WO2008028996A1

WO2008028996A1 - Arrangement for cooling an electrical machine

Info

Publication number: WO2008028996A1
Application number: PCT/FI2007/000226
Authority: WO
Inventors: Jouni Ikäheimo; Alpo Hauru; Ville KIVELÄ
Original assignee: Abb Oy
Priority date: 2006-09-07
Filing date: 2007-09-07
Publication date: 2008-03-13
Also published as: FI119458B; FI20060800A0; EP2059996A4; FI20060800A; EP2059996A1

Abstract

The object of the invention is a cooling arrangement for an electrical machine in which a main magnetic field affects the stator and the rotor. In the arrangement, an auxiliary rotor (16, 16') is fitted onto the same shaft (8) with the rotor (6) of the machine and arranged to rotate in relation to the stator (4) and drive shaft (8) of the electrical machine. The number of poles in the auxiliary rotor (16, 16') is substantially smaller than the number of poles in the electrical machine, and a subharmonic magnetic field induced on the stator (6) of the electrical machine is arranged to rotate the auxiliary rotor (16, 16') and a fan (20) arranged to it.

Description

ARRANGEMENT FOR COOLING AN ELECTRICAL MACHINE

The object of the invention is an arrangement for the cooling of an electrical machine according to the preamble of claim 1.

During operation, an electrical machine produces heat that must be conveyed out of the machine. Air cooling is a commonly used cooling method in which a fan is fitted on the shaft of the machine for blowing air through the electrical machine. If necessary, special air deflectors are used to route the cooling air to the objects that have the greatest need for cooling. Such objects include winding ends, for example. If the flow-through of ambient air is not sufficient, cooling is intensified using heat exchangers that cool down the cooling air. This increases the temperature difference between the cooling air and the parts of the electrical machine that are to be cooled, improving cooling efficiency.

The simplest solution is a fan connected directly to the shaft of the electrical machine that rotates at the same speed as the machine. In speed-controlled drives, the fan speed will drop simultaneously with the rotational speed of the machine, which causes the cooling effect to decrease at speeds lower than the rated speed. However, the need for cooling capacity may remain unchanged or even increase in comparison with uncontrolled speed. On electrical machines that rotate at a slow speed and are under heavy load, a fan coupled directly to the drive shaft of the machine is not a sufficient solution even at rated speed and rated load. In order to keep the machine operating temperature within specified limits, ad- ditional cooling must be generated using heat exchangers or separate fans, for example.

Permanent-magnet motors with fractional slot windings are preferred electric motors in applications that require a slow speed of rotation. The internal air circulation needed for cooling these slow motors is also weakened when the rotational speed decreases. Solutions have been proposed in which an electrical machine is fitted with a second rotor for rotating the fan that is coaxial to the drive shaft and rotates at a higher speed compared to the drive shaft. This increases the cooling air flow rate and thus intensifies cooling.

For example, a solution is known from the patent publication GB 948013 in which the slip speed of a squirrel cage motor is utilised. In the publication, some of the rotor's squirrel cage bars are used as the "stator" of the fan motor. The fan rotor is supported on the drive shaft rotor by bearings, which allows the fan to rotate at a different speed compared to the drive shaft. The publication GB 2139011 proposes a solution for an asynchronous machine in which one of the drive shaft ends is supported on one of the end plates of the electrical machine through the fan motor's bearings. The drive motor is controlled over a wide range of speeds while the speed of the fan motor varies relatively little.

According to prior art, the speed of the rotor driving the fan is higher than that of the actual drive motor. This is accomplished by either controlling or constructing the motor driving the fan in a different way compared to the drive motor. In practice, such arrangements will always require additional components such as additional bearings and windings, as well as additional space.

The objective of the invention is to create advantageous and efficient air cooling for an electrical machine in which a motor driving a fan is arranged coaxially with the shaft of the electrical machine serving as the drive machine. In order to achieve this, the electrical machine according to the invention is characterised by the features specified in the characteristics section of claim 1. Certain particular embodiments of the invention are characterised by the features specified in the dependent claims.

In a solution according to the invention, the means for cooling an electrical machine constitute an integrated and compact structure with the electrical machine. Compared to the basic solution in which a fan is coupled directly to the drive shaft of the electrical machine, the present solution only requires an auxiliary rotor driving the fan and the bearings to fit it on the drive shaft of the electrical machine. The longitudinal space requirement of the fan is equal to that of the basic solution referred to in the above. At the same time, the solution according to the invention provides sufficient cooling effect for applications that have previously required cooling solutions external to the drive motor.

According to a preferred embodiment of the invention all the stator teeth do not extend to the whole length of the stator. The axial length of some teeth, e.g. every second tooth is shorter than stator' s length. The main flux of the machine will diminish in the area of the auxiliary rotor whereas the subharmonic flux will strengthen.

According to another preferred embodiment of the invention the first and second poles of the auxiliary rotor have different breadth in the circumferential direction. The edges of the first pole and the second pole will not simultaneously pass the stator slot of the same phase and therefore the torque will be more even. The invention will be described in detail with the help of certain embodiments by referring to the enclosed drawings, where

- Figure 1 illustrates an electrical machine according to the invention,

- Figure 2 illustrates the cross-section A - A of an electrical machine according to the invention at the drive rotor,

- Figure 3 illustrates the cross-section B - B of an electrical machine according to the invention at the auxiliary rotor,

- Figure 4a illustrates the cross-section of an electrical machine according to another embodiment of the invention at the drive rotor,

- Figure 4b illustrates the cross-section of an electrical machine according to third embodiment of the invention at the auxiliary rotor.

Figure 1 illustrates the cross-section of an electrical machine according to an embodiment of the invention. The frame structure 2 of the electrical machine surrounds the sheet pack of the stator 4 that is manufactured of magnetically conductive sheets. Slots for fitting the windings have been formed in the stator. The coil ends 14 of the windings protrude out of the sheet pack at both ends of the stator. The ends of the electrical machine have end shields 12 that are fixed to the stator frame structure 2. Bearings 10 are fitted in the centre of the end shields 12 to support the drive shaft 8 of the electrical machine that is coupled in a well-known way to the device driven by the electrical machine. Inside the stator 4, the electrical machine's rotor 6 is mounted on the drive shaft 8; in this embodiment, the rotor is manufactured from an iron core with permanent magnets fitted on its surface to excite the electrical machine. The electrical machine is a low-speed motor with a high number of poles and a fractional slot winding as described in more detail below.

In Figure 1, an auxiliary rotor 16 is fitted to the right of the electrical machine's rotor to drive a fan 20 directly mounted on it. The auxiliary rotor 16 is supported on the shaft 8 by bearings 18 and is thus rotating in relation to both the stator 4 and the electrical machine's rotor 6. The fan 20 routes cooling air to the coil ends 14, the stator 4 and the rotor 6 as is known in the context of air cooling for electrical machines. In Figure 1, a second auxiliary rotor 16' and a second fan 20' coupled to it have been installed to the left of the electrical machine's rotor. The second auxiliary rotor 16' is also mounted on the shaft 8 with a bearing 18' and therefore the second fan 20' operates substantially in the same way as the fan 20 installed at the other end of the electrical machine. Compared to the rotor 6, the auxiliary rotors 16 and 16' are significantly shorter in the axial direction. The structure of the auxiliary rotor 16 can be similar to that of the rotor 6, in which case it is manufactured from an iron core, such as a cast iron core, with permanent magnets glued on the outer surface. The appearance of the auxiliary rotors can be disc-shaped.

The electrical machine is connected either directly to the mains, in which case the supply frequency is generally 50 Hz, or in controlled operation through a frequency converter, in which case the supply frequency varies within the control range. Alternating current supplied to the stator winding of the electrical machine generates a magnetic field in the electrical machine, the frequency of which corresponds to the supply voltage frequency. This main magnetic flux is closed through the stator, the air gap and the rotor, and takes part to produce the operating power of the machine. It is well known that the rotational speed of an alternating current machine is determined by the number of pairs of poles in the machine. While the rotational speed of a two-pole machine at a supply frequency of 50 Hz is 3000 revolutions per minute, the speed of a 30-pole machine at the same frequency is only 200 revolutions per minute, for example.

A preferred solution in slow operation is a permanent-magnet motor with a small factor q (factor q<l, q is number of slots per pole per phase). The motor winding is simple and it is easy to create a corresponding high number of poles on the rotor, e.g. by gluing magnets onto the surface of a solid iron rotor. The stator winding is manufactured as a so-called fractional slot winding in which the number of slots per phase and pole is a fractional number. An example of fractional slot winding with a small factor q is an electrical ma- chine in which the number of poles is 2p=34, the number of phases is m=3 and the number of stator slots is Ql=36. In this case, the factor q is q = Ql/(2*p*m) = 36/(34*3) = 6/17. The main field is 34-pole and rotates at 176 rpm at a supply frequency of 50 Hz. In addition to the main field, the fractional slot winding generates several subharmonic fields that are also closed through the magnetic circuits of the stator and rotor. In particular, a 34-pole main field generates magnetic fields that have 2, 4, 6 and 8 poles. When an auxiliary rotor with 2 poles, for example, is bearing-mounted on the shaft of the drive rotor, the corresponding subharmonic component imposes torque on the auxiliary rotor and rotates it at 3000 rpm. The circumferential speed of the inside fan attached to the auxiliary rotor reaches a value high enough to cool the electrical machine.

A two-pole auxiliary rotor is preferred because the rotational speed is highest. However, other subharmonic fields rotating faster than the electrical machine's field are also avail- able for use in accordance with the invention because an auxiliary rotor attached to the fan and rotated by these fields will rotate faster than the drive rotor. The extent and intensity of the subharmonic fields is a machine-specific characteristic and therefore the most feasible number of poles for the auxiliary rotor must be determined separately for each application. Correspondingly, the rotational speed of the auxiliary rotor affects the internal air deflec- tors for the fan and the machine, their locations and shapes, as well as the air intake openings.

In Figure 1, an auxiliary or additional rotor is installed on both sides of the electrical machine's drive rotor. The guiding of cooling air within the electrical machine is implemented in accordance with the dimensioned cooling requirement of the electrical machine. There- fore an electrical machine may have one or two fans on an application-specific basis. The air flow of the fans can be guided inside through both ends and out from the middle of the electrical machine. Or, fans installed at opposite ends can be used to guide the cooling air flow in through one end and out from the other, and the air can be guided within the electrical machine to the objects that generate most heat. The guiding of cooling air can be im- plemented by generally known methods such as internal cooling ducts.

Figures 2 and 3 illustrate the operation of a solution according to the invention in an electrical machine excited by permanent magnets in which the auxiliary rotor driving the fan is also excited by permanent magnets. Where applicable, the reference numbers used in Figures 2 and 3 are the same as in Figure 1. Figure 2 illustrates one half of the axial cross- section A - A of a 34-pole electrical machine at the machine's stator 4 and rotor 6. For the sake of clarity, the cross-section does not show the stator winding slots or teeth. On the outer surface of the rotor - that is, on the surface facing the air gap 22, permanent magnets 26 and 28 have been fitted, and the embodiment of Figure 2 has 34 of these. The permanent magnets 26 are excited so that on the outer circumference of the rotor, the S pole of permanent magnet 26 and the N pole of permanent magnet 28 are alternately facing the air gap 22. The main magnetic field of the electrical machine is illustrated by flux lines 24 that are closed through the stator 4 and the rotor 6. The fluxes are closed through adjacent counter-clockwise and clockwise poles, which makes the flux direction correspond to the arrows in the flux lines 24. At a supply frequency of 50 Hz, the rotational speed of this 34- pole machine is approximately 176 revolutions per minute.

Figure 3 illustrates a cross-section B - B at the auxiliary rotor 16. A permanent magnet 30 is fitted on the outer surface of the auxiliary rotor - that is, the circumferential surface facing the air gap 22. The auxiliary rotor 16 is supported on the shaft 8 in a rotating manner by the bearing 10. The subharmonic magnetic field induced by the electrical machine's sta- tor winding (not shown in Figure 3), which has two poles, is closed in accordance with the flux lines 32 through the back of the stator 4, the back of the auxiliary rotor 16, the air gap and the permanent magnet 30. Correspondingly to permanent magnet 30, a permanent magnet in the opposite direction is installed on the other half of the circumference of the auxiliary rotor. Even though the permanent magnet illustrated in Figure 3 is formed of one piece, it may consist of several permanent magnet pieces that are parallel in the direction of the outer circumference of the rotor and are excited in the same direction. In Figure 3, the auxiliary rotor is formed with two poles but it can equally well have four, six or eight poles in connection with the 34-pole machine of Figure 2. Similarly the number of poles in the auxiliary rotor can be any number corresponding to a subharmonic field of the magnetic field of the actual electric motor.

Figures 4a and 4b illustrate some other advantageous features according to the invention. Where applicable, the reference numbers used in Figures 4a and 4b are the same as in Figures 1 to 3. Figure 4a illustrates the cross-section of the electric machine like the Figure 2, but the structure of the stator teeth 38 and stator slots 40 are illustrated in Figure 4a. The stator 4 consists of the laminated core wherein the outer portion forms the stator yoke 42. The stator slots 40 and the stator teeth 38 are formed on the inner portion of the stator. The stator windings 44 are arranged into the slots 40 as well-known in the art. Only some of the windings are shown. An electrical machine is excited by permanent magnets 28 that are fixed onto the surface of the rotor as stated above in connection of Figure 2. Figure 4b shows the cross-section of the auxiliary rotor 16 and the end portion of the stator 4 that surrounds the auxiliary rotor 16. In this embodiment of the invention every second tooth of the stator is absent at the end portion of the stator and thereby there are fewer teeth 38', i.e. half of the number of the teeth in the stator and the slots 46 are broader. The breadth of the slot 46 is equal to the sum of one tooth 38' of the stator and two slots 40 of the electric machine. The windings 44 extend to the end of the stator and there is a space 48 between two windings that are positioned in the same broader slot 46. The main magnetic flux in the section that surrounds the auxiliary rotor 16 will diminish and subharmonic flux will strengthen. Thus the torque and efficiency of the auxiliary rotor will grow.

A further advantageous embodiment of the invention is shown in the Figure 4b, too. The first and the second pole of the auxiliary rotor are not symmetrical. The first permanent magnet 50 forms the first pole of the auxiliary rotor 16 and the breadth of the first pole in the circumferential direction of the rotor is approximately seven teeth 38' and six slots 46. The breadth of the second permanent magnet 52 forming the second pole of the auxiliary pole is smaller and the second magnet extends approximately over five teeth 38' and over five slots 46. The edges of the first and the second will not pass simultaneously the stator winding of the same phase. The effective magnetic flux of the auxiliary rotor remains almost the same but the torque produced will be more even.

In the embodiments of the invention as described above permanent magnets has been fixed onto the surface of the rotors. The rotors with totally embedded or partly embedded mag- nets can be used respectively. Further the magnetizing energy of the main rotor and the auxiliary rotor may be implemented in different ways.

In the above the invention has been described with the help of a certain embodiment. However, the description should not be considered as limiting the scope of patent protection; the embodiments of the invention may vary within the scope of the following claims.

Claims

1. An arrangement for cooling an electrical machine, the electrical machine comprising a stator (4), the number of poles in the electrical machine being substantially high and the rotor (6) of the electrical machine being mounted on the drive shaft (8) of the electrical machine, and a main magnetic field affecting the stator and rotor being induced in the electrical machine, whereby on the same shaft (8) with the rotor (6) of the machine, substantially inside the stator (4) of the machine, there is fitted an auxiliary rotor (16, 16') that is arranged to rotate in relation to the stator (4) and drive shaft (8) of the electrical machine, and the auxiliary rotor has a substantially smaller number of poles than the electrical ma- chine, characterised in that at least one subharmonic magnetic field is arranged to affect the stator (6) of the electrical machine, said subharmonic magnetic field being arranged to affect at least the auxiliary rotor (16, 16') so that the subharmonic magnetic field is arranged to rotate the auxiliary rotor (16, 16') and a fan (20) arranged to it.

2. An arrangement according to claim 1, characterised in that the number of slots per pole per phase is less than one.

3. An arrangement according to claim 1 or 2, characterised in that the number of poles in the electrical machine is greater than 20 and the number of poles in the auxiliary rotor is no more than 8.

4. An arrangement according to any of the claims from 1 to 3, characterised in that the number of poles in the electrical machine is 34 and the number of poles in the auxiliary rotor is no more than 8.

5. An arrangement according to any of the claims from 1 to 4, characterised in that the auxiliary rotor (16) is fitted and bearing-mounted onto the first end of the drive shaft (8).

6. An arrangement according to any of the claims from 1 to 4, characterised in that there are two auxiliary rotors (16, 16') arranged in the electrical machine and fitted onto the first and second ends of the drive shaft.

7. An arrangement according to any of the claims from 1 to 6, characterised in that the electrical machine is excited by permanent magnets (26, 28).

8. An arrangement according to any of the claims from 1 to 7, characterised in that the auxiliary rotor (16, 16') of the electrical machine is excited by permanent magnets (30).

9. An arrangement according to any of the claims from 1 to 8, characterised in that number of slots (38') and teeth (46) is smaller at the end of the stator (4) surrounding the auxiliary rotor (16).

10. An arrangement according to the claims from 9, characterised in that number of teeth (38') at the end of the stator is half of the number of the teeth (38) in stator and there are two parallel windings (44) in one slot (46).

11. An arrangement according to any of the claims from 1 to 10, characterised in that the breadth of the first pole (50) in the auxiliary rotor is broader than the breadth of the second pole (52) in the auxiliary rotor (16) in the circumferential direction of the rotor.