Field coil for an electric machine

A field coil segment for an electric machine including a rotor and a stator includes a first wire element having a first cross sectional area electrically connected in parallel with a second wire element having a second cross sectional area greater than the first cross sectional area.

TECHNICAL FIELD

This disclosure is related to field coils for electric machines.

BACKGROUND

An electric-powered machine transforms electric power to mechanical torque by inducing rotating magnetic fields with a field coil between a static element, i.e., a stator, and a rotatable element, i.e., a rotor. The rotating magnetic fields impose a torque upon the rotor. The torque is transferred to a shaft coupled to the rotor through conductor bars. The field coil may be associated with the stator.

Known electric-powered machines include a rotor having a stack of steel sheets assembled onto a rotatable shaft, and a plurality of conductor bars fabricated from conductive material, e.g., copper or aluminum. The conductor bars are preferably connected at both axial ends of the rotors using shorting end rings. Known field coils induce current flows through the conductor bars on the rotor that are preferably parallel to an axis of rotation of the rotor.

A known stator includes field coils for carrying a supply current to induce the magnetic field. The quantity of field coils may be varied and are preferably arranged in pairs. The most common types of electric machines are driven with single-phase or three-phase electrical power. A single-phase electric machine requires a starter to begin rotating the rotor as the magnetic field does not rotate. A three-phase electric machine rotates the rotor without a starter by sequentially rotating the magnetic field between the phases of the field coils.

Known field coils are wire-wound or bar-wound. A wire wound field coil is created from bundles of small diameter electrically conductive wires that are inserted into the stator. A bar-wound field coil is created from a series of bars of electrically conductive material that are inserted into the stator.

SUMMARY

A field coil segment for an electric machine including a rotor and a stator includes a first wire element having a first cross sectional area electrically connected in parallel with a second wire element having a second cross sectional area greater than the first cross sectional area.

DETAILED DESCRIPTION

Referring now to the drawings, wherein the showings are for the purpose of illustrating certain exemplary embodiments only and not for the purpose of limiting the same,FIG. 1illustrates a partial section view through a three-phase electric machine12including a stator10and a rotor22. It is appreciated that although three-phase electrical machines are discussed in detail, the disclosure is not so limited and additional phasing schemes may be employed with similar benefits achieved.

The rotor22has an axis of rotation36about a point that is susceptible to a magnetic field through a series of steel laminate stacked plates32and a plurality of conductor bars34. The rotor22is rotatable about the axis of rotation36that is concentric with a shaft25. The stator10is an annular device that is concentric to the axis of rotation36, and includes a plurality of radially oriented slots16with respect to the axis of rotation36, each which is of a shape and size to accommodate an electrically conductive winding referred to herein as a field coil segment14. The plurality of field coil segments14are depicted as elements of the stator10, but it is appreciated that the concepts described herein apply electric machines employing the field coil segments14as elements of a rotor.

The field coil segments14are electrically arranged to form suitable field coils that electrically connect between a switching circuit30and a sink19, shown with reference toFIG. 3. Each phase includes at least a pair of opposing field coils capable of creating a magnetic pole. The phased power may be supplied in single-phase or in multiple phases, with three-phase being a common multi-phase configuration. The single-phase configuration includes a pair of opposing field coils for every magnetic pole and the three-phase configuration includes three pairs of opposing field coils for each magnetic pole. For example, a single-phase single-pole stator has one pair of opposing field coils and a single-phase three-pole stator has three pairs of opposing field coils. Similarly, a three-phase single-pole stator has three pairs of opposing field coils and a three-phase three-pole stator has nine pairs of opposing field coils. Three-phase electrical machines are able to urge rotation in the rotor22without a starter device whereas single-phase electrical machines require a starter device.

As shown, the electric machine12including stator10is arranged to include a set of opposing first-phase field coils24, a set of opposing second-phase field coils26, and a set of opposing third-phase field coils28sequentially located about a circumference of the stator10relative to the axis of rotation36in close proximity to and separated from the rotor22by an air gap. Each set of the first-phase field coils24, second-phase field coils26, and third-phase field coils28electrically connects to between a power source and a sink, e.g., a three phase switching circuit and a sink shown with reference toFIG. 3.

Each of the first-phase field coils24, second-phase field coils26, and third-phase field coils28are formed by electrically connecting selected ones of the field coil segments14. This includes serially connecting corresponding wire elements for the field coil segments14associated with the first-phase field coils24, serially connecting corresponding wire elements for the field coil segments14associated with the second-phase field coils26, and serially connecting corresponding wire elements for the field coil segments14associated with the third-phase field coils28. Such connections are known and not discussed in detail herein. The serially connected wire elements of each of the first-phase field coils24, the second-phase field coils26, and the third-phase field coils28are electrically connected in parallel between a power source and a sink.

In the embodiment shown with reference toFIG. 1, each of the field coil segments14has two small wire elements18and two large wire elements20. The small wire elements18and the large wire elements20are arranged in a stack formation relative to radial lines projecting orthogonal to the axis of rotation36. The small wire element18has a cross-sectional area that is less than the cross-sectional area of the large wire element20. The small wire elements18are preferably relatively proximal to the rotor22and the axis of rotation36, and the large wire elements20are preferably relatively distal to the rotor22and the axis of rotation36. It is appreciated that the cross-sectional shapes of the small and large wire elements18,20, respectively, may be any suitable cross-sectional shape, e.g., round, square, or rectangular. It is appreciated that each cross-sectional area is identified for a plane that is orthogonal to electric current flow. Further, although field coil segments14are described as being arranged on a stator10, the disclosure is not so limited and contemplates field coil segments14being arranged on the rotor22.

FIGS. 2-1through2-6each schematically illustrates a winding arrangement for a single field coil segment14inserted into a slot16of the rotor10, with each figure illustrating a different winding arrangement. It is understood that the illustrated field coil segments14and associated winding arrangements are not exhaustive but are illustrative of winding arrangements of field coil segments14that may be employed.

FIGS. 2-1through2-6each schematically illustrates a winding arrangement for a field coil segment including small wire elements18, large wire elements20, and in several of the illustrations, intermediate wire elements38. It is appreciated that the terms small, large, and intermediate indicate relative dimensions for cross-sectional areas. The winding arrangements for field coil segments shown with reference toFIGS. 2-1through2-6are illustrative. Other winding arrangements for the field coil segments14arranged in a radially-oriented stack and inserted into one of the slots16using small, intermediate, and large wire elements consistent with the disclosure are contemplated.

FIG. 2-1schematically shows a first winding arrangement for a single one of the field coil segments14inserted into one of the slots16. The first winding arrangement includes a radially-oriented stack having two adjacent small wire elements18proximal to the rotor22and two adjacent large wire elements20distal to the rotor22. The two small wire elements18and the two large wire elements20are electrically connected in parallel between a power source and a sink.

FIG. 2-2schematically shows a second winding arrangement for a single one of the field coil segments14inserted into one of the slots16. The second winding arrangement includes a radially-oriented stack having two small wire elements18and two large wire elements20arranged in alternating order with a first one of the small wire elements18proximal to the rotor22, a first one of the large wire elements20adjacent thereto, a second one of the small wire elements18adjacent to the first one of the large wire elements20, and a second one of the large wire elements20distal to the rotor22. The two small and two large wire elements18,20are electrically connected in parallel between a power source and a sink.

FIG. 2-3schematically shows a third winding arrangement for one of the field coil segments14inserted into one of the slots16. The third winding arrangement includes a radially-oriented stack including the small wire element18, an intermediate wire element38, and the large wire element20. The small wire element18is proximal to the rotor22with the intermediate wire element38adjacent thereto, followed by the large wire element20distal to the rotor22. The small, medium, and large wire elements18,38,20, respectively, are electrically connected in parallel between a power source and a sink.

FIG. 2-4schematically shows a fourth winding arrangement for one of the field coil segments14inserted into one of the slots16. The fourth winding arrangement includes a radially-oriented stack having the small wire element18proximal to the rotor22, followed by two of the intermediate wire elements38, and the large wire element20distal to the rotor22. The small, medium, and large wire elements18,38,20are electrically connected in parallel between a power source and a sink.

FIG. 2-5schematically shows a fifth winding arrangement for one of the field coil segments14inserted into one of the slots16. The fifth winding arrangement includes a radially-oriented stack having two small wire elements18arranged side-by-side within the slot proximal to the rotor22adjacent to one of two stacked large wire elements20. The small and large wire elements18,20are electrically connected in parallel between a power source and a sink.

FIG. 2-6schematically shows a sixth winding arrangement for one of the field coil segments14inserted into one of the slots16. The sixth winding arrangement includes a radially-oriented stack having two small wire elements18arranged side-by-side proximal to the rotor22, with an intermediate wire element38adjacent thereto. A large wire element20is adjacent to the intermediate wire element38and distal to the rotor22. The small, medium, and large wire elements18,38,20, respectively, are electrically connected in parallel between a power source and a sink.

In each of the winding arrangements depicted with reference toFIGS. 2-1through2-6, the corresponding wire elements for the field coil segments14associated with each phase are preferably connected in series, and the serially connected wire elements are electrically connected in parallel between a power source and a sink. This is schematically shown inFIG. 3.

FIG. 3schematically show a wiring diagram of the first-phase field coils24, second-phase field coils26, and third-phase field coils28, each constructed from a plurality of serially-connected field coil segments14employing the small wire elements18and the large wire elements20. The small wire elements18and the large wire elements20of the field coil segments14are arranged in one of the aforementioned winding arrangements for a stator of an exemplary three-phase electrical machine12that is connected to a power source that includes a switching circuit30. The small and large wire elements18,20are electrically connected in parallel between the switching circuit30and sink19. The switching circuit30passes electrical current through one of the three phases, preferably in sequential order, to create successive magnetic fields in each of the first, second, and third field coils24,26,28. Each of the first, second, and third field coils24,26, and28includes the small wire elements18and large wire elements20. For example, the switching circuit30may be a three-phase switching circuit or inverter that controls the amplitude and frequency of electrical current passing through the first, second, and third field coils24,26, and28, respectively, to create a rotating magnetic field that acts upon a rotor.

The rotor is magnetically susceptible and is urged to rotate about the axis of rotation36to align with the magnetic pole when a magnetic pole is created. The electrical current provided by the switching circuit30causes the magnetic poles to rotate around the stator10. The rotation of the magnetic poles causes the rotor to align with the rotating magnetic pole created. The creation of rotating magnetic poles thereby urges the rotor to rotate when the switching circuit30sequentially controls electrical current through each of the first, second, and third field coils24,26,28, respectively. Rotational speed of the rotor is controlled by the frequency of the electrical current output from the switching circuit30.

Each wire element has an internal alternating current (AC) resistance that is affected by a skin effect. The skin effect is the tendency of the electrical current to travel near the surface of a wire element as the frequency increases. A wire element with a larger cross-sectional area in a plane orthogonal to a direction of current flow incurs more skin effect due to a larger surface area than a wire element with a smaller cross-sectional area due to a smaller surface area. The skin effect is discussed in terms of a skin depth, which is the depth of the electrical current traveling from an edge of the wire elements. As the frequency of the current increases, the skin depth decreases as the current attempts to travel near the surface thereby causing higher AC resistance in a wire element. A large wire has little skin effect during low frequency current transfer allowing the skin depth to penetrate the wire resulting in an overall low AC resistance.

Increasing the frequency of the AC current results in increasing the skin effect due to the large surface area of the large wire element, thereby reducing the skin depth and increasing the AC resistance. A smaller wire element has a small skin effect at low frequency with resulting low AC resistance but is unable to transfer as much current as a larger wire due to its smaller cross-sectional area. Increasing the AC current frequency increases the skin effect by a small amount due to the small surface area of the small wire element. The small wire element has a low skin depth loss and a lower increase in AC resistance. Thus, the small wire element carries more current than the larger wire element at higher frequencies.

The effective AC resistance caused by the skin effect is approximated by the following equation:
R≈(L*ρ)/(π*(D−δ)*δ)  [1]
wherein L is wire length,

ρ is material resistivity,

D is the wire diameter, and

δ is the effective skin depth.

An analysis using EQ. 1 indicates that AC resistance is affected by larger wire elements at low frequency operation due to skin effect, whereas AC resistance is affected in smaller wire elements at high frequency operation due to skin effect.

FIG. 4is a data graph plotting AC resistance (ohm)80in relation to a frequency (Hz)90for wire elements in a first arrangement82, a second arrangement84, a third arrangement86, and a fourth arrangement88. The first arrangement82is a single wire element having a relatively large cross-sectional area. The second arrangement84is a single wire element having a relatively small cross-sectional area. The third arrangement86is two wire elements having the same cross-sectional area and arranged in a parallel circuit. The fourth arrangement88is two wire elements including a first wire having a relatively small cross-sectional area and a second wire having a relatively large cross-sectional area. The first arrangement82has a lower AC resistance80than the second arrangement84at low operating frequencies. The first arrangement82has greater AC resistance than either the third arrangement86or the fourth arrangement88at low operating frequencies. As indicated, the AC resistance of the first arrangement82increases to be greater than the second arrangement84at approximately 470 Hz, and has the greatest AC resistance of all the arrangements at higher frequencies up to at least 1200 Hz.

The third arrangement86and the fourth arrangement88have approximately the same AC resistance at low operating frequencies, and begin diverging at approximately 200 Hz. The AC resistance of the third arrangement86increases in relation to the AC resistance of the fourth arrangement88. The AC resistance of the third arrangement86is less than the AC resistance of the second arrangement84at lower frequencies, is equal thereto at approximately 980 Hz and is greater than the AC resistance of the second arrangement84thereafter. The AC resistance of the fourth arrangement88is the least across the reported frequency range, i.e., 0-1200 Hz. Therefore, the graph indicates no loss of performance at a low frequency range and the best performance at higher frequency ranges for the fourth arrangement88, which includes two wire elements including a first wire having a relatively small cross-sectional area and a second wire having a relatively large cross-sectional area.

Table 1 provides an AC resistance reduction relationship for electric machines configured in the third arrangement86(Resistance Same Size Wire) and the fourth arrangement88(Resistance Different Size Wire) across the frequency range between 0 Hz and 1200 Hz.

The AC resistance of the fourth arrangement88and the AC resistance of the third arrangement86are shown for each operational frequency listed. An AC resistance ratio (Resistance Ratio) is calculated, and is a ratio of the resistance of the third arrangement86relative to the fourth arrangement88at each frequency. A power loss reduction (Loss Reduction at Peak Torque (W)) is determined, and is a calculated reduction in power between operating with the third arrangement86relative to the fourth arrangement88due to the change in resistance at different frequencies. Each power loss reduction has a negative value, indicating an increase in power when operating with the fourth arrangement88relative to the third arrangement86. The large wire element of the fourth arrangement88is dominant at low frequencies, approximately 0 to 200 Hz, as the AC resistance is similar between the third arrangement86and the fourth arrangement88. As the frequency increases from approximately 200 Hz, the skin effect becomes more pronounced for the large wire element of the third arrangement86relative to the fourth arrangement88. However, the skin effect is less on the small wire element of the fourth arrangement88, and thus the small wire element transfers current more efficiently. The skin effect is particularly pronounced as the frequency increases, as may be seen by the AC resistance ratio at 1200 Hz of 0.56 and a loss reduction at peak torque of −1765.99 W.

FIG. 5is a data graph plotting efficiency (%)100on the vertical axis in relation to a frequency (Hz)102on the horizontal axis for an exemplary electric machine configured with field coil segments14arranged as described with reference toFIG. 2-1, with two smaller wire elements proximal to a rotor and two larger wire elements distal from the rotor.

The depicted data includes a first variation104, a second variation106, and a third variation108, each which is compared to a base field coil segment having uniform sized wire elements. Each of the first variation104, second variation106, and third variation108are based upon a percentage decrease and increase to create smaller and larger wire cross-sections, respectively while maintaining the same overall amount of material for a field coil segment. For the particular example, the smaller wire cross-section is associated with the small wire elements and the larger wire cross-section is associated with the large wire elements described herein.

The first variation104includes the cross-sectional area of the small wire element reduced by 4.5%, and the cross-sectional area of the large wire element increased by 4.5%. Below approximately 300 Hz, a reduction in efficiency occurs over the base field coil segment, i.e., approximately −0.5% at 0.0 Hz. At high frequency, i.e., above approximately 300 Hz, a steadily increasing advantage is shown to 1200 Hz at which point there is a 3.0% efficiency increase.

The second variation106includes the cross-sectional area of the small wire element reduced by 9.0%, and the cross-sectional area of the large wire element increased by 9.0%. Below approximately 300 Hz, a reduction in efficiency occurs over the base field coil segment, i.e., approximately −1.0% at 0.0 Hz. At high frequency, i.e., above approximately 300 Hz, a steadily increasing advantage is shown to 1200 Hz at which point there is a 6.0% efficiency increase.

The third variation108includes the cross-sectional area of the small wire element reduced by 13.5%, and the cross-sectional area of the large wire element increased by 13.5%. Below approximately 400 Hz, a reduction in efficiency occurs over the base field coil segment, i.e., approximately −2.0% at 0.0 Hz. At high frequency, i.e., above approximately 400 Hz, a steadily increasing advantage is shown to 1200 Hz at which point an 8.0% efficiency increase is realized.

It will be apparent that the size variation and benefits achieved thereby is not limited to the specific examples illustrated. It will be further understood the variations illustrated may be used to select appropriate field coil segment sizes for a specific application. For example, if an electric machine is expected to operate at speeds that are predominantly greater than 800 Hz, the third variation or a larger cross-sectional difference may be selected. However, if an electric machine is expected to operate at speeds that are predominantly between approximately 300 Hz and 600 Hz, either the first variation or the second variation may be selected.