WO1997009770A1

WO1997009770A1 - Axial air gap electric motor

Info

Publication number: WO1997009770A1
Application number: PCT/GB1996/002168
Authority: WO
Inventors: Michael Short; Kevin Pickering; Robert Orton
Original assignee: Morris Mechanical Handling Limited
Priority date: 1995-09-06
Filing date: 1996-09-03
Publication date: 1997-03-13

Abstract

An electric motor has a fixed stator assembly and a rotatable rotor assembly and the air gap between them extends in the axial direction of the motor. The motor assembly is displaceable axially away from the stator assembly into engagement with a brake. The axial position of the brake and hence of the rotor assembly is adjustable. On energising the motor the rotor assembly moves out of engagement with the brake and takes up a position with a predetermined air gap between the rotor and stator assemblies.

Description

AXIAL AIR GAP ELECTRIC MOTOR

This invention relates to an electric motor having stator and rotor assemblies with an axially extending air gap between them.

Electric chain or rope hoists have traditionally employed a squirrel-cage induction motor for driving the hoist and a separate brake mechanism is supplied to hold the hoist and prevent slippage when the motor is not energised. This results in a large motor-hoist-brake combination.

An electric motor of the type to which the invention relates can have a rotor assembly which is displaceable axially of the stator assembly and the present invention is concerned with such a motor which could be used with or incorporated in a hoist or the like which requires braking when the drive motor is de-energised.

According to the present invention an electric motor has a fixed stator assembly and a rotatable rotor assembly with an axial air gap between them, the rotor assembly being displaceable axially towards and away from the stator assembly to vary the air gap, means urging the rotor assembly axially away from the stator assembly and into engagement with a brake, means for adjusting the axial position of the brake relative to the stator assembly and wherein on energising the motor the rotor assembly moves out of engagement with the brake to a running position where there is an axial air gap of predetermined dimension between the rotor and stator assemblies.

The axial air gap motor is designed such that the force of attraction between the energised stator assembly and the rotor assembly is used to overcome the spring loaded brake and to cause the rotor assembly to move to the running position relative to the stator winding with a predetermined air gap between them. As soon as the motor is de-energised either intentionally or in the event of a supply failure, the rotor assembly is displaced into engagement with the brake and further rotation of the rotor assembly and any load driven by it is stopped. In the design of a hoist, safety is paramount, and so a "fail to safe" is a necessity.

The brake springs are designed to cause the brake to arrest the rotor assembly in a smooth and controlled manner and the motor has to be designed so that the force of attraction is sufficient to overcome the springs.

It is convenient for the brake to comprises a first non-rotatable part which is adjustable axially with respect to the stator assembly and a second part forming part of the rotatable rotor assembly.

It is convenient for the first part of the brake to be mounted on a seat having an outer periphery in threaded relation with a stationary housing whereby rotation of the seat relative to the housing causes it to be moved axially relative to the stator assembly.

In an alternative arrangement, the axial air gap in the running position can be determined by the thickness of a shim, located during assembly of the motor, between said housing and the fixed support for the stator assembly.

Alternatively, the rotor assembly can be mounted on a shaft, a closed sleeve fitted over the end of the shaft and spring means in the sleeve urging the sleeve axially relative to the shaft away from the stator assembly, said rotor assembly being attached to the sleeve.

In order that the invention may be more readily understood it will now be described, by way of example only, with reference to the accompanying drawings in which:-

Figure 1 is a sectional plan of an electric motor in accordance with the invention and incorporated in a hoist,

Figure 2 is a plan view of a laminated pack of magnetic material forming part of the rotor assembly of the motor.

Figure 3 is a plan view of a laminated pack of magnetic material forming part of the stator assembly of the motor,

Figure 4 is a sectional plane of part of an electric motor similar to that shown in figure 1 and in accordance with an alternative embodiment of the invention,

Figure 5 is an end view in the direction 5-5 of figure 4,

Figures 6 and 7 are similar to figure 4 and illustrate the two axial end positions of the rotor assembly of the motor and

Figure 8 is a sectional plan of part of an electric motor similar to that shown in figure 1 and in accordance with an alternative embodiment of the invention.

Referring to figure 1, an elongate rotatable shaft 1 has an end portion 3 which projects into a tubular sleeve 5 having an annular flange 7 at one end. The flange carries an annular brake pad 9. The sleeve is mounted for rotation in bearings in a rotor housing 11. The outer end of the tubular sleeve is closed and a spring 13 in the sleeve acts between the end of the sleeve and the adjacent end of the shaft to urge the sleeve 5 axially away from the end of the shaft so that the brake pad 9 engages with a brake seat 15 on the rotor housing 11.

Adjacent the annular flange 7 the shaft 1 carries a rotor assembly 17 which is εplined onto the shaft and coacts with a stator assembly 19 fixed in the housing and separated from the shaft by a central opening. An air gap 21 between the rotor and stator assemblies extends parallel to the longitudinal axis of the shaft 1.

When the motor is de-energised and there is no attraction force across the air gap 21 between the rotor and stator assemblies, the spring 13 urges the sleeve 5 axially in the direction away from the stator 19 until the brake pad 9 engages with the brake seat 15 thus preventing rotation of the sleeve relative to the rotor housing. Since the sleeve 5 is mechanically connected to the rotor assembly, the rotor assembly is also prevented from rotating. When the stator is energised the rotor assembly is attracted towards the stator assembly and the rotor assembly is moved axially relative to the shaft. This movement is limited by the engagement of a nut 18 on the sleeve 5 abutting against a free anvil 19 on a rotatable bearing stop 22 and sets the running air gap 21 to a predetermined efficient dimension.

The brake seat 15 can be moved axially towards and away from the stator in order to take up wear of the brake pad by adjusting screws 60 extending through the housing 11.

The motor includes a gearbox assembly and the remote end of the shaft 1 is a gear cut pinion and a spur gear 23 mounted on a lay-shaft 25 meshes with the shaft to be in driving relation therewith. The lay-shaft is rotatably mounted in an extension 11A of the rotor housing. The spur gear 23 includes a clutch mechanism 27 which permits the gear to rotate relative to the lay-shaft 25 in the event of serious overload.

A chain sprocket assembly 29 surrounds the shaft 1 and includes an output spur gear 31. The assembly is free to rotate independently of the shaft 1. The spur gear 31 meshes with a pinion 33 on the lay-shaft 25.

Referring to figures 1 and 2, the rotor assembly 17 comprises a steel insert 40 which is internally splined and which is cast into an aluminium ring 42. Surrounding the ring 42 is a toroidal lamination pack 44 of magnetic material which is insulated on both of its opposite faces. A further aluminium outer ring 46 is cast onto the outer periphery of the lamination pack 44. A multiplicity of substantially radially directed slots 43 are formed in the lamination pack. The slots are filled with aluminium conductors which are connected to the inner and outer rings 42 and 46 and serve as rotor bars.

The cross-sectional shape of the rotor slots are specifically designed to optimise the surface area of the rotor by minimising the dimension of the mouth of the slot. The neck of the slot is tapered at an angle to optimise the cross-sectional area variation in length to achieve optimum double-bar technology whilst maintaining a simple slot shape.

Referring to figures 1 and 3 , the stator structure 19 comprises a toroidal lamination pack 50 rigidly secured to the hoist body by mechanical means not shown. A multiplicity of slots 52 extend substantially radially through the pack. The slot cross-sectional dimensions are constructed to afford a narrow opening to assist coil insertion with sufficient cross- sectional area to accommodate both fast and slow speed windings 54 with the fast speed winding being positioned at the base of the slots. The two windings comprise multi-turn insulated copper conductors. In a typical motor, for example, the slow speed winding could comprise an 8 pole per phase winding distributed over 24 slots and the fast speed winding could be 2 pole per phase winding distributed over the same 24 slots.

By increasing the slot depth of the stator winding it allows more turns of wire, or larger conductors to be fitted. The disadvantage is that the fast speed winding is pushed further away from the rotor, thereby reducing its overall effect. The volume of backing iron reduces behind this winding and unless the motor is extended in length the level of saturation will increase.

A similar effect is noted when the slot width is altered, but here the level of iron surrounding the windings [conveniently 2 and 8 poles per phase] is reduced, forcing the motor diameter to increase for the same level of saturation. This is particularly evident whilst starting conditions exist, due to the high levels of current present. For this reason, the two pole winding is placed at the bottom of the slot, where most of the iron exists.

One of the most important features of this type of motor is the attraction force between the stator and rotor assemblies. If this resource is to be utilised effectively this force has to be quantified.

When looking at the mechanical design of the hoist, the attraction force which exists between the rotor and stator assemblies must be quantified from its maximum to minimum values. This data is required to enable full calculations of loadings to be carried out on equipment such as bearings, as well as impact forces on end stops.

Using computer analysis, theoretical values are calculated for various air gaps. When manufacturing this design of motor it is important to incorporate some mechanism to take up system tolerances, therefore providing the optimum attraction and running air gaps.

The attraction air gap may be varied to adjust both the time taken to lift the brake and the force of impact when it arrives at the predetermined running air gap. If this parameter is varied by brake wear, then consideration must be given to wear rates and limits.

The rating of dual speed hoist motors is split over both sets of windings in the order of 33 percent on the slow and 66 percent on the fast. Hence the fast winding is used more often than the slow speed.

Table 1 shows the characteristics of the axial air gap motor compared with a standard squirrel cage motor.

Tabl e 1

Comparison of Standard Squirrel Cage Motor and the Axial Air Gap Motor.

By consulting Table 1 it can be seen that the axial motor has a starting torque, for the fast speed winding, over 50 percent larger than the equivalent standard motor. This will allow the motor to accelerate the load up to speed in a shorter period of time, therefore reducing the time the control switchgear sees the starting currents.

Stator copper losses for the axial motor slow speed winding are higher as a result of the slot fill requirements of the fast speed winding. This is obviously not as critical because the low duty requirements and the reduction of performance given by standard motors. This effect is offset by the fast winding having a lower loss level than a standard motor.

Providing a cavity throughout the stator bore in this application, produces a reduction in the active air gap surface area. This has resulted in an increase in the axial motor's core losses. If a closed rotor slot is used, this value is reduced.

Tests were completed on the motor of the invention at 415 and 440 volts. These are illustrated in Table 2.

Tabl e 2

Summary of Test Results for a Self Ventilated Axial Air Gap Motor

An alternative form of adjusting means for the air gap is shown in figures 4-7.

When the motor is de-energised and there is no attraction force across the air gap 21, the spring 3 urges the sleeve in the direction away from the stator 19 so that the brake pad engages with the brake seat thus preventing rotation of the sleeve, and hence the rotor assembly, relative to the housing 11. The brake seat 15 is mounted in the housing 11 by means of a plurality of screws 22, one of which is shown in the figures. By adjusting the screws in a manner to be described, the axial position of the brake seat relative to the housing 11 is varied. The position of the seat determines the dimension of the air gap 21 when the rotor assembly is in abutting relation with the brake seat. This dimension of the air gap is referred to as the pull-in gap because it is from this position of the rotor that the rotor is pulled into driving relation with the stator when the stator is energised and the motor is in use.

When the rotor is pulled into driving relation with the stator, as shown in figure 7, the nominal air gap between the rotor and stator also has to be adjusted to give a predetermined value. The tubular sleeve 5 has an axially threaded outer portion and two lock-nuts 23 are mounted on this portion. By adjusting the lock-nuts towards or away from the adjacent surface of the rotor housing 11, the travel T of the tubular sleeve and hence the rotor in the axial direction is determined when the lock-nuts engage the housing and as the nominal Running air gap = Pull in air gap - the travel T of the sleeve, the adjustment of the lock-buts 23 brings about ar. adjustment of the nominal air gap when the motor is running.

It will be appreciated that the adjustment of the pull- in gap by the screws 22 is independent of the adjustment of the running gap by the lock-nuts 23. Hence, as the brake wears with use, it can be adjusted without affecting the running air gap.

The adjustment of the brake seat is brought about as follows. Referring to figures 4 and 5, the rotor housing 11 has three slots 25 which are of arcuate form and lie on a circle coaxial with the shaft 1. The three screws 22 extend through respective slots and are threaded into holes in the brake seat 15. The periphery of the brake seat is threaded and the seat is mounted on a screw thread in the housing 11. To adjust the brake seat the screws 22 are loosened and the screws are used to rotate the brake seat thus causing it to move axially of the housing. The screws move within the slots and are then tightened when the brake seat has been rotated sufficiently. It is convenient for the brake seat to have six threaded holes equi-spaced around the axis of the central opening nd to allow for greater rotation of the brake seat, it is possible to remove screws which reach the end of the slot 25 and place them in the next threaded hole.

In the alternative arrangement shown in figure 8, the end portion 3 of the shaft 1 is surrounded by a sleeve 70 which is secured, conveniently by welding, at an end to a splined seating 72 on which the rotor is mounted. The sleeve, seating and the rotor assembly 17 can be displaced axially of the shaft 1 towards and away from the stator assembly 19 to adjust the air gap 21. The sleeve 70 carries a brake pad 74 which is urged into engagement with a brake seat 76. The brake seat 76 is threaded within a fixed housing 78 and rotation of the brake seat within the housing causes the brake seat to be moved axially, thereby enabling wear of the brake seat and the brake pad to be compensated.

The housing 78 has a generally hollow cylindrical portion 80 which surrounds the rotor assembly and the free end of the cylindrical portion is turned outwardly with a flange 82. This flange projects into an annular slot formed by adjacent ends of two parts of the casing of the motor and a shim 84 is positioned in the slot during assembly of the motor.

The shim is positioned between the flange 82 on the housing and the part of the motor casing which carries the stator assembly. By choosing a shim of an appropriate axial thickness during the assembly of the motor, the axial position of the brake seat 76 relative to the stator assembly is determined.

Acting between the sleeve 70 and the support for the brake pad 74, there are a plurality of springs 86 which are compressed when the motor is energised and which urge the rotor assembly into its braked position when the motor is de- energised.

Claims

Claims :

1. An electric motor having a fixed stator assembly and a rotatable rotor assembly with an axial air gap between them, the rotor assembly being displaceable axially towards and away from the stator assembly to vary the air gap, means urging the rotor assembly axially away from the stator assembly and into engagement with a brake, means for adjusting the axial position of the brake relative to the stator assembly and wherein on energising the motor the rotor assembly moves out of engagement with the brake to a running position where there is an axial air gap of predetermined dimension between the rotor and stator assemblies.

2. An electric motor as claimed in claim 1 in which the brake comprises a first non-rotatable part which is adjustable axially with respect to the stator assembly and a second part forming part of the rotatable rotor assembly.

3. An electric motor as claimed in claim 2 in which the first part of the brake is mounted on a seat having an outer periphery in threaded relation with a stationary housing whereby rotation of the seat relative to the housing causes it to be moved axially relative to the stator assembly.

4. An electric motor as claimed in claim 3 in which the axial air gap in the running position is determined by the thickness of a shim, located during assembly of the motor, between said housing and the fixed support for the stator assembly.

5. An electric motor as claimed in claim 1 in which the rotor assembly is mounted on a shaft, a closed sleeve fits over the end of the shaft and spring means in the sleeve urges the sleeve axially relative to the shaft away from the stator assembly, said rotor assembly being attached to the sleeve.

6. An electric motor as claimed in claim 5 including adjusting means on the sleeve for limiting the axial movement of the sleeve towards the stator assembly when the motor is energised to set the air gap of predetermined dimension between the rotor and stator assemblies.

7. An electric motor as claimed in any preceding claim in which the rotor assembly includes a squirrel-cage winding.

8. An electric motor as claimed in claim 7 in which the squirrel-cage winding comprises inner and outer aluminium rings connected by aluminium conductors extending through radial slots formed in a laminated pack of magnetic material positioned between the inner and outer rings.

9. An electric motor as claimed in any preceding claim in which the stator assembly comprises a pack of laminated magnetic material, having a central opening for receiving the rotor shaft and substantially radial slots containing the electrical coils for the stator winding.

10. An electric motor as claimed in claim 9 in which each slot contains two coils, the coils constituting parts of two separate windings of different poles per phase to allow the motor to operate at two different speeds.

11. An electric motor as claimed in claim 10 in which the two windings have 8 poles per phase and 2 poles per phase respectively and are distributed over the same 24 slots.

12. An electric motor as claimed in any preceding claim in which it forms part of a hoist.