Abstract:
A double insulated rotor ( 10 ) for an electric motor has a rotor core ( 14 ) fitted to a shaft ( 12 ) by way of an insulating sleeve ( 24 ) moulded to the shaft ( 12 ).

Description:
FIELD AND BACKGROUND OF THE INVENTION 
     This invention relates to electric motor and in particular, to a double insulated rotor for an electric motor such as a universal motor or high voltage d.c. motor. 
     Generally, wound rotors comprise a shaft and a rotor core fixed to the shaft. Rotor windings are wound around poles of the rotor core to form the armature. The power of the motor comes from the interaction between the magnetic fields of the stator and rotor and the rotational force applied to the rotor core is transferred to the shaft for doing useful work such as driving a load. 
     The rotor core may be fixed directly onto the shaft as a press fit which usually gives a strong connection for transferring the power or torque. However, in some applications, safety specifications require that the rotor core be electrically insulated from the shaft, known as a double insulated rotor. This is a common requirement for mains voltage motors. 
     One way this is achieved is by insert moulding an insulator between the rotor core and the shaft. In this process, the rotor core and shaft are loosely assembled and placed in an injection mould. The mould is closed and hot plastics resin is injected into the spaces between the rotor core and the shaft to fix the core to the shaft. At the same time, end spacers and lamination end protectors or spiders may be formed integral with the insulator. 
     While this provides an excellent solution to the problem with the rotor core securely fixed to the shaft, it is time consuming and any change to the rotor core, such as diameter or length, requires a new mould to be produced. 
     A second solution involves placing the rotor core on a pre-fabricated plastics material tube. The shaft is then pressed into the tube which expands and locks the core to the shaft. This is effective and allows the size of the rotor core to be changed. However, the process requires significant force to press in the shaft to expand the tube and the longer the tube, the more difficult it is. Also, as the shaft is pressed into the tube, the shaft cannot be keyed to the tube. Under severe conditions of torque and vibration, the rotor core has been known to slip with respect to the shaft. Exact tolerances are required for the rotor core i.d., the shaft o.d. and the tube&#39;s i.d. and o.d. The smaller the rotor and shaft, the more exactly the tolerances are, giving rise to extensive post forming machining to meet the tolerance requirements to avoid slippage. 
     Thus, there is a need for a double insulated rotor which is easy to assemble and can accommodate variations in the rotor size, i.e., length and diameter. 
     SUMMARY OF THE INVENTION 
     Accordingly, in one aspect thereof, the present invention provides a double insulated rotor for an electric motor comprising: a shaft; an insulating sleeve moulded to the shaft; a stack of stamped laminations forming a rotor core, the rotor core having a plurality of salient poles and a central aperture receiving the shaft and insulating sleeve; a commutator mounted on the shaft; windings wound around the salient poles and terminated on terminals of the commutator, wherein: the insulating sleeve is injection moulded onto the shaft and has a substantially polygonal outer cross-section where it contacts the rotor core; and the aperture in the rotor core has a corresponding profile adapted to receive the shaft and insulating sleeve as a press fit. 
     According to a second aspect, the present invention provides a method of making a double insulated rotor for an electric motor, comprising the steps of: injection moulding an insulating sleeve onto a shaft; stacking a plurality of stamped motor laminations to form a rotor core having a plurality of poles and a central aperture; pressing the shaft and sleeve through the central aperture of the rotor core to position the rotor core onto a core portion of the insulating sleeve; mounting a commutator onto the sleeve adjacent the rotor core; winding coils about the poles of the rotor core and connecting the coils to terminals of the commutator, wherein the core section is formed with a substantially polygonal cross-section and the central aperture is formed with a corresponding profile. 
     Preferred and/or optional features are set out in the dependent claims. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     One preferred embodiment will now be described, by way of example only, in which: 
     FIG. 1 is a perspective view of a partial rotor according to the preferred embodiment; 
     FIG. 2 is a part sectional view of the rotor of FIG. 1; 
     FIG. 3 illustrates a lamination of the rotor of FIG. 1; 
     FIG. 4 is a part sectional view of a shaft and overmoulded sleeve of the rotor of FIG. 1; 
     FIG. 5 is a cross section along V—V of FIG. 4; and 
     FIG. 6 is a cross section along VI—VI of FIG.  4 . 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     FIG. 1 is a perspective view of a rotor  10  according to the preferred embodiment with the rotor windings and commutator omitted for clarity. While the rotor windings and commutator are essential for the operation of the motor, they are well known and play no part in understanding the invention. In the complete rotor, windings would be wound around poles of the rotor and connected to terminals of the commutator. 
     As can be seen in FIGS. 1 and 2, the rotor has a shaft  12  and a rotor core  14 . The rotor core  14  is formed by stacking together laminations  16  stamped from sheet electrical steel. A representative lamination is shown in FIG.  3 . The laminations have a central hole  18  and a number of radially extending T-shaped fingers  20  which form the salient poles of the rotor core. The central hole is hexagonal. 
     Referring back to FIGS. 1 and 2, at each end of the rotor core is a rotor core end protector or spider  22 . The spider  22  protects the windings from the sharp corners of the rotor core during winding and use. For a high voltage motor, slot insulation or liners would be provided to provide further insulation between the windings and the rotor core, as is well known. 
     The shaft  12  has an overmoulded insulating sleeve  24 . The shaft also has four portions which are knurled. The knurling  26  aids the grip between the shaft and the overmoulded sleeve. 
     The sleeve  24  has a core section  28  adapted to receive the rotor core as a press fit. This core section  28  has a hexagonal shape with rounded edges in cross section as shown FIG.  5 . The rounded edges allow for deformation of the sleeve during the fitting of the rotor core and accommodates adhesive, if required to glue the rotor core to the sleeve. 
     The sleeve  24  also has a stop section  30  which forms an axial abutment for the correct axial alignment of the rotor core with the sleeve  24  and hence with the shaft  12 . 
     Adjacent the stop section  30  is a commutator section  32  for receiving a commutator. Section  32 , as shown in section in FIG. 6, has a plurality of small axially running ridges  34 . The ridges  34  form an interference fit with the body of the commutator allowing the commutator to be pressed into place. The valleys between the ridges provide space for adhesive used to fix the commutator to the sleeve. 
     At the opposite end of the sleeve  24  is a collar section  36  for supporting a collar  38  shown in FIG.  2 . The collar section  36  has a cross section similar to the commutator section  32 , shown in FIG. 6, with ridges for gripping the collar and valleys for accommodating the glue used to fix the collar to the sleeve. The collar provides a second axial abutment for the rotor core, preventing axial movement of the rotor core  14  with respect to the shaft  12 . 
     Thus, to assemble the rotor  10 , the shaft  12  is placed in a die of an injection moulding machine and the sleeve  24  is overmoulded directly onto the shaft  12 . A first spider  22  is placed on the sleeve  24  against the stop  30 . The rotor core  14  is pressed onto the core section  28  of the overmoulded sleeve until it is firmly pressing the spider  22  against the stop  30 . The second spider  22  is placed in position at the other end of the rotor core  14  and the collar  38  is pressed onto the collar section  36  of the overmoulded sleeve thereby fixing the rotor  14  to the shaft  12 . Adhesive such as a suitable epoxy may be added to the sleeve  24  before the rotor core  14  and collar  38  are pressed onto their respective sections of the sleeve  24 . 
     A commutator (not shown) is now pressed onto the commutator section  36  of the sleeve  24  with adhesive being applied to the commutator section  36  beforehand. 
     The rotor core may now be coated with an insulating epoxy or slot liners fitted to the winding slots before the motor windings are wound about the poles of the rotor core. The windings are connected to terminals of the commutator and slot sticks may be fitted to secure the windings. 
     As can be appreciated, the length of the rotor core and/or the diameter of the rotor core can be changed easily without affecting the mould for the overmoulded sleeve. Indeed, even changes in the length of the shaft can be accommodated by minor changes to the die. Similarly, as the sleeve is moulded onto the shaft, the shaft can include special features to increase the grip between the shaft and the sleeve without affecting the design of the rotor core, e.g., by providing splines, knurls, keyways, projections, dimples, deformations, flats and combinations thereof or other engagement features, preventing slippage between the shaft and the sleeve.