Apparatus and method for cooling

Abstract

To provide an apparatus and a method for cooling an electric motor.SOLUTION: To obtain simple and sufficient cooling, an electric motor is provided with a dual cooling system including: a cooling space (11) provided to the stator (2) for receiving and passing on a cooling fluid which cools the stator; and an inlet (12) and an outlet (13) provided to the electric motor for receiving and passing on a cooling gas flow which cools at least a rotor (4) before exiting the electric motor.SELECTED DRAWING: Figure 3

Description

[Technical Field]

[0001]

[0001] The present invention relates to a solution for cooling electric motors. In the following, the invention is explained by reference to an electric motor of a cycloidal propulsion unit as an example, but the cooling method can also be used for other electric motors. [Background technology]

[0002]

[0002] A problem with electric motors, such as those in cycloidal propulsion units, is providing sufficient cooling for the various parts of the electric motor. Sufficient cooling is essential because the load on the electric motor can be increased if temperature increases can be avoided. Therefore, if the cooling is efficient, more power can be made available from the electric motor without the need to increase the motor diameter or length. Summary of the Invention

[0003]

[0003] The object of the present invention is to provide a solution which ensures sufficient cooling for an electric motor. This object is achieved by an apparatus according to independent claim 1 and by a method according to independent claim 14.

[0004]

[0004] Efficient dual cooling for an electric motor can be obtained by circulating a cooling fluid through the stator and cooling other components of the electric motor with a cooling gas flow that enters the electric motor at an inlet, flows along the rotor, and then exits the electric motor via an outlet. Such a solution ensures that the cooling is efficient enough to ensure that a significant amount of power is available from the electric motor without having to increase the size of the electric motor.

[0005]

[0005] Preferred embodiments of the invention are disclosed in the dependent claims.

[0006]

[0006] In the following, the device and method will be described in more detail, by way of example, with reference to the accompanying drawings, in which: [Brief explanation of the drawings]

[0007] [Figure 1]

[0007] An example of an electric motor installed in a cycloidal propulsion unit is shown. [Figure 2] 1 illustrates an electric motor installed in a cycloidal propulsion unit. [Figure 3] 1 illustrates an electric motor installed in a cycloidal propulsion unit. [Figure 4]

[0008] 2 illustrates a second embodiment of an electric motor. [Figure 5]

[0009] 1 illustrates a third embodiment of an electric motor. [Figure 6]

[0010] 1 illustrates a fourth embodiment of an electric motor. [Figure 7] 1 illustrates a fourth embodiment of an electric motor. [Figure 8]

[0011] 1 illustrates a fifth embodiment of an electric motor. DETAILED DESCRIPTION OF THE INVENTION

[0008]

[0012] 1 to 3 illustrate an electric motor installed in a cycloidal propulsion unit, seen from the side in FIG. 1, from above in FIG. 2 and in cross section with the addition of a cooling device in FIG. 3.

[0009]

[0013] The electric motor comprises a stator 2, which in this case is fixedly mounted within the hull 3 of the ship. A rotor 4 is rotatably arranged within the stator 2. The rotor 4 extends downwards as a rotating casing 5, which rotates together with the rotor 4 about a rotation axis 6 during operation. When the electric motor is installed in a cycloidal propulsion unit, the rotation axis is in fact oriented vertically.

[0010]

[0014] The hollow rotating casing 5 is provided with blades 7 at its lower part. These blades 7 are located underwater outside the hull 3 and rotate together with the rotating casing around a rotation axis 6. Each blade is further provided with a drive mechanism 8 that rotates the blade 7 independently relative to the rotating casing 5 around a rotation axis 9. Due to the blade-specific drive mechanism 8, each blade can be individually controlled to rotate in an optimal manner. This allows the vessel to be driven and operated as needed using a cycloidal propulsion unit. Therefore, the illustrated electric motor may be the vessel's main motor. The drive mechanism may be implemented using suitable mechanical devices, such as gears and shafts. However, in the following description, it is assumed, by way of example, that the drive mechanism 8 includes an electric motor driven by a motor drive 10, such as a frequency converter. Thus, there is one electric motor for each blade 7. The motor drive 10 may be installed, for example, within the rotating casing 5.

[0011]

[0015] A challenge for the illustrated type of electric motor 1 is cooling. In the illustrated example, a hybrid cooling solution is used to cool the electric motor 1. This hybrid cooling solution includes cooling spaces 11 provided in the stator 2 for receiving and passing a cooling fluid that cools the stator. Furthermore, the electric motor 1 is provided with inlets 12 and outlets 13 for receiving and passing a cooling gas flow within the stator casing of the electric motor for cooling at least the rotor 4. For simplicity, the inlets, outlets, fans, heat exchangers, and cooling spaces are not illustrated in Figures 1 and 2.

[0012]

[0016] The cooling space 11, in which a cooling fluid such as water circulates, may be implemented, for example, as a cooling jacket. In this case, the cooling space 11 may be implemented as a relatively high-volume annular container that surrounds at least a portion of the stator 2. Alternatively, depending on the implementation, the cooling space may be implemented as one or more tubes that partially surround the stator. The cooling space may be implemented as a separate part that surrounds and contacts the outer surface of the stator, or alternatively as a part that is integrated into the outer wall of the stator casing. If integrated into the outer wall, the wall of the stator is at least partially hollow.

[0013]

[0017] 3, it is assumed by way of example that a heat exchanger 14 is arranged to cool a gas flow, such as an air flow, entering the electric motor 1 via the inlet 12. Preferably, this heat exchanger is at least partially embedded in the illustrated electric motor, which ensures a space-saving, compact design. However, a heat exchanger is not required in all embodiments. In FIG. 3 , the heat exchanger is connected to a fluid circuit in which a cryogenic fluid is used to cool the gas entering the electric motor 1. This gas flow circulates within the electric motor 1, where it contacts at least the rotor 4, before exiting the electric motor via the outlet 13. In the illustrated example, two outlets are provided by way of example, but in some implementations, one outlet may be sufficient. FIG. 3 also illustrates, by way of example, that both outlets 13 are provided with fans 15, such as centrifugal fans, but only one fan may be sufficient. In some implementations, a fan may not be required at all if the fluid flow can be generated elsewhere, such as by a building or ship's ventilation system, which can generate the gas flow required for cooling. If a fan is included, it is also preferably at least partially embedded in the illustrated electric motor to provide a compact design.

[0014]

[0018] 3, a closed loop for circulating the gas flow from outlet 13 to inlet 12 is implemented by duct 21, which feeds the heated air flow from outlet 13 via heat exchanger 14 back into electric motor 1. In addition to cooling rotor 4, the gas flow may also cool slip ring unit 16, rotor magnets 17, and windings 18 of stator 2. Main bearings 29, which rotatably support the rotor and rotating casing 5, are further cooled to some extent by the gas flow, and may further be provided with a lubricating oil feed device that also cools the main bearings.

[0015]

[0019] Efficient cooling of the magnets and windings is achieved by directing gas flow through the rotor 4 through at least one opening 19, shown in FIG. 3, located between the magnets 17 and the rotating shaft 6. Additionally, gas flow is directed through an annular gap 20 formed around the outer periphery of the rotor 4, which is shown in FIG. 3 between the magnets 17 of the rotor 4 and the windings 18 of the stator 2. To ensure that gas flow passes through the at least one opening 19 and the annular gap 20, a wall 23 is provided extending from a position very close to the outer periphery of the rotor 4 to the top wall of the motor. This wall 23 prevents the gas flow from passing directly from the inlet to the outlet without first passing through the rotor 4 and the annular gap 20.

[0016]

[0020] 1-3 or other embodiments, if another type of electric motor is utilized, such as an induction motor having a wound or squirrel-cage induction rotor, magnets are not required on the rotor, and in this case it is advantageous to direct the gas flow through an annular gap 20 formed around the periphery of the rotor 4, because in this case the gas flow cools the rotor windings or squirrel-cage.

[0017]

[0021] When the electric motor 1 is used in a cycloidal propulsion unit having a rotating casing with a drive mechanism 8 for the blades, it is advantageous to direct the air flow also through the rotating casing 5, as illustrated in FIG. 3. This makes it possible to ensure that the temperature of the space in which the drive mechanism is installed remains sufficiently low. In some installations, the drive mechanism 8 may also be cooled by a separate cooling fluid (not illustrated). However, even in this case, it is advantageous to ensure that the temperature of the interior space of the rotating casing 5 is at an acceptable level by a cooling gas flow, since fluid cooling makes it easier to keep the most important components in the rotating casing at an appropriate temperature.

[0018]

[0022] In Figure 3, an opening 22 through the rotor 4 is provided near the rotating shaft 6. This opening 22 facilitates gas flow from above through the rotor 4 into the rotating casing 5 and back. This opening 22 may be quite large to facilitate the ability of maintenance personnel to enter the rotating casing 5 through this opening 22 for inspection purposes.

[0019]

[0023] 3, the inlet 12 is located near the axis of rotation 6, while one or more outlets 13 are located closer to the periphery of the rotor 4. One advantage of this is that the cooled gas flow entering the electric motor via a heat exchanger 14 close to the axis of rotation 6 keeps the ambient temperature of the slip ring unit 16 and the drive mechanism 8 for the blades 7 lower. However, in some installations, the direction of gas flow may be opposite, with one or more inlets at the periphery of the rotor and one or more outlets near the axis of rotation.

[0020]

[0024] 3, the heat exchanger 14 is located after the one or more fans 15 in the flow direction. However, this is merely an example. In other implementations, the heat exchanger 14 may be located before the one or more fans 15 in the flow direction. In still other implementations, the gas flow may be arranged so that the gas is fed into the outer circulation at the top of the electric motor and flows downward through the air gap 20 into the electric motor in the opposite direction compared to FIG. 3.

[0021]

[0025] Figure 4 illustrates a second embodiment of an electric motor 31. The embodiment of Figure 4 is very similar to the first embodiment described in relation to Figures 1 to 3. Therefore, the embodiment of Figure 4 will be described below mainly by pointing out the differences from these embodiments.

[0022]

[0026] In FIG. 4 , the gas flow is directed differently from the inlet 12 to the outlet 13. The wall 23 shown in FIG. 3 is absent in FIG. 4 . Because of this, there is nothing forcing the gas flow through the openings 19 (if provided) and the annular gap 20 in the rotor 4. Instead, the gas flow passes from the space above the rotor 4 to the space below the rotor through the openings 22 and along the slip ring unit 16 near the rotating shaft 6. However, if airflow through the openings 19 to the space below the rotor 4 is preferred, this can be achieved in some implementations by reversing the flow direction and positioning the openings 13 above the openings 19. In this case, the gas flow enters the electric motor through the openings 13, and part of the gas flow passes through the openings 19 to the space below the rotor 4.

[0023]

[0027] If the electric motor 31 is used in a cycloidal propulsion unit, the rotating casing 5 and the devices located inside the rotating casing may be similar to those described in relation to Figure 3. For simplicity, only the upper part of the rotating casing is shown in Figure 4, but also in this second embodiment the gas flow may reach the devices installed in the rotating casing 5 and provide them with cooling.

[0024]

[0028] Figure 5 illustrates a third embodiment of an electric motor. Figure 5 is very similar to the first embodiment described in relation to Figures 1 to 3 and the second embodiment described in relation to Figure 4. Therefore, the embodiment of Figure 5 will be described below mainly by pointing out the differences from these embodiments.

[0025]

[0029] In FIG. 5, the electric motor 41 is provided with two inlets 12 and also two heat exchangers 14 that provide cooled gas flows to the electric motor 41. Furthermore, two ducts 21 are provided for passing the gas flow leaving the outlet 13 through the respective inlets 12 to provide closed circulation. The advantage of such a solution, which can also be used in the previously described embodiments, is an increased cooling capacity. As a result, a larger amount of cooling gas at a suitable temperature can be provided in the stator of the electric motor 41 compared to the solutions illustrated in FIGS. 3 and 4. Furthermore, such a solution makes the system more redundant in the event of a failure, such as a failure of the fan motor. It also allows the use of smaller components (fans and / or heat exchangers), thereby saving space.

[0026]

[0030] As can be seen from figure 5, the rotor 4 has a diameter D which is greater than its length L in the direction of the rotation axis 6. This can also be applied to all other embodiments.

[0027]

[0031] If the electric motor 41 is used in a cycloidal propulsion unit, the rotating casing 5 and the devices located inside the rotating casing may be similar to those described in relation to Fig. 3. For simplicity, Fig. 5 shows only the top of the rotating casing, but also in this second embodiment, the gas flow may reach devices installed in the rotating casing 5 and provide them with cooling.

[0028]

[0032] 6 and 7 illustrate a fourth embodiment of the electric motor.

[0029]

[0033] The embodiment of Figures 6 and 7 is very similar to the first embodiment described in relation to Figures 1 to 3. Therefore, the embodiment of Figures 6 and 7 will be described below mainly by pointing out the differences from these embodiments.

[0030]

[0034] In Figure 6, only the upper right portion of the electric motor is illustrated to provide an enlarged view of the area around the annular gap 20 between the magnets 17 of the rotor 54 and the windings 18 of the stator 2. Figure 7 illustrates an enlarged view of the same area from above.

[0031]

[0035] 6 and 7, as in the embodiment of Figure 3, the cooling gas flow passes through the annular gap 20 between the rotor 54 and the windings 18 of the stator 2. However, to further improve the cooling of the rotor 54, the rotor 54 is provided with holes 55 along the periphery of the rotor 54. In practice, these holes 55 may form channels extending in a direction parallel or substantially parallel to the direction of the axis of rotation 6.

[0032]

[0036] 6 and 7, it is assumed by way of example that the electric motor is of the type in which the magnets 17 are provided in the rotor 54. In that case, holes 55 or channels are provided through the rotor in the magnets 17. In practice, these holes 55 are provided in close proximity to the magnets 17, between them and the openings 19. With such a solution, a cooling flow in the direction of the rotation axis 6 is obtained on two sides of the magnets 17, via the annular gap 20 and via the holes 55 in the magnets. This makes the cooling even more efficient.

[0033]

[0037] However, if the electric motor is of a type in which the rotor does not have magnets, such as in the case of an induction motor, it is still beneficial to have the illustrated holes 55 near the outer periphery of the rotor 54 to provide cooling flow through the holes 55 in the direction of the rotation axis 6. In that case, the holes 55 may be, for example, as close as possible to the cage of the induction motor. Especially for low-cost induction motors, heat losses are significant and efficient cooling is required. This can be achieved by the configuration illustrated in FIGS. 6 and 7, which combines the holes 55 in the rotor 54 with an annular gap between the rotor 54 and the stator 2. It may be beneficial to have the holes in two circles in close proximity to the cage of the induction motor to maximize the surface area of ​​the holes.

[0034]

[0038] Figure 8 illustrates a fifth embodiment of an electric motor. The embodiment of Figure 8 is very similar to those described in relation to Figures 1 to 3. Therefore, the embodiment of Figure 8 will be described below mainly by pointing out the differences from these embodiments.

[0035]

[0039] In Figure 8, the electric motor 61 is installed in an enclosed room 63 (or space), for example of a ship or building. The room 63 is connected to the ventilation system of the ship or building via an outlet 64 and an inlet 65. Therefore, a separate heat exchanger for cooling the gas flow passed through the electric motor 61 is not required for the electric motor, since such a heat exchanger for cooling the gas flow is provided in the ventilation system. One advantage of such a solution is the space savings. In a cycloidal propulsion unit, for example, this can be an important advantage.

[0036]

[0040] 8, it is assumed by way of example that the electric motor 61 is provided with a fan 15 which generates a gas flow into the electric motor 61. However, this fan can be eliminated if the air flow from the inlet 65 into the room provides a sufficiently strong air flow, and this inlet 65 can be connected directly to the inlet 12 of the electric motor 61.

[0037]

[0041] In FIG. 8 , the outlets 13 from the stator 62 are not provided at the top of the stator, but instead at the side of the stator. In this case, two outlets are provided. The first outlet is provided directly below the stator windings 18, and the second outlet is provided directly above the stator windings 18. In this way, the ends 66 of the stator windings are also efficiently cooled. Due to the location of the lower outlets 13 below the stator windings 18, the gas flow enters the space below the rotor 4 through openings 19. In some implementations, it may be advantageous to have several outlets 13 in one or more rows around the side of the stator. In this case, the outlets in this one or more rows of the motor casing may be aligned with the winding ends on both ends of the electric motor.

[0038]

[0042] In Figure 8, the outlet 13 is located in close proximity to the cooling space 11, in which a cooling fluid circulation is provided for cooling the stator 62. This makes it possible to provide cooling fins (not shown) extending from the cooling space 11 in the gas flow passing through the outlet 13. In this way, the gas flow can be partially cooled by the cooling fluid circulation in the cooling space 11. Such cooling of the gas flow can also be obtained by reversing the gas flow in the electric motor 61, in which case the inlet 12 and the outlet 13 are interchanged. As a result, the gas flow entering the electric motor via the inlet in the side of the stator is cooled by the fins in the cooling space 11 just before entering the electric motor.

[0039]

[0043] The inlet or outlet may be provided on the side of the stator 62, as illustrated in Figure 8 and also in the previous embodiment, in which case the electric motor may still be provided with the aforementioned fan and heat exchanger as described in the previous embodiment.

[0040]

[0044] If the electric motor 61 is used in a cycloidal propulsion unit, the rotating casing 5 and the devices located inside the rotating casing may be similar to those described in relation to Figure 3. For simplicity, only the upper part of the rotating casing is shown in Figure 8, but also in this second embodiment the gas flow may reach the devices installed in the rotating casing 5 and provide them with cooling.

[0041]

[0045] It should be understood that the above description and accompanying drawings are only intended to illustrate the present invention. It will be apparent to those skilled in the art that changes and modifications can be made to the present invention without departing from the scope of the invention. The following is a summary of the claims as originally filed: [1] An apparatus comprising a cycloidal propulsion unit having an electric motor (1, 31, 41, 51, 61) having a stator (2, 32, 62) and a rotor (4, 54), a cooling space (11) provided in the stator (2, 32, 62) for receiving and passing a cooling fluid for cooling the stator; an inlet (12) and an outlet (13) provided in the electric motor for receiving and passing a cooling gas flow that cools at least the rotor (4, 54) before exiting the electric motor; The electric motor is provided with a dual cooling system comprising: [2] The apparatus according to [1], further comprising a fan (15) for circulating the cooling gas flow from the inlet (12) to the outlet (13). [3] The apparatus according to [1] or [2], further comprising a heat exchanger (14) through which the cooling gas flow passes, the heat exchanger (14) receiving and passing a cooling fluid that cools the cooling gas flow in the heat exchanger. [4] The device according to any one of [1] to [3], wherein an annular gap (20) is provided between the outer periphery of the rotor (4, 54) and the stator (2), and the cooling gas flow between the inlet (12) and the outlet (13) is directed to pass through the annular gap (20). [5] The apparatus described in any one of [1] to [4], wherein the cooling gas flow is directed to flow through at least one opening (19, 22) in the rotor (4, 54) before exiting the electric motor (1). [6] The rotor (54) has a hole (55) formed along the outer periphery of the rotor and is configured to rotate around a rotation axis (6); the cooling gas flow is directed to flow between the inlet (12) and the outlet (13) by passing through the holes (55) in the rotor (54) located in close proximity to the outer periphery of the rotor (54); The device according to any one of [1] to [5]. [7] The device described in any one of [1] to [6], wherein one of the inlet (12) and the outlet (13) is located in proximity to the cooling space (11) so that the cooling fluid in the cooling space (11) cools the cooling gas flow entering or exiting the electric motor (61). [8] The device according to any one of [1] to [7], wherein the cooling space (11) provided in the stator (2, 32, 62) includes a cooling jacket that at least partially surrounds the stator (2, 32, 62). [9] The device according to any one of [1] to [8], wherein the cooling space (11) provided in the stator (2, 32, 62) includes a cooling pipe provided in the stator (2, 32, 62).

[10] The cycloidal propulsion unit comprises a rotating casing (5) arranged as an extension to the stator (2, 32, 62) coaxially with the rotor (4, 54) for rotation therewith about a vertical axis of rotation (6); The rotating casing (5) is provided with a drive mechanism (8) arranged to rotate the blades (7) of the cycloidal propulsion unit projecting outward from the rotating casing (5) in a direction away from the rotor (4, 54). The device described in any one of [1] to [9].

[11] The apparatus according to

[10] , wherein at least a portion of the cooling gas flow is directed to flow from the inlet (12) through the rotating casing (5) to the outlet (13) to cool the drive mechanism.

[12] The device according to

[10] or

[11] , wherein the rotor (4, 54) is provided with an opening (22) that provides access for an inspector to enter the rotating casing (5) by passing through the stator.

[13] The device according to any one of [1] to

[12] , wherein the rotor (4, 54) has a diameter (D) greater than the length (L) of the rotor in the direction of the rotation axis (6).

[14] A method for cooling an electric motor (1, 31, 41, 51, 61) having a stator (2, 32, 62) and a rotor (4, 54), comprising: circulating a cooling fluid through the stator (2, 32, 62) to cool the stator; Passing a cooling gas flow into the electric motor (1, 31, 41, 51, 61) via an inlet (12), through the rotor (4, 54), and out of the electric motor via an outlet (13); 10. A method comprising providing dual cooling by

[15] The method according to

[14] , wherein the cooling gas flow is directed along the outer periphery of the rotor (4, 54) and through an annular gap (20) provided between the rotor and the stator (2).

[16] The method according to

[14] or

[15] , wherein the cooling gas flow is directed along the periphery of the rotor (54) and through holes (55) provided in the rotor (54).

Claims

1. 1. An apparatus comprising a cycloidal propulsion unit having an electric motor (1, 31, 41, 51, 61) having a stator (2, 32, 62) and a rotor (4, 54), and a rotating casing (5) arranged as an extension to the stator (2, 32, 62) and coaxially with the rotor (4, 54) for rotation therewith about a vertical axis of rotation (6), wherein the rotating casing (5) comprises a drive mechanism (8) arranged to rotate blades (7) of the cycloidal propulsion unit projecting outwardly from the rotating casing (5) in a direction away from the rotor (4, 54), The electric motor a cooling space (11) provided in the stator (2, 32, 62) for receiving and passing a cooling fluid for cooling the stator; an inlet (12) and an outlet (13) provided in the electric motor for receiving and passing a cooling gas flow that cools at least the rotor (4, 54) before leaving the electric motor; and wherein at least a portion of the cooling gas flow is directed to flow from the inlet (12) through the rotating casing (5) to the outlet (13) to cool the drive mechanism.

2. 2. The apparatus of claim 1, further comprising a fan (15) for circulating the cooling gas flow from the inlet (12) to the outlet (13).

3. 2. The apparatus of claim 1, wherein the apparatus comprises a heat exchanger (14) through which the cooling gas flow passes, the heat exchanger (14) receiving and passing a cooling fluid that cools the cooling gas flow in the heat exchanger.

4. 2. The apparatus of claim 1, wherein an annular gap (20) is provided between an outer periphery of the rotor (4, 54) and the stator (2), and the cooling gas flow between the inlet (12) and the outlet (13) is directed to pass through the annular gap (20).

5. 2. The apparatus of claim 1, wherein the cooling gas flow is directed to flow through at least one opening (19, 22) in the rotor (4, 54) before exiting the electric motor (1).

6. The rotor (54) has a hole (55) formed along the outer periphery of the rotor and is configured to rotate around a rotation axis (6); the cooling gas flow is directed to flow between the inlet (12) and the outlet (13) by passing through the holes (55) in the rotor (54) located in close proximity to the outer periphery of the rotor (54); 10. The apparatus of claim 1.

7. 2. The apparatus of claim 1, wherein one of the inlet (12) and the outlet (13) is located in proximity to the cooling space (11) so that cooling fluid in the cooling space (11) cools the cooling gas flow entering or exiting the electric motor (61).

8. 2. The device according to claim 1, wherein the cooling space (11) provided in the stator (2, 32, 62) comprises a cooling jacket at least partially surrounding the stator (2, 32, 62).

9. 2. The device according to claim 1, wherein the cooling space (11) provided in the stator (2, 32, 62) comprises a cooling pipe provided in the stator (2, 32, 62).

10. 2. The device of claim 1, wherein the rotor (4, 54) is provided with an opening (22) that provides access for service personnel to enter the rotating casing (5) by passing through the stator.

11. 2. The device according to claim 1, wherein the rotor (4, 54) has a diameter (D) greater than the length (L) of the rotor in the direction of the axis of rotation (6).

12. 1. A method for cooling an electric motor (1, 31, 41, 51, 61) having a stator (2, 32, 62), a rotor (4, 54), and a rotating casing (5) arranged as an extension to the stator (2, 32, 62) coaxially with the rotor (4, 54) for rotation therewith about a vertical axis of rotation (6), the rotating casing (5) comprising a drive mechanism (8) arranged to rotate blades (7) of a cycloidal propulsion unit projecting outwardly from the rotating casing (5) in a direction away from the rotor (4, 54), the method comprising: circulating a cooling fluid through the stator (2, 32, 62) to cool the stator; Passing a cooling gas flow into the electric motor (1, 31, 41, 51, 61) via an inlet (12), through the rotor (4, 54), and out of the electric motor via an outlet (13); and wherein at least a portion of the cooling gas flow is directed to flow from the inlet (12) through the rotating casing (5) to the outlet (13) to cool the drive mechanism.

13. 13. The method of claim 12, wherein the cooling gas flow is directed along the periphery of the rotor (4, 54) and through an annular gap (20) provided between the rotor and the stator (2).

14. 13. The method of claim 12, wherein the cooling gas flow is directed around the periphery of the rotor (54) and through holes (55) in the rotor (54).

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