Electrical generator or motor structure – Dynamoelectric – Rotary
Reexamination Certificate
1998-09-17
2001-05-15
Ramirez, Nestor (Department: 2834)
Electrical generator or motor structure
Dynamoelectric
Rotary
C310S061000, C310S113000, C310S184000, C310S198000, C310S210000, C310S214000, C310S216055, C310S269000
Reexamination Certificate
active
06232691
ABSTRACT:
FIELD OF THE INVENTION
This invention relates to an improved DC electric starter-generator having compact size, low weight and increased reliability, and which is capable of delivering high starting torque with good efficiency. In a preferred version, it relates to an improved DC electric starter-generator which can be used in starting the engine of aircraft, and subsequently generating electrical power for use by the aircraft.
BACKGROUND OF THE INVENTION
A DC electric starter-generator (“S/G”) is typically used for bringing aircraft engines from a full-stop up to a rotational velocity sufficient to achieve engine ignition, and for subsequently generating electrical power. The S/G must be capable of producing high starting torque in order to turn an aircraft engine having substantial compression and inertial mass. In addition, the S/G must meet certain size and weight criteria in order to be acceptable for use on aircraft. While known aircraft S/G designs are capable of providing the necessary torque, they suffer from a number of significant drawbacks.
One major drawback is the collection of harmful, abrasive dust at points of rotation. Typically, known aircraft S/Gs include an armature assembly mounted on an armature shaft, the armature shaft being supported by bearings located at either side of the armature assembly. It is generally necessary to cool a S/G unit using a fan mounted to the armature shaft at one end, so that air is directed over the armature assembly and over a corresponding stator assembly whenever the S/G unit is in operation. In addition, it is common to commutate the armature windings by using commutation brushes. The commutation brushes generate dust particles which are directed downstream and which accumulate at the support bearings, causing the bearings to wear down prematurely. Typically, bearings must be replaced after every 1,000 hours of operation due to the dust collection problem. In addition, due to wear, the commutation brushes must be changed after every 400-600 hours of operation. This results in the need for frequent maintenance and service for known aircraft S/G designs.
Another major drawback of known aircraft S/G designs is their relatively low operating efficiency, typically hovering in the 70% range. Low efficiency S/Gs require higher current input, and take a longer time to achieve the necessary rotational velocity for engine ignition. This generally results in lower performance and possibly a shorter service life for the S/G and the aircraft engine, due to increased operating temperatures. A key factor which limits the performance of known S/Gs, both in terms of available starting torque and motor efficiency, is the limiting effect of counter electromotive forces (e.m.f.) produced in the armature—i.e. as the armature increases in rotational velocity and flux, the counter e.m.f. produced by the armature and acting on the stator field coils also increases, causing the current flowing in the stator field coils to be reduced, thereby decreasing the magnetic flux generated around the armature windings which produce the torque. The limiting effect of counter e.m.f. is inherent to all S/Gs, and known aircraft S/G designs have not been able to achieve efficiency much above the 70% range largely due to this problem. Other factors which decrease the efficiency of prior art aircraft S/G designs include friction introduced by commutation brushes, and high resistive copper losses in the series armature and stator windings.
Related to the problem of efficiency and service life is the issue of adequate cooling for the stator and armature assemblies and other components located within the S/G housing. In typical S/G designs, the air space between the armature and the stator is minimal. While the problem of air-cooling is minimized once the aircraft is airborne, the limited air space between armature and stator in present S/G designs limits the effectiveness of air-cooling while the aircraft is still on the ground. Consequently, a design which increases the air space and improves the airflow, while still being capable of producing an effective flux field, is desirable.
SUMMARY OF THE INVENTION
The present invention provides an improved DC S/G design having a higher starting torque, greater efficiency, and improved reliability in comparison to prior art devices. In particular, the improved S/G is designed to overcome a number of major drawbacks which are found in known aircraft S/G designs. Although a preferred embodiment particularly suited for use in an aircraft is described, it should be noted that the improvements taught by this invention are easily adaptable to S/Gs for other applications, including use as a S/G for virtually any machine which requires such a device.
In one aspect of the present invention, there is provided a DC electric starter comprising:
(i) a rotor assembly with a rotor core having a cross-sectional silhouette formed by a hub with F equally spaced apart appendages extending radially from said hub, each of said appendages comprising an arm attached to said hub and having an outer end and a flanged formation at said outer end of said arm, said flanged formation having a width contained within a sector defined by a first pair of straight lines extending from the center of the rotor assembly to each edge of said flanged formation, the angle formed by said first pair of straight lines defining a first angle between them, said first angle being between 30°×(4/F) and 40°×(4/F) wherein F is the number of rotor poles and the value of F is one of four and six; and
(ii) a generally hollow cylindrically shaped stator assembly encircling said rotor assembly, said stator assembly having an inner surface and including a plurality of stator slots extending substantially lengthwise along said inner surface, each of said slots having at least two main stator phase winding segments located therein, said two main stator phase winding segments in each stator slot being oppositely wound;
wherein, said stator assembly has a forward direction of rotation, and there are twelve main stator phase winding segments and 6×F stator slots, said twelve main phase winding segments being identifiable as main phase winding segments one through twelve, each of said main phase winding segments one through six being routed through six consecutive stator slots and in the same direction within said stator slots, each of said main phase winding segments one through six being further routed through every sixth stator slot at substantially 360°/F intervals from a corresponding one of said six adjacent stator slots, for a plurality of turns, each turn covering 360° in said forward direction, each of said main phase winding segments seven through twelve being routed through said six consecutive stator slots but in a direction within said stator slots opposite from main phase winding segments one through six, each of said main phase winding segments seven through twelve being further routed through every sixth stator slot at substantially 360°/F intervals from the corresponding one of said six consecutive stator slots, for said plurality of turns, each turn covering 360° in said forward direction, so as to form said two oppositely wound main phase winding segments in every stator slot.
In another embodiment, every (N+1)
th
turn of each of said phase winding segments is offset by one stator slot position in said forward direction, so as to form a secondary phase winding segment corresponding to each of said main phase winding segments one through twelve, the turn ratio of said secondary phase winding segments to said main phase winding segments being 1:N, where N is an integer between 3 and 7.
In another embodiment, the turn ratio of said secondary phase winding segments to said main phase winding segments is 1:5.
In yet another embodiment, the DC electric starter also operates as a generator in generator mode.
REFERENCES:
patent: 3489939 (1970-01-01), McDougald et al.
patent: 3740600 (1973-06-01), Turley
patent: 3908130 (1975-09-01), Lafuze
patent:
Bereskin & Parr
Dellcom Aviation Inc.
Dinh Le Dang
Ramirez Nestor
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