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Claims  |
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I claim:
1. A polyphase electric motor comprising:
a stator including a core of magnetic material;
a rotor;
a main polyphase stator winding wound on said core and encompassing said
magnetic material, said winding comprising a plurality of coils, each coil
representing a single phase;
a plurality of input terminals adapted to be connected to a source of
polyphase AC voltage;
a plurality of capacitors; and
means connecting each of said coils in a series circuit with one of said
capacitors and said series circuits being connected with said input
terminals;
each of said capacitors being capable of being charged to a voltage
sufficient, when added to said AC voltage, to develop a volt-second value
across the magnetic material of said stator core greater than the
volt-second capacity of said magnetic material so that said magnetic
material will periodically be driven into saturation in opposite
directions upon changes in polarity of the phases of said polyphase AC
voltage.
2. The motor of claim 1 in which switch means are provided in shunt with
each of said capacitors.
3. The motor of claim 2 in which said switch means are centrifugally
operated and are closed at starting.
4. The motor of claim 1 in which said coils and their associated capacitors
are connected in a wye configuration.
5. The motor of claim 1 in which said coils and their associated capacitors
are connected in a delta configuration.
6. The motor of claim 1 in which at least one auxiliary winding is wound on
said core to encompass said magnetic material and connected to a pair of
said input terminals.
7. A polyphase electric motor comprising:
a stator including a core of magnetic material;
a rotor;
a main polyphase stator winding wound on said core and encompassing said
magnetic material, said winding comprising a plurality of coils, each coil
representing a single phase;
a plurality of input terminals adapted to be connected to a source of
polyphase AC voltage;
a plurality of capacitors;
means connecting each of said coils in a series circuit with one of said
capacitors, and said series circuits being connected in a wye
configuration with said input terminals, and said capacitors being capable
of being charged so that the stator core will periodically change
nonlinearly from a non-saturated to a saturated condition, and
an auxiliary polyphase auxiliary winding comprising a plurality of coils
with each coil representing a single phase, said coils of the auxiliary
winding being connected in a wye configuration with said input terminals.
8. A polyphase electric motor comprising:
a stator including a core of magnetic material;
a rotor;
a main polyphase stator winding wound on said core and encompassing said
magnetic material, said winding comprising a plurality of coils, each coil
representing a single phase;
a plurality of input terminals adapted to be connected to a source of
polyphase AC voltage;
a plurality of capacitors;
means connecting each of said coils in a series circuit with one of said
capacitors, and said series circuits being connected in a delta
configuration with said input terminals, and said capacitors being capable
of being charged so that the stator core will periodically change
nonlinearly from a nonsaturated to a saturated condition, and
an auxiliary polyphase auxiliary winding comprising a plurality of coils
with each coil representing a single phase, said coils of the auxiliary
winding being connected in a delta configuration with said input
terminals.
9. A polyphase electric motor comprising:
a stator including a core of magnetic material;
a rotor;
a main polyphase stator winding wound on said core and encompassing said
magnetic material, said winding comprising a plurality of coils, each coil
representing a single phase;
a plurality of input terminals adapted to be connected to a source of
polyphase AC voltage;
a plurality of capacitors;
means connecting each of said coils in a series circuit with one of said
capacitors, and said series circuits being connected in a delta
configuration with said input terminals, and
an auxiliary polyphase auxiliary winding comprising a plurality of coils
with each coil representing a single phase, said coils of the auxiliary
winding being connected in a wye configuration with said input terminals.
10. An electric motor as in claim 9 wherein
each of said capacitors is capable of being charged so that the stator core
will periodically change nonlinearly from a non-saturated to a saturated
condition.
11. A polyphase electric motor comprising:
a stator including a core of magnetic material;
a rotor;
a main polyphase stator winding wound on said core and encompassing said
magnetic material, said winding comprising a plurality of coils, each coil
representing a single phase;
a plurality of input terminals adapted to be connected to a source of
polyphase AC voltage;
a plurality of capacitors;
means connecting each of said coils in a series circuit with one of said
capacitors, and said series circuits being connected in a wye
configuration with said input terminals, and
an auxiliary polyphase auxiliary winding comprising a plurality of coils
with each coil representing a single phase, said coils of the auxiliary
winding being connected in a delta configuration with said input
terminals.
12. An electric motor as in claim 11 wherein
each of said capacitors is capable of being charged so that the stator core
will periodically change nonlinearly from a non-saturated to a saturated
condition.
13. A polyphase electric motor comprising:
a stator including a core of magnetic material;
a rotor;
a main polyphase stator winding wound on said core and encompassing said
magnetic material, said winding comprising a plurality of coils, each coil
representing a single phase;
a plurality of input terminals adapted to be connected to a source of
polyphase AC voltage;
a plurality of capacitors;
means connecting each of said coils in a series circuit with one of said
capacitors and said series circuits being connected with said input
terminals;
each of said capacitors being capable of being charged to a voltage
sufficient, when added to said AC voltage, to develop a volt-second value
across the magnetic material of said stator core greater than the
volt-second capacity of said magnetic material so that said magnetic
material will periodically be driven into saturation in opposite
directions upon changes in polarity of the phases of said polyphase AC
voltage;
said coils and their associated capicators being connected to said input
terminals in a wye configuration; and
a plurality of auxiliary windings wound on said core to encompass said
magnetic material, each of said auxiliary windings being connected in
parallel with one of said series circuits.
14. A polyphase electric motor comprising:
a stator including a core of magnetic material;
a rotor;
a main polyphase stator winding wound on said core and encompassing said
magnetic material, said winding comprising a plurality of coils, each coil
representing a single phase;
a plurality of input terminals adapted to be connected to a source of
polyphase AC voltage;
a plurality of capacitors;
means connecting each of said coils in a series circuit with one of said
capacitors and said series circuits being connected with said input
terminals;
each of said capacitors being capable of being charged to a voltage
sufficient, when added to said AC voltage, to develop a volt-second value
across the magnetic material of said stator core greater than the
volt-second capacity of said magnetic material so that said magnetic
material will periodically be driven into saturation in opposite
directions upon changes in polarity of the phases of said polyphase AC
voltage;
said coils and their associated capacitors being connected to said input
terminals in a delta configuration; and
a plurality of auxiliary windings being wound on said core to encompass
said magnetic material, each of said auxiliary winding being connected in
parallel with one of said series circuits.
15. A polyphase electric motor comprising:
a stator including a core of magnetic material;
a rotor;
a main polyphase stator winding wound on said core and encompassing said
magnetic material, said winding comprising a plurality of coils, each coil
representing a single phase;
a plurality of input terminals adapted to be connected to a source of
polyphase AC voltage;
a plurality of capacitors;
means connecting each of said coils in a series circuit with one of said
capacitors and said series circuits being connected with said input
terminals;
each of said capacitors being capable of being charged to a voltage
sufficient, when added to said AC voltage, to develop a volt-second value
across the magnetic material of said core greater than the volt-second
capacity of said magnetic material so that said magnetic material will
periodically be driven into saturation in opposite directions upon changes
in polarity of the phases of said polyphase AC voltage; and
a plurality of auxiliary windings wound on said core to encompass said
magnetic material, said coils and their associated capacitors being
connected to said input terminals in a wye configuration and said
auxiliary windings being connected to said input terminals in a delta
configuration.
16. A polyphase electric motor comprising:
a stator including a core of magnetic material;
a rotor;
a main polyphase stator winding wound on said core and encompassing said
magnetic material, said winding comprising a plurality of coils, each coil
representing a single phase;
a plurality of input terminals adapted to be connected to a source of
polyphase AC voltage;
a plurality of capacitors;
means connecting each of said coils in a series circuit with one of said
capacitors and said series circuits being connected with said input
terminals;
each of said capacitors being capable of being charged to a voltage
sufficient, when added to said AC voltage, to develop a volt-second value
across the magnetic material of said stator core greater than the
volt-second capacity of said magnetic material so that said magnetic
material will periodically be driven into saturation in opposite
directions upon changes in polarity of the phases of said polyphase AC
voltage; and
a plurality of auxiliary windings wound on said core to encompass said
magnetic material, said coils and their associated capacitors being
connected to said input terminals in a delta configuration and said
auxiliary windings being connected to said input terminals in a wye
configuration.
17. A polyphase electric motor comprising:
a stator including a core of magnetic material;
a rotor;
a main polyphase stator winding wound on said core and encompassing said
magnetic material, said winding comprising a plurality of coils, each coil
representing a single phase;
a plurality of input terminals adapted to be connected to a source of
polyphase AC voltage;
a plurality of capacitors;
means connecting each of said coils in a series circuit with one of said
capacitors and said series circuits being connected with said input
terminals, said capacitors being capable of being charged so that the
stator core will periodically change nonlinearly from nonsaturated to a
saturated condition;
said coils and their associated capacitors being connected to said input
terminals in a wye configuration; and
a plurality of auxiliary windings being wound on said core to encompass
said magnetic material, each of said auxiliary windings being connected
and parallel to one of said series circuits.
18. A polyphase electric motor comprising:
a stator including a core of magnetic material;
a rotor;
a main polyphase stator winding wound on said core and encompassing said
magnetic material, said winding comprising a plurality of coils, each coil
representing a single phase;
a plurality of input terminals adapted to be connected to a source of
polyphase AC voltage;
a plurality of capacitors;
means connecting each of said coils in a series circuit with one of said
capacitors and said series circuits being connected with said input
terminals, said capacitors being capable of being charged so that the
stator core will periodically change nonlinearly from a nonsaturated to a
saturated condition;
said coils and their associated capacitors being connected to said input
terminals in a delta configuration; and
a plurality of auxiliary windings being wound on said core to encompass
said magnetic material, each of said auxiliary windings being connected in
parallel with one of said series circuits.
19. A polyphase electric motor comprising:
a stator including a core of magnetic material;
a rotor;
a main polyphase stator winding wound on said core and encompassing said
magnetic material, said winding comprising a plurality of coils, each coil
representing a single phrase;
a plurality of input terminals adapted to be connected to a source of
polyphase AC voltage;
a plurality of capacitors;
means connecting each of said coils in a series circuit with one of said
capacitors and said series circuits being connected with said input
terminals, said capacitors being capable of being charged so that the
stator core will periodically change nonlinearly from a nonsaturated to a
saturated condition; and
a plurality of auxiliary windings being wound on said core to encompass
said magnetic material, said coils and their associated capacitors being
connected to said input terminals in a wye configuration and said
auxiliary windings being connected to said input terminals in a delta
configuration.
20. A polyphase electric motor comprising:
a stator including a core of magnetic material;
a rotor;
a main polyphase stator winding wound on said core and encompassing said
magnetic material, said winding comprising a plurality of coils, each coil
representing a single phase;
a plurality of input terminals adapted to be connected to a source of
polyphase AC voltage;
a plurality of capacitors;
means connecting each of said coils in a series circuit with one of said
capacitors and said series circuits being connected with said input
terminals, said capacitors being charged so that the stator core will
periodically change nonlinearly from a nonsaturated to a saturated
condition; and
a plurality of auxiliary windings being wound on said core to encompass
said magnetic material, said cores and their associated capacitors being
connected to said input terminals in a delta configuration and said
auxiliary windings being connected to said input terminals, in a wye
configuration.
21. A polyphase electric motor comprising:
a stator including a core of magnetic material;
a rotor;
a main polyphase stator winding wound on said core and encompassing said
magnetic material, said winding comprising a plurality of coils, each coil
representing a single phase;
a plurality of input terminals adapted to be connected to a source of
polyphase AC voltage;
a plurality of capacitors;
means connecting each of said coils in a series circuit with one of said
capacitors and said series circuits being connected with said input
terminals; and
an auxiliary winding wound on said core and encompassing said magnetic
material to function as a generator winding to counteract, when the back
e.m.f. exceeds input voltage, some of the current drawn by the main
polyphase stator winding, said auxiliary winding being connected with said
input terminals and comprising at least one coil.
22. An electric motor as in claim 21 wherein:
said auxiliary winding comprises a plurality of coils with each coil
representing a single phase.
23. An electric motor as in either claim 21 or claim 22 wherein the
capacitors are capable of being charged so that the stator core will
periodically change nonlinearly from a nonsaturated to a saturated
condition.
24. A polyphase electric motor comprising:
a stator including a core of magnetic material;
a rotor;
a main polyphase stator winding wound on said core and encompassing said
magnetic material, said winding comprising a plurality of coils, each coil
representing a single phase;
a plurality of input terminals adapted to be connected to a source of
polyphase AC voltage;
a plurality of capacitors;
means connecting each of said coils in a series circuit with one of said
capacitors and said series circuits being connected with said input
terminals;
each of said capacitors being capable of being charged to a voltage
sufficient, when added to said AC voltage, to develop a volt-second value
across the magnetic material of said stator core greater than the
volt-second capacity of said magnetic core so that said magnetic material
will periodically be driven into saturation in opposite directions upon
changes in polarity of the phases of said polyphase AC voltage; and
an auxiliary winding wound on said core and encompassing said magnetic
material to function as a generator winding to counteract, when the back
e.m.f. exceeds input voltage, some of the current drawn by the main
polyphase stator winding, said auxiliary winding being connected with said
input terminal and comprising at least one coil.
25. An electric motor as claimed in claim 24 wherein:
said auxiliary winding comprises a plurality of coils with each coil
representing a single phase.
26. A motor as claimed in claim 2 wherein said coils and their associated
capacitors are connected in wye configuration.
27. A motor as claimed in claim 2 wherein said coils and their associated
capacitors are connected in delta configuration.
28. A motor as claimed in claim 6 wherein the auxiliary winding has a
higher inductance than each of said coils of the stator windings.
29. A motor as claimed in claim 27, wherein each auxiliary winding has a
higher inductance than the coil of the stator winding representing each
respective phase.
30. A motor as claimed in claim 28, wherein each auxiliary winding has a
higher inductance than the coil of the stator winding representing each
respective phase.
31. A motor as claimed in claim 29, wherein each auxiliary winding has a
higher inductance than the coil of the stator winding representing each
respective phase.
32. A motor as claimed in claim 30, wherein each auxiliary winding has a
higher inductance than the coil of the stator winding representing each
respective phase.
33. A motor as claimed in claim 31 wherein each auxiliary winding has a
higher inductance than the coil of the stator winding representing each
respective phase.
34. A motor as claimed in claim 32 wherein each auxiliary winding has a
higher inductance than the coil of the stator winding representing each
respective phase.
35. A motor as claimed in claim 33 wherein each auxiliary winding has a
higher inductance than the coil of the stator winding representing each
respective phase.
36. A motor as claimed in claim 34 wherein each auxiliary winding has a
higher inductance than the coil of the stator winding representing each
respective phase.
37. A motor as claimed in claim 23 wherein each auxiliary winding has a
higher inductance than the coil of the stator winding representing each
respective phase.
38. A motor as claimed in claim 25 wherein each auxiliary winding has a
higher inductance than the coil of the stator winding representing each
respective phase.
39. A motor as claimed in claim 1 wherein the respective capacitors have a
capacitance large enough to maintain capacitive power factors in the
respective series circuits defined by the respective coils of the main
stator winding and the capacitors.
40. A motor as claimed in claim 2 wherein the respective capacitors have a
capacitance large enough to maintain capacitive power factors in the
respective series circuits defined by the respective coils of the main
stator winding and the capacitors.
41. A motor as claimed in claim 4 wherein the respective capacitors have a
capacitance large enough to maintain capacitive power factors in the
respective series circuits defined by the respective coils of the main
stator winding and the capacitors.
42. A motor as claimed in claim 5 wherein the respective capacitors have a
capacitance large enough to maintain capacitive power factors in the
respective series circuits defined by the respective coils of the main
stator winding and the capacitors.
43. A motor as claimed in claim 23 wherein the respective capacitors have a
capacitance large enough to maintain capacitance power factors in the
respective series circuits defined by the respective coils of the main
stator winding and the capacitors.
44. A motor as claimed in claim 25 wherein the respective capacitors have a
capacitance large enough to maintain capacitive power factors in the
respective series circuits defined by the respective coils of the main
stator winding and the capacitors.
45. A motor as claimed in claim 27 wherein the respective capacitors have a
capacitance large enough to maintain capacitive power factors in the
respective series circuits defined by the respective coils of the stator
winding and the capacitors.
46. A motor as claimed in claim 28 wherein the respective capacitors have a
capacitance large enough to maintain capacitive power factors in the
respective series circuits defined by the respective coils of the main
stator winding and the capacitors.
47. A motor as claimed in claim 29 wherein the respective capacitors have a
capacitance large enough to maintain capacitive power factors in the
respective series circuits defined by the respective coils of the main
stator winding and the capacitors.
48. A motor as claimed in claim 30 wherein the respective capacitors have a
capacitance large enough to maintain capacitive power factors in the
respective series circuits defined by the respective coils of the main
stator winding and the capacitors.
49. A motor as claimed in claim 31 wherein the respective capacitors have a
capacitance large enough to maintain capacitive power factors in the
respective series circuits defined by the respective coils of the main
stator winding and the capacitors.
50. A motor as claimed in claim 32 wherein the respective capacitors have a
capacitance large enough to maintain capacitive power factors in the
respective series circuits defined by the respective coils of the main
stator winding and the capacitor.
51. A motor as claimed in claim 33 wherein the respective capacitors have a
capacitance large enough to maintain capacitive power factors in the
respective series circuits defined by the respective coils of the main
stator winding and the capacitors.
52. A motor as claimed in claim 34 wherein the respective capacitors have a
capacitance large enough to maintain capacitive power factors in the
respective series circuits defined by the respective coils of the main
stator winding and the capacitors. |
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Claims |
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Description |
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BACKGROUND OF THE INVENTION
Present day induction motors such as the squirrel cage type have numerous
limitations. For example, when heavily loaded, they draw excessive
currents as the rotor slows down, which currents can result in motor burn
out unless the motor is protected by auxiliary equipment. Such motors must
have a high breakaway torque to running torque ratio to prevent motor
damage in the event of motor overload, and as a result the flux density
must be maintained at considerably less than saturation levels. This
relatively low flux density during normal operation is also necessitated
by potential input voltage variations. Because the flux density must be
kept relatively low, the motor size must be substantially larger than
would theoretically be necessary in an ideal motor in order to obtain the
desired output horsepower. In addition, the output horsepower available
from such motors is significantly dependent on the line voltage, and to
some extent, line frequency. Another problem encountered in conventional
induction motors is the high starting currents inherent in their
operation. Ordinarily, in motors of any size, external current limiting
devices must be used, or special and expensive rotor designs employed.
Similar problems exist with regard to polyphase motors.
SUMMARY OF THE INVENTION
The present invention overcomes or reduces the foregoing disadvantages of
conventional electric motors by providing a system in which the magnetic
flux density in the stator is maintained at a maximum level. In addition,
the system permits the current in the rotor also to be maintained at a
large magnitude relative to those permitted in conventional electric
motors of the induction type. Since the force generated in a conductor is
defined by the equation:
F=BlI
where F=force
where B=flux density
where l=length of the conductor
where I=current in the conductor
it can be seen that maximizing the terms B and I for a given l maximizes
the force and consequently the torque and horsepower of a motor.
According to the present invention, flux density is maximized by
controlling the flux density in the stator core by means of a capacitor
coupled in series with the main stator winding, the capacitor having a
value such that the voltage stored therein will, in combination with the
input voltage, periodically cause the volt-second capacity of the stator
core to be exceeded with the result that the core will periodically change
non-linearly from a nonsaturated to a saturated condition and back again.
The average flux density in the stator core is thus maintained quite high
without the danger of high input voltages resulting in extremely high
input currents. The capacitor limits the amount of energy that can be
transferred to the rotor even if the rotor has a very low impedance so
rotor current can also be maximized. The rotor inductance can be made
lower than in a conventional motor and the current induced at zero motor
speed can be made greater than is conventional; yet this current will
still have proper value at normal motor operating speeds and normal loads.
Thus, the motor of the present invention can be optimized much better than
conventional motors for a large number of applications or for any given
application.
By using a capacitor in series with the motor stator winding and operating
the motor magnetic path in soft saturation due to the limiting effect of
total energy transfer of the capacitor, the end result is a motor that can
be operated at maximum flux density under most conditions of line voltage
without resulting in extremely high input currents for high input
voltages. In other words, the input current and flux density in the device
would not be extremely non-linear as a function of the line voltage as is
presently the case with conventional AC induction and other motors. The
present invention makes use of the fact that the inductances of the motor
winding can only absorb so much energy before the magnetic material of the
motor stator saturates and discharges the capacitor. When the motor
magnetic material saturates, the capacitor discharges through the motor
winding and the power line source and charges up the capacitor in the
opposite polarity. The current through the winding then reverses and the
capacitor is then the source of energy and maintains the current flowing
through the winding. This continues until the voltage of the input line
changes in polarity. The volt-seconds of the input voltage from the line
then adds to the volt-seconds that have been applied by the capacitor to
the motor winding. This continues until the total volt-seconds applied to
the motor winding exceeds the volt-second capacity of the winding and
magnetic material of the motor stator, and then the magnetic material of
the motor again saturates. The capacitor then discharges through the motor
winding since it has saturated and the line power source charges up the
capacitor in the opposite polarity again. The current then reverses once
more through the motor winding and the capacitor again provides the source
of current through the motor winding. This continues until the line
voltage again changes polarity. As the line voltage amplitude continues to
increase the volt-second of the line voltage plus that of the capacitor
again are in phase and add until the volt-second capacity of the motor
winding and its associated magnetic material are exceeded. The winding
magnetic material again saturates and the inductance of the motor winding
decreases considerably again causing the capacitor to discharge through
the winding. This process is repeated each half-cycle and results in the
motor running at maximum flux density and thus maximum force, torque and
horsepower.
The use of the present invention allows for maximum flux density and since
the voltage across the capacitor is usually much higher (although it need
not be) than the line voltage, the flux density in the stator core is
relatively independent of the line voltage over fairly wide ranges of
amplitude. Furthermore, the capacitor prevents excessive currents from
passing through the motor winding when the magnetic material saturates
since only the energy in the capacitor, i.e., 1/2 CV.sup.2, can be
transferred through the winding. This limited energy transfer prevents
excessive currents from the line through the motor winding.
The result is an AC motor that will operate over wide ranges of input
voltage and operate at high efficiency and possess excellent operating
characteristics. Since the capacitor limits the amount of energy
transferred through the motor winding each half cycle, motor burn out is
not normally possible. In the case of motor overload all that will occur
is that the motor will stall and the input power to the motor will be
greatly reduced. This is due to the fact that the series capacitor will
have a much lower voltage across it than normal since the motor is not
operating in the controlled phase, and the 1/2 CV.sup.2 energy level is
greatly reduced.
The present invention can also be applied to polyphase AC motors by
connecting a suitable capacitor in series with each of the coils making up
a single phase of the polyphase stator winding.
It has been found that even better operating characteristics can be
achieved if an auxiliary winding is provided on the stator core, this
auxiliary winding being connected in parallel with the main winding and
capacitor. It has been found that the auxiliary winding provides the
necessary rotating field for starting a single phase motor and in addition
provides considerably more starting torque for the motor. It has further
been found that once the motor is up to rated speed at rated load, the
auxiliary winding plays no appreciable part in the operation of the motor.
If, however, the load increases, the auxiliary winding once again draws
current, acts as a motor winding, and provides additional torque to the
motor. In the event of a substantial overload, the motor will still stall
without the damage due to large currents but as soon as the load is
removed the motor will again come up to speed. This auxiliary winding is
usually much greater in impedance than the main winding and therefore the
current through the auxiliary winding is relatively low compared, for
example, with the main winding of an induction motor.
Furthermore, the auxiliary winding serves to limit the input current,
because as the input voltage increases, or the motor speed increases, this
winding begins to act as a generator winding due to the back e.m.f.
exceeding the input voltage, and generates a current which counteracts
some of the current drawn by the main winding. This, of course, is made
possible by the fact that the main winding is the primary source of power
to the motor. One or more auxiliary windings can be used in the case of a
polyphase motor.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is a schematic diagram of a preferred embodiment of a single phase
motor according to the present invention;
FIG. 2 is a schematic diagram of a modification of the embodiment of FIG.
1.
FIG. 3 is a schematic diagram of a first embodiment of a polyphase motor
according to the present invention;
FIG. 4 is a schematic diagram of a second embodiment of a polyphase motor
according to the present invention;
FIG. 5 is a schematic diagram of a third embodiment of a polyphase motor
according to the present invention;
FIG. 6 is a schematic diagram of a fourth embodiment of a polyphase motor
according to the present invention;
FIG. 7 is a schematic diagram of a fifth embodiment of a polyphase motor
according to the present invention; and
FIG. 8 is a schematic diagram of a sixth embodiment of a polyphase motor
according to the present invention.
DESCRIPTION OF THE INVENTION
FIG. 1 illustrates in schematic form the preferred embodiment of the
present invention. An AC induction motor of the squirrel cage type is
generally indicated at 10 and is diagrammatically shown to have a stator
12 of magnetic material and a squirrel cage rotor 14. The stator is shown
as having four pole pieces, 16, 18, 20 and 22 although more or less pole
pieces may be used if desired, as will be apparent to those skilled in the
art. It will also be apparent to those skilled in the art that the
configuration of the pole pieces shown is diagrammatic only. In most
applications, it would probably be desirable to provide the pole pieces
with a constriction so that saturation would occur at these points only.
No attempt is made herein to optimize the physical construction of the
motor. The main stator winding 24 is shown as wound on poles 16 and 20 and
is connected to input terminals 26 by means of a series capacitor 28. The
capacitor 28 need have no particular value, but its capacitance must be
large enough to maintain a capacitive power factor in the series circuit
comprising this capacitor and the winding 24 during the motor's normal
operating mode. An auxiliary winding 30 is wound on pole pieces 18 and 22
and is connected in parallel with winding 24 and capacitor 28. The winding
30 is preferably of considerably higher inductance and impedance than the
winding 24. It may, for example, have considerably more turns of finer
wire. A starting capacitor 32 is connected across the capacitor 28 by a
centrifugal switch 34.
The operation of the motor shown in FIG. 1 is as described above. Briefly,
when an AC voltage is applied to the terminals 26, the capacitor 28 begins
to charge and a current flows through the winding 24. A current also flows
through the winding 30 which is out of phase with the primarily capacitive
current in the winding 24 with the result that a rotating field is created
which causes the rotor 14 to begin rotating. At this time, a substantial
amount of the driving force is produced by the winding 30 inasmuch as the
main winding 24 and capacitor 28 has not yet entered into its normal
operating mode. As the rotor speed and the back e.m.f. increase, the
effective inductance of the winding 24 becomes such that this winding 24,
together with the capacitor 28, goes into its operating mode. In other
words, the effective volt-second capacity of the winding 24 and its
associated magnetic material becomes sufficiently large to permit the
operation of the device in the manner described previously, i.e., the
capacitor 28 will periodically charge, discharge and recharge in the
opposite direction causing the magnetic material associated with the
winding 24 to switch from a non-saturated to a saturated condition while
maintaining the average flux density quite large.
As the rotor approaches rated speed, the current in the auxiliary winding
30 becomes less and less. Preferably, this winding is designed to have
minimum current at rated speed and load and nominal input voltage. In the
event the load should increase or the speed otherwise decrease, the
winding 30 will draw more current and again contribute to the driving
force of the motor. This is very desirable as it provides additional
torque for periods of overload, which overload, if the winding 30 was not
present, might cause the capacitor 28 and winding 24 to be driven out of
its operating mode and the motor to stall.
The capacitor 32, while not necessary, is helpful for increasing starting
torque by initially allowing more current to flow through the main winding
24. After the motor reaches a predetermined speed, the centrifugal switch
34 opens, removing the capacitor 32 from the circuit.
The advantages of the present invention can be seen from the following
example. A Dayton squirrel cage induction motor Model 5K989A rated at 1/4
horsepower at 1725 RPM was modified in accordance with the present
invention by connecting a 70 microfarad capacitor in series with the main
stator winding, the capacitor running at 180 to 190 volts. The start
winding was used as the auxiliary winding and was connected directly
across the input line, i.e., the centrifugal switch normally employed in
the start winding circuit was bypassed. This switch was then used to
connect an additional start capacitor of 120 microfarads (the capacitor 32
in FIG. 1) into the circuit. No internal modifications were made to the
motor. Before modification, the efficiency of the motor, i.e., power out
to power in, at rated load and speed was about 35%; after modification, at
the same speed and load, the efficiency was approximately 60%. In
addition, because of the larger internal losses of the unmodified motor,
operation at higher output power levels which would theoretically produce
greater efficiency is not possible for any significant period of time
because the motor would overheat and possibly burn out. Because the
internal losses of the modified motor are less, the same motor may be
operated at considerably higher power output levels with a corresponding
increase in efficiency. Thus the modified motor was operated to produce
0.4 horsepower at which level it had an efficiency of about 75% without
any overheating. In fact, the power dissipated internally in the modified
motor under these conditions was less than that dissipated in the
unmodified motor at rated conditions.
In the unmodified motor, at no load, the input current at an input voltage
of 120 volts is approximately 6.3 amps, the rated current of the motor. At
140 volts, however, the current rises to over 9.0 amps and rises rapidly
with additional input voltage so that motor burn out would occur. The
modified motor had a current of about 3.4 amps at 120 volts input, and was
approximately the same at 140 volts, the curve being almost flat beyond
that point.
The starting torque of the modified motor was somewhat less than the
unmodified motor, but was entirely adequate for input voltages greater
than 80 volts. This starting torque could be increased by increasing the
capacitance of the start capacitor 32. The motor of the present invention
is thus unlike a split-phase motor in that it has adequate starting torque
at all normal line voltages and for all normal applications, even without
additional starting capacitance.
The same Dayton motor was then again modified to substitute a 100
microfarad capacitor for the 70 microfarad capacitor (the capacitor 28),
the capacitor again running at about 180 to 190 volts. In this case, motor
efficiency was found to be about 51% at rated load and speed, and again
the motor could be run at higher output levels without danger of burn out,
e.g., at approximately 0.4 horsepower, with an efficiency of about 75%.
The input current at 120 volts was about 5.1 amps and rose to about 5.3
amps at 140 volts with the current rise being quite gradual for higher
coltages. The starting torque of the motor as modified in this example was
still less than the unmodified motor, but was greater than that of the
modified motor of example 1.
FIG. 2 shows a modification of the motor of FIG. 1, with the same reference
numerals being used for the same elements. As can be seen, the centrifugal
switch 34 now acts to remove both the start capacitor 32 and the auxiliary
winding 30 from the circuit after the motor gets up to speed. This circuit
may be used where it is desirable that the motor stall on overload and not
start again until the overload condition is corrected. In such a case, the
centrifugal switch 34 may be of the conventional type that will not
re-close until the power has been removed. While the start capacitor 32 is
not necessary to the operation of the motor, the auxiliary winding 30 must
be present in a single phase motor in order that a rotating field can be
created to start the motor. Once the motor has gotten up to a speed
sufficient to enable the capacitor 28-winding 24 circuit to go into its
operating mode, the winding 30 is no longer necessary to the operation of
the motor, although it is generally desirable.
The present invention can equally well be used in connection with three
phase or other polyphase motors with one capacitor being provided per
phase. FIGS. 3 through 8 illustrate in schematic form various embodiments
of three phase motors according to the present invention. In each of FIGS.
3 through 8, the three coils making up the main stator winding are
designated 24a, 24b and 24c while the three capacitors connected in series
with these coils are designated 28a, 28b and 28c, respectively. In the
case of such polyphase motors, no starting winding is necessary, but the
use of an auxiliary winding is still beneficial for the reasons previously
stated. FIGS. 3, 4, 5 and 6 show such auxiliary windings, one winding or
coil for each phase, these coils being designated as 30a, 30b and 30c.
Although three auxiliary windings are illustrated, it appears that only
one such winding would be necessary to obtain many of the benefits
desired. In each figure, the windings are shown connected to appropriate
input terminals A, B, C and D which correspond to the input terminals 26
in FIGS. 1 and 2 except, of course, that they are adapted to be connected
to a source of three phase voltage rather than single phase voltage.
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