Electric motor structure to minimize electro-magnetic interference

An electric motor is configured with a stator core assembly that includes a stator core having a plurality of winding slots. A plurality of stator windings pass through the plurality of winding slots that include slot liners configured to provide electrostatic shields surrounding the plurality of stator windings. The electrostatic shields are referenced to an electrical location to reduce common mode currents associated with the electric motor.

BACKGROUND

The present invention is directed to electric motors, and more particularly to a structure for an electric motor that makes the electric motor more immune to creating electro-magnetic interference (EMI).

The stator of an electric motor12is energized by a switching inverter10such as depicted inFIG. 1. The switching inverter10has very fast transitions, with a fast dv/dt applied to the stator windings. The electric motor12has multiple capacitive paths. Two of these capacitive paths are of primary concern. They include the path between the stator windings and the motor frame, Cwf, (ground) capacitance (14), and the path between the stator windings and the rotor (17), Cwr, capacitance (16). A very rapid dv/dt is applied across each of these capacitances, Cwf, Cwr, causing an electrical current to flow through these capacitive paths when the inverter10switches.

Current flowing in these capacitive paths causes two major problems. These problems are associated with EMI as well as excessive bearing currents. If for example, the inverter10is connected to a 250 VDC link, and the semiconductor switches turn on and off in 50 nsec, the dv/dt will be 5×109V/sec. Typical stator winding to ground capacitance is about one (1) to about ten (10) nF. Thus, assuming 2 nF of capacitance, 10 A of peak current will be flowing through this path. This is a substantial amount of common-mode current; and it requires very large, expensive, heavy common-mode filters to attenuate this current. Current that flows through the Cwr path will flow to ground through the rotor bearings. This current can cause degradation of the bearings.

In view of the foregoing, it would be advantageous and beneficial to provide a motor structure that reduces EMI problems as well as common-mode currents that flow into the rotor bearings and electrical ground structures generally associated with conventional electric motor structures.

BRIEF DESCRIPTION

The present invention is directed to an electric motor structure. According to one embodiment, an electric motor comprises:a stator core comprising a plurality of winding slots; anda plurality of slot liners, each slot liner disposed at least partially within a corresponding winding slot, wherein each slot liner comprises an electrically conductive layer electrically insulated from the stator core, and further wherein the plurality of conductive layers are electrically connected together in parallel to provide a common connection point.

According to another embodiment, an electric motor comprises:a stator core comprising a plurality of winding slots;a plurality of stator windings passing through the plurality of winding slots; anda plurality of slot liners, each slot liner comprising an electrically conductive layer disposed between a corresponding stator winding and the stator core, each conductive layer electrically insulated from its corresponding stator winding and the stator core, wherein the plurality of conductive layers being electrically connected together in parallel to provide a common connection point.

According to yet another embodiment, an electric motor comprises:a stator core comprising a plurality of winding slots;a plurality of stator windings passing through the plurality of winding slots; anda plurality of stator slot liners, each slot liner configured to provide an electrostatic shield at least partially surrounding a corresponding stator slot winding, wherein the plurality of electrostatic shields are connected together in parallel to provide a common connection point.

Still another embodiment comprises an electric motor configured with a plurality of stator slot liners, each slot liner configured to provide an electrostatic shield at least partially surrounding a corresponding stator winding, wherein the plurality of electrostatic shields are connected together in parallel and referenced to an electrical location to reduce common mode currents associated with the electric motor.

While the above-identified drawing figures set forth alternative embodiments, other embodiments of the present invention are also contemplated, as noted in the discussion. In all cases, this disclosure presents illustrated embodiments of the present invention by way of representation and not limitation. Numerous other modifications and embodiments can be devised by those skilled in the art which fall within the scope and spirit of the principles of this invention.

DETAILED DESCRIPTION

FIG. 2illustrates a typical electric motor stator lamination structure20. Lamination structure20comprises a plurality of winding slots22.

FIG. 3illustrates a typical electric motor stator lamination stackup structure30comprising a plurality of laminations20. Lamination stackup structure30further comprises a plurality of slot liners32.

FIG. 5illustrates a top unfolded view of a stator slot liner50, according to one embodiment. The use of a stator slot liner having a structure such as shown inFIG. 5provides one or more electrostatic shields around at least a portion of the stator windings42of a corresponding electric motor. The stator slot liner/shield52provides a conduction path for capacitive currents that would otherwise flow into an electric motor stator and rotor assemblies.

More specifically, the electrostatic shield52provided via stator slot liner50allows problematic common-mode currents to instead flow to a much better electrical path, thus greatly reducing the associated EMI problems. This approach also reduces the common-mode currents that would flow into the bearings of the motor and cause bearing failures. The electrostatic shield provided by each slot liner50advantageously prevents capacitive currents from flowing into the ground structure and creating common-mode EMI currents, and also prevents capacitive currents from flowing into the bearing assembly and causing bearing fluting and pitting problems, thus leading to increased motor bearing life. According to one aspect, the capacitive common-mode current is redirected to flow directly back to the inverter10instead of ground. In this way, any required common-mode filter is smaller, and the system is much less prone to unintentional electromagnetic radiation.FIG. 6illustrates a side view of the stator slot liner50depicted inFIG. 5.

According to another embodiment, stator slot liner50comprises an electrostatic shield52and only one layer of insulation material. The single layer of insulation material may consist of layer54or layer56. In this embodiment, the surface of the electrostatic shield52that is devoid of any insulation material relies on the winding insulation itself to provide the requisite insulation between the electrostatic shield and any corresponding windings. Insulation layers54,56may each have a thickness between about 0.001 inch and about 0.05 inch according to one embodiment.

FIG. 7illustrates a side cutout view of the motor stator windings42inside the slot liner50depicted inFIGS. 5 and 6, according to one embodiment. Looking again atFIGS. 5 and 6, slot liner50comprises a thin layer52of copper or other suitable electrical conductor, e.g. copper or aluminum, that may comprise a thickness between about 0.0001 inch and 0.005 inch, sandwiched between two layers of insulation material54,56. The electrical conducting layer52may be very thin to substantially reduce induced eddy currents in the conducting layer52, and may comprise a thickness between about 0.0001 inch and 0.005 inch as stated herein, while retaining the desired electrostatic shielding properties. According to one aspect, the insulation layer between the motor stator windings42and the thin conducting layer52, e.g. copper foil layer, may comprise a thickness between about 0.001 inch and about 0.05 inch as stated herein, and constructed of a material with a low relative permittivity, such as without limitation, Polytetrafluoroethylene (PTFE), which has a relative permittivity of about 2.1. These properties substantially minimize the capacitance between the stator windings42and the copper foil shield/conducting layer52.

Stator slot liner50may comprise an electrostatic shield52and only one layer of insulation material, as stated herein. The single layer of insulation material may consist of only layer56for the embodiment depicted inFIG. 7. In this embodiment, the inner surface of the electrostatic shield52that is devoid of any insulation material relies on the winding insulation itself to provide the requisite insulation between the electrostatic shield52and the corresponding windings. The remaining layer of insulation56sandwiching the conducting layer52prevents the conducting layer52from being in electrical contact with the stator laminations20.

Slot liner50is then used in place of a traditional stator slot liner (such as nomex). Each stator slot liner50behaves in a fashion similar to a Faraday shield; and the windings42in each slot22exhibit a capacitance to the conducting layer, e.g. copper foil, of the corresponding slot liner50.

According to one aspect, a slot liner50is placed inside each stator slot22. The motor windings42are then positioned inside the shielded slot liners50in a fashion similar to that employed when installing traditional insulating slot liner windings. During assembly of the stator lamination stackup assembly30, one side of each slot liner50is electrically connected in common with one side of each of the remaining slot liners50such that the plurality of slot liners50are electrically connected in parallel with one another. According to one aspect, these connections may be implemented using electrical wires. According to another aspect, these connections may be implemented using a conductive ring assembly. Subsequent to connecting the slot liners50together, the shielded slot liners50are referenced to an electrical location where the C*dv/dt currents can flow into a better electrical path, such as the return node124of an electrical inverter126driving the motor128such as described in further detail herein with reference toFIG. 12. This feature is made possible since the capacitive currents from the stator windings42now flow into the thin conductive sheet52sandwiched between the insulator layers54,56of the slot liners50.

FIG. 8illustrates an electrically conductive end cap80for covering the end winding portions of the stator assembly windings42depicted inFIG. 4. Since a small capacitance remains between the end winding portions of the stator assembly windings42and both the motor frame and the rotor, the conductive end caps80assist in recovering these currents. According to one aspect, conductive end cap80completely covers the end winding portions of the stator assembly windings42. According to one embodiment, the electrically conductive end cap80is not connected to the frame of the motor, but instead is connected to a path to return the dv/dt currents in a better EMI path that is more immune to EMI, such as, for example, to return node124ofFIG. 12. This feature advantageously reduces capacitance from the stator windings42to the motor rotor, resulting in improved electromagnetic interference performance and reduced bearing currents.

FIG. 9illustrates another electrically conductive end cap90for covering the end windings depicted inFIG. 4. According to one aspect, these conductive end caps80,90can be constructed from a thin conductive material, that may comprise, according to one embodiment, a thickness between about 0.001 inch and 0.05 inch. According to another aspect, these conductive end caps80,90can be constructed from a conductive mesh to allow airflow, and spray oil if required, to pass through the cap80,90.

According to one aspect, the purpose of the caps80,90is to capture the capacitive currents, and redirect them to a better electrical path. According to one embodiment, the stator assembly40depicted inFIG. 4employs four (4) conductive end caps, two at each end of the motor stator assembly40. One end cap is located beneath the stator windings42, while another end cap is located over the stator windings42. Since this end cap must not be electrically connected to any other metal parts of the motor, it needs to be electrically insulated using, for example, and without limitation, non-conducting standoffs.

Each end cap80,90may comprise a corresponding hole82,92in the center of the respective cap to accommodate the rotor shaft of the motor12depicted inFIG. 1. These end caps80,90then fully cover the end winding portions of the stator windings42from both inside and outside regions of the windings42to capture the corresponding capacitive currents. Each end cap80,90is referenced to a preferred location such that the corresponding C*dv/dt current flow can be redirected.

In summary explanation, an electric motor structure has been described that reduces common-mode currents and that reduces motor bearing currents. The reduced currents advantageously result in reduced filter requirements and increased motor bearing life. Thus, the need for large, heavy electromagnetic common-mode filters is greatly reduced. According to one aspect, use of thin insulated copper foil advantageously reduces motor assembly cost, and provides a motor assembly that is reliable and electrically robust from electrical transients, and that further is simple and compatible with existing motor assembly approaches. The embodied motor structures advantageously reduce bearing currents without the need for expensive ceramic bearings, or the use of electrical rotating brushes to shunt capacitive currents to eliminate undesired current flow through the metallic motor bearings. The embodied motor structures further greatly reduce EMI complexity by redirecting stator winding capacitive currents into an electrical shield assembly, allowing these currents to be directed to a more desirable electrical path.

FIG. 10illustrates an electric motor stator lamination stackup structure100with slot liners50such as depicted inFIGS. 5 and 6, according to one embodiment. Each slot liner50has a portion of its corresponding electrical conducting layer52exposed to allow one side of each slot liner50to be electrically connected in common with one side of each of the remaining slot liners50such that the plurality of slot liners50are electrically connected in parallel with one another. According to one aspect, these connections may be implemented using electrical wires such as depicted for one embodiment inFIG. 11. According to another aspect, these connections may be implemented using a conductive ring assembly as stated herein. Subsequent to connecting the slot liners50together, the shielded slot liners50are referenced to an electrical location where the C*dv/dt currents can flow into a better electrical path, such as the return node124of an electrical inverter driving a motor such as described in further detail herein with reference toFIG. 12. This feature is made possible since the capacitive currents from the stator windings now flow into the thin conductive sheet52sandwiched between the insulator layers54,56of the slot liners50, as stated herein before.

FIG. 11illustrates the stator lamination stackup structure100depicted inFIG. 10with stator windings102placed in the stator slots and the slot liners50electrically connected together with electrical wires104, according to one embodiment.

FIG. 12illustrates an equivalent circuit depiction of an electric motor switching inverter system120configured with a return node124for common-mode currents using stator slot liner shields52such as depicted inFIGS. 5-7, according to one embodiment. Subsequent to electrically connecting the slot liner electrostatic shields52together in parallel as stated herein before, the shields52are referenced to an electrical location where the C*dv/dt currents can flow into a better electrical path, such as return node124to reduce common mode currents associated with the electric motor128, such as the return node of an electrical inverter126driving the electric motor128, as stated herein before. According to one aspect, a common-mode inductor122is installed around the three motor winding power lead wires and the additional common mode current return path wire123connected to the EMI slot liner50to further enhance and promote elimination and/or reduction of common mode currents. According to another aspect, a small resistor129that may be in the range from about 0.1 Ohm to about 100 Ohms is placed in series with the slot liner wire121, to reduce any resonance between the slot liner capacitance and the inductance of the corresponding wire assembly.

FIG. 13is a block diagram of a motor control system130that employs a motor132with slot liners according to one embodiment. Motor control system130comprises a power converter stage134, gate drive electronics136that activate the power source140that may be an AC or DC power source, a controller142such as a digital signal processor for controlling the power source140and motor132, filters144,146placed at the input and output of the power stage134, and cables that interconnect the various components. The electrostatic shield provided by each motor slot liner50/conducting layer52for the motor132advantageously prevents capacitive currents from flowing into the ground structure and creating common-mode EMI currents, and also prevents capacitive currents from flowing into the bearing assembly and causing bearing fluting and pitting problems, thus leading to increased motor bearing life. According to one aspect, the capacitive common-mode current is redirected to flow directly back to the inverter that is one portion of the power stage134instead of ground. In this way, any required common-mode filter is smaller, and the motor control system130is much less prone to unintentional electromagnetic radiation.

FIG. 14is a more detailed block diagram of a motor control system150that employs a motor152with slot liners50according to one embodiment. Motor control system150is configured to control a corresponding airfoil154in response to commands received via an aircraft interface156.

Looking now atFIG. 15, a cutout view of a stator slot160illustrates a stator slot winding162disposed within a corresponding stator slot liner50. In this embodiment, a stator slot cap164is inserted into the top portion of the stator slot160such that the stator slot liner50and the stator slot cap form an EMI shield surrounding the stator slot winding162. According to one embodiment, the stator slot cap164comprises an electrically conductive layer52sandwiched between a pair of insulation layers, in similar fashion to stator slot liner50. Some embodiments may include slot liners and stator slot caps that employ only a single layer of insulation that electrically insulates the electrically conductive layer52from the stator core. In such embodiment, the electrically conductive layer relies on the stator winding insulation itself to provide the requisite insulation between the conductive layer and the stator winding(s).

FIG. 16is a cutout view of a stator slot170illustrating a stator slot winding162disposed within a corresponding stator slot liner50. A stator slot cap172is inserted into the top portion of the stator slot170to provide an added layer of insulation in the open slot portion of the stator slot170. The slot cap172in this embodiment does not have any electrically conductive shield layer, and thus serves only to provide an added layer of insulation.

The embodiments described herein importantly do not interconnect the electrostatic shields/conductive layers52directly to a common motor ground location such as, for example, the motor stator core. Instead, the plurality of conductive layers52are electrically connected together in parallel via an interconnecting wire or conductive ring, to provide a common connection point that can be referenced to an electrical location such as a return path/node to a motor inverter, to reduce common mode currents associated with the electric motor. The present inventors discovered that connecting the electrostatic shields52to a motor ground location such as a stator core or other motor ground location, results in disadvantageously increasing common mode currents associated with the electric motor.