of burnout of the winding 14, since it never
carries any high current for any prolonged period of time. In fact, during
normal run operation of the motor, the start winding 14 could be switched
entirely out of the circuit if desired. This is not necessary, however,
since the current through the winding 14 automatically drops to a low
current (approximately 25% to 50% of the current in the winding 13) due to
the inherent operating characteristics of the motor. The three-position
switch in the position shown, with the contacts 20 and 23 interconnected,
causes the motor of FIG. 1 to operate in its high-speed, high-torque mode
of operation. In this mode, at full load, the currents in the windings 13
and 14 are electrically dephased or out of phase by 90.degree..
When the moveable contact of the switch 20 is moved to the next
counterclockwise position, it interconnects the contact 24 with the
primary on/off switch 10/12. In this position of operation, it is readily
apparent that the winding 14 and the series connected winding 13 and
capacitor 17 are disconnected from the power supply 9. At the same time,
however, a third run winding 25 is connected in series across the power
supply 9. The number of turns of the winding 25 is selected to provide a
desired low speed operation of the motor from the power supply 9. The wire
size of the winding 25 is a relatively smaller wire size comparable to the
wire size of the winding section 13.
When the winding 13 and capacitor 17 are switched out of the circuit, the
capacitor 17 discharges through a relatively large power dissipating
resistor 18, in the manner described in U.S. Pat. Nos. 4,794,288 and
4,734,601, to prevent discharge of the capacitor 17 through the winding
13. Consequently, no noise or chattering of the rotor 15 takes place when
the high speed winding sections 13 and 14 are disconnected from the power
supply. To turn the motor off, the switch moveable contact 20 is rotated
further counterclockwise to again engage the contact 22, thereby
disconnecting all of the windings from the power supply 9.
The switch 20, 22, 23, 24, is located externally of the motor, so it is not
subject to the operating environment of the windings 13, 14, and 25.
Reference now should be made to FIG. 2, which also discloses a two-speed
motor utilizing many of the same operating characteristics which have been
described above in conjunction with the motor of FIG. 1. The motor of FIG.
2, however, is directed to a parallel resonant, single-phase motor of the
general type disclosed in U.S. Pat. No. 4,772,814. Reference should be
made to that patent for a description of the operating characteristics of
this motor. The parallel resonant motor of FIG. 2, however, has been
modified so that it operates as a parallel resonant motor in the high
speed mode of operation only. It operates with a separate low speed run
winding in the same manner as described in conjunction with the embodiment
of FIG. 1 when it is switched to the low speed mode of operation.
The speed selection switch of the motor of FIG. 2 is a three-pole
double-throw switch comprising interconnected contact arms 30 and 31,
which are connected to the switch contact 10, and a third contact arm 32,
which is connected to the contact 12 of the switch 10/12. When the switch
30, 31, 32, is operated to the right-hand position shown in FIG. 2, the
motor is operated in its high speed mode. The windings 13 and 14 are
connected in series with one another across the power supply 9 through the
switch 10/12 to rotate the rotor 15 of the motor. A capacitor 27 is
connected in parallel with the winding 13 and forms a parallel resonant
circuit with the winding 13 at the operating frequency of the motor. A
power dissipating resistor 28 is connected across the capacitor 27 and
functions in a manner comparable to the function of the resistor 18,
described in conjunction with FIG. 1. So long as the switch 30, 31, 32, is
in the right-hand position shown in FIG. 2, the motor operates in its high
speed mode of operation with operating characteristics of the type
described in U.S. Pat. No. 4,772,814.
When it is desired to operate the motor of FIG. 2 in its low speed mode of
operation, the switch 30, 31, 32, is switched to the left-hand position.
In this position, the windings 13 and 14, and the capacitor 27 are
disconnected from the circuit. A low speed run winding 25, however, then
is connected across the power supply 9 through the switch 10/12 and by
means of the contacts 31 and 32 to continue operation of the motor. The
operation in this low speed mode is identical to that described in
conjunction with the embodiment of FIG. 1 when that motor is operated in
its low speed mode of operation.
FIG. 3 is another variation of a parallel resonant motor similar to the one
of FIG. 2. The motor of FIG. 3, however, employs a split winding 13A, 13B
in place of the single start winding 13 of the motor of FIG. 2. A
capacitor 27 is connected in series with the winding 13B and this series
circuit in turn is connected in parallel with the winding 13A when the
switch 30, 31 is closed to the right-hand position shown in FIG. 3. The
circuit including the winding 13B and capacitor 27 is in parallel
resonance with the winding 13A at the operating frequency of the motor. In
all other respects, the embodiment of FIG. 3 operates in the same manner
as that of FIG. 2. In the low speed operation of the motor, the winding 25
is energized and the windings 13A, 13B and 14 are disconnected from the
circuit. The interconnections of the switch 30, 31, and 32, are the same
as used for the circuit of FIG. 2.
It should be noted that the switch 30, 31, 32, of FIGS. 2 and 3 is located
externally of the motor. Normal operation of the motor is such that the
rest position of the switch 30, 31, and 32, of the motors of FIGS. 2 and
3, after the motor has been turned off by opening the switch 10/12, is to
the right-hand position. Consequently, when the switch 10/12 next is
turned on, the motors of FIGS. 2 and 3 start operation in the high speed
mode. This ensures that overheating of the low speed coil 25 does not
occur. This also provides maximum starting torque for the motor. The
manner in which the switch 30, 31, and 32 reverts to its right-hand
position after opening of the switch 10/12 is not shown, but this may be
effected in any of a number of conventional readily available techniques.
The switch is a standard three-pole, double-throw switch having this
characteristic.
FIGS. 4, 5, and 6, all are variations of series resonant capacitor motors
of the general type disclosed in FIG. 1. Each of the motors of these three
different circuits, however, has operating characteristics which differ
somewhat from those of the motor of FIG. 1. The motor of FIG. 4 employs a
split start winding 14A, 14B, and utilizes a large wire size (No. 17) for
the windings 13 and 14B and a small wire size (No. 22) for the winding
14A.
In place of the three-position rotary switch shown in FIG. 1, the speed
selection switch illustrated in FIGS. 4, 5, and 6, is a three-pole,
double-throw switch similar to the one used in the embodiments of FIGS. 2
and 3. The wiring interconnections of the various poles of the switch,
however, differ from those shown in FIGS. 2 and 3. In FIG. 4, the moveable
contact 30 of the three-pole switch is connected to the contact 10 of the
on/off switch 10/12. Similarly, the moveable contact 31 of the three-pole
switch is connected to the contact 12 of the on/off switch. The moveable
contact 32 of the three-pole switch is connected to the capacitor 17.
Consequently, in the right-hand position of the motor, current flows
through the contact 30, the winding 13, the series resonant capacitor 17,
the contact 32, and the winding 14B (in the upward direction shown in FIG.
4), and through the contact 31 back to the switch 12. In addition, a
parallel circuit path is provided in the downward direction (as viewed in
FIG. 4) through the winding section 14A. Thus, current flow through the
winding sections 14A and 14B in the high speed mode of operation of the
motor is in opposite directions.
The purpose of using opposite current flow through the winding sections 14A
and 14B, is to employ a transformer effect to produce a
counter-electromotive force secondary induction voltage into the circuit
of the run winding 13 and the capacitor 17 of the motor of FIG. 4. This
generates a high voltage vectorially and across the capacitor 17, thereby
increasing the voltage of the capacitor 17, so that the size of the
capacitor is smaller than would otherwise be required. The winding 14B of
FIG. 4 effectively replaces a reactor transformer for the purpose of
increasing the capacitor voltage of the capacitor 17.
When the motor of FIG. 4 is to be operated in the low speed mode, the
three-pole switch is operated to its left position. This position does not
change the current flowing through the series resonant circuit consisting
of the winding 13 and the capacitor 17. This position, however, does cause
the two winding sections 14A and 14B to be connected in electrical series
with one another and in parallel with the series-connected capacitor 17
and winding 13 across the power supply 9. The turns ratios of the windings
13, 14A and 14B are selected to provide the desired high and low speeds of
operation of the motor. As with the motors described above in conjunction
with FIGS. 1, 2 and 3, the switch 30, 31, 32, is located externally of the
motor.
FIG. 4A is a vector diagram of the voltages occurring across various ones
of the components of the circuit of FIG. 4 at the start-up condition of
operation. When power initially is applied to the motor at start-up, the
line voltage is applied across the winding section 14A, and is illustrated
by vector A in FIG. 4A. This vector coincides with the positive "X" axis
of the vector diagram, and is substantially the full line voltage,
approximately 230 volts. At the same time, because of the transformer
effect which takes place in the circuit, the voltage vector B across the
winding 13 is out of phase with the voltage vector across the winding 14A,
and is in the negative quadrant of the vector diagram. Similarly, the
voltage across the winding 14B is shown in vector C of the diagram of FIG.
4A. The composite of these vectors, B and C, produces the resultant vector
D to establish the starting point of the voltage across the capacitor 17,
as illustrated in the vector E. An examination of FIG. 4A clearly shows
that the voltage across the capacitor 17 is substantially more than the
line voltage applied across the winding 14A. This effect is produced in
the manner described above, in the description of FIG. 4.
FIG. 5 is directed to a series capacitor motor which is a variation of the
motor shown in FIG. 4. A single winding 14, however, is used in place of
the split-winding of FIG. 4. In addition, a second capacitor 37 is
connected in parallel with the capacitor 17 when the motor is operated in
its high-speed mode of operation. The capacitor 37 comprises the sole
series resonant capacitor for the motor when it is operated in its
low-speed mode of operation. In the motor of FIG. 5, all three of the
moveable contacts 30, 31, and 32 of the three-pole double-throw switch are
interconnected in common to the terminal 12 of the on/off switch 10/12.
The terminal 10 of the switch is connected to the junction of the windings
13 and 14 of the motor. Consequently, when the three-pole switch is
operated to the right-hand position shown in FIG. 5, the capacitors 17 and
37 are in parallel with one another and in series with the winding 13. The
winding 14 then is connected in parallel with the circuit comprising the
winding 13 and the capacitors 17 and 37. In the high-speed mode of
operation, at both start and run, the two capacitors 17 and 37 are
connected in parallel to enable the motor to develop high starting torque
and low current. During the high-speed mode of operation from start to
operating frequency, this circuit remains.
At low speed, the three-pole double-throw switch 30, 31, 32, is moved to
the left-hand position shown in FIG. 5. The winding 4 continues to be
supplied with power, as when the switch is in the right-hand position.
Similarly, the series resonant circuit, consisting of the winding 13 and
the capacitor 37, remains in the circuit. The capacitor 17, however, has
been disconnected from the circuit and discharges through the high
impedance discharge resistance 18 when the switch is moved from the
right-hand position to the left-hand position.
When the capacitor 17 is removed from the circuit for low speed operation,
the current in the windings 13 and 14 is reduced; and the motor develops a
high impedance in the circuit which produces a high resistance rotor slip
to permit variable speed operation of the motor. The circuit of FIG. 5
employs a variable capacitance to change the impedance (increase it) at
low speed when the motor is under its load condition of operation. It is
necessary for this motor to be loaded to vary the speed from its high
speed mode of operation to a low speed mode.
The motor shown in FIG. 6 also is directed to a two-speed parallel
capacitor motor, which is a variation of the motor shown in FIG. 3. As
with the motor of FIG. 3, the motor of FIG. 6 employs a split winding 13A,
13B in place of a single start winding 13. The split winding section,
however, has the two winding portions electrically connected in series,
with the capacitor 27 connected in series with the winding 13B. This
series circuit in turn is connected in parallel with the winding 13A when
the switch 30 is closed to the right-hand position shown in FIG. 6. The
motor of FIG. 6 otherwise operates in a manner similar to the operation of
the motor of FIG. 3, but a high voltage, small-size capacitor 27 is used
in the parallel resonant circuit.
FIGS. 7 and 8 are directed to variations of a motor using a speed
responsive switch which performs two functions. First, irrespective of
whether the speed control switch 40 is in its high speed or low speed
position when power is first applied through the switch 10/12, the motors
of FIGS. 7 and 8 always start in the high-speed mode of operation. If the
control switch 40 is in its low-speed (left-hand) position, the motor
automatically drops back to its low speed mode of operation once a
predetermined rotational speed of the rotor 15 is attained. Conversely, if
the speed selection switch 40 is in the high-speed (right-hand) position
at motor start-up when the power supply switch 10/12 is closed, it
continues to operate in the high-speed mode of operation; and the speed
responsive switch has no affect on the motor operation. In all other
respects, the motors of FIGS. 7 and 8 operate in a manner identical to the
operation of the motor described in conjunction with FIG. 1. Both of these
motors are series resonant motors. The motor of FIG. 8 also includes a
separate capacitor start circuit for improving the starting torque of the
motor.
Reference now should be made to FIG. 7. The components of the motor of FIG.
7 which differ from those of the motor of FIG. 1 include a centrifugal
switch having an operator 41 and a moveable contact 43. In addition, the
switch 20 of FIG. 1 has been replaced by a single-pole double-throw switch
40. When the switch 40 is in the position shown in FIG. 7, the motor is in
its high speed mode of operation. At start-up, the moveable contact 43 of
the centrifugal switch 41, 43 is in the lower position, as illustrated in
FIG. 7. Thus, when power is applied to the motor by closure of the switch
10/12, the initial starting conditions are identical to those described
previously for the high speed mode of operation of the circuit of FIG. 1.
When a predetermined operating speed of the motor has been attained, the
centrifugal switch 41 moves the contact 43 from the lower position to the
upper position to interconnect the winding 25 with the left-hand or low
speed terminal of the switch 40. So long as the switch 40, however, is in
the right-hand position, as shown in FIG. 7, this operation has no affect
whatsoever on the motor operation. The high speed mode continues, so long
as the switch 40 remains in the right-hand position.
If, however, after attaining a speed which is greater than the
preestablished speed for operation of the switch 40/43, the switch 40 is
moved to the left-hand position, the winding 25 is energized from the
switch 10/12 in the same manner as described in conjunction with the
circuit of FIG. 1 when the switch contactor 20 is moved to the contact 24
of the circuit shown in FIG. 1. In this condition of operation, the low
speed winding 25 is supplied with operating power, but no current flow
takes place through the windings 13 and 14.
Now assume that the switch 40 of FIG. 7 is in the left-hand position at
motor start-up. When power is applied through the on/off terminals 10/12
from the source of power 9, no current flows through the winding 25 since
the moveable contact 43 is in the position shown in FIG. 7. Current,
however, does flow through the windings 13 and 14 and the capacitor 17
connected in series with the winding 13 through the contact 43. Thus, the
same conditions of operation which are established for start-up in the
high speed mode of operation of the motor of FIG. 7 also are present when
the switch 40 is switched to the left (the low speed side) at start-up.
When the preestablished operating speed for operating the centrifugal
switch 41/43, however, is attained, movement of the contact 43 from the
lower position to the upper position interconnects the run winding 25 with
the power source; but the windings 13 and 14 then are disconnected since
there is no circuit connection made between these windings and the switch
40 in the left-hand position.
Consequently, the motor of FIG. 7 provides an added degree of protection
against overheating of the low speed winding 25 which is not present in
the circuit of FIG. 1. If the motor of FIG. 1 is started in the low speed
position, current flow passes through the low speed winding 25 during the
start-up condition of operation. For the circuit of FIG. 1 this can lead
to overheating of the winding 25. That is the reason the switch 20 of FIG.
1 also is used as the primary switch for turning off the motor rather than
the main power supply switch 10/12. With the circuit of FIG. 7, it is not
possible to start the motor through the run winding 25, since the motor
always starts in the high speed mode of operation, irrespective of the
position of the switch 40.
The circuit of FIG. 8 is similar to that of FIG. 7, but instead of using a
centrifugal switch 41/43, a voltage responsive switch 42 is employed. The
voltage through the operating coil 43 of this switch is used to move a
pair of interconnected moveable contacts 45 and 47 from a lower position
to an upper position when a predetermined operating speed of the rotor 15
of the motor is attained. The operation of the voltage responsive relay 42
is the same as the operation of the centrifugal switch 41/43 of the
circuit of FIG. 7, so far as the results obtained are concerned. The
insurance against starting the motor with power applied only through the
low speed run winding 25 is provided by the circuit of FIG. 8 in the same
manner as the circuit of FIG. 7.
In addition, the circuit of FIG. 8 also includes a separate capacitor start
circuit, comprising the capacitor 37 and the high impedance discharge
resistor 38. This circuit operates in the same manner as the circuit
discribed in conjunction with FIG. 5 for the same purposes. The capacitor
37 is in the circuit at start-up, irrespective of the position of the
switch 40. Once the desired operating speed of the motor has been
attained, the capacitor 37 is switched out of the circuit by movement of
the moveable contact 45 from the lower position shown in FIG. 8 to the
upper position. The motor of FIG. 8 otherwise operates in the same manner
as the motor of FIG. 7.
FIG. 9 illustrates another example of a three-speed, series resonant motor
using a rotary switch of the type employed with the embodiment of FIG. 1.
The motor of FIG. 9, however, is capable of being started in any of its
three operating speeds, and produces high starting torque as a result of
the series resonant circuit produced by the run winding 13 and the
capacitor 17. The motor also must be loaded at all times for proper
operation in a manner comparable to the operation of known DC series wound
motors.
In addition to the run winding 13, which always is in the operating circuit
for the motor, from start-up though selected run speed, three start
windings, 54, 55, and 56, are provided for the corresponding running
speed. The highest speed is obtained by connecting the rotary switch 50
with the contact 53, placing the winding 54 in parallel with the winding
13 and capacitor 17. When the middle position of the three-position switch
50 is selected by connecting the movable switch arm to the medium speed
terminal 52, a pair of windings 54 and 55 comprise a series connected
winding in parallel with the winding 13 and capacitor 17. Finally, when
the movable switch is moved to contact the low speed terminal 51, three
windings, 54, 55, and 56, are connected in series with one another, and in
parallel with the winding 13 and capacitor 17.
The winding 13, in conjunction with the capacitor 17, forms a series
resonant circuit throughout the operation of the motor through its
start-up condition, as well as full load operating conditions, any one of
the three speeds which may be selected by the movable contact 50. The wire
size of the winding 13 is the largest wire size, and, typically, is No. 16
or No. 17 wire. The winding 54 also is a relatively large size winding,
equal in size to the wire of the winding 13, or slightly smaller. The wire
size used for the winding 55 is somewhat smaller than that used in the
winding 54, and, typically, is No. 18 or No. 20 wire. Finally, the wire
size of the winding 56 is still smaller than the size of the winding 55,
to further reduce the speed of the motor and increase impedence of the
starting winding circuit consisting of all three windings, 54, 55, and 56,
when the switch 50 selects the terminal 51.
It is apparent from consideration of the manner in which the windings 54,
55, and 56 are arranged, that the impedence of the composite start winding
comprising one, two or all three of these w | | |
