1. Field of the Invention
The present invention relates to electric motors and in particular to universal motors in which both the rotor and the stator have a winding arrangement.
2. Description of the Prior Art
It is known that there are various different ways to configure an electric motor, each of which has its own associated advantages and disadvantages. FIG. 1 schematically illustrates an axial view of a permanent magnet DC motor, wherein a set of permanent magnets are arranged around the circumference of the central rotor, whilst fixed windings on the stator are energised to cause rotation of the rotor with respect to the stator. Such DC motors are known to be relatively powerful but suffer from the magnetic drag of the permanent magnet mounted on the rotor and thereby lose efficiency.
FIG. 2 schematically illustrates an induction motor having the same stator arrangement as that illustrated in FIG. 1, but wherein the iron rotor has embedded conducting bars (e.g. made out of copper) in place of the permanent magnets. The energization of the stator winding arrangements induces currents in the conducting bars of the rotor. The interaction between the electromagnetic fields generated by the stator winding arrangement and these induced currents thus causes the rotor to rotate with respect to the stator. Induction motors are known to be very efficient at a given design speed, but lose efficiency when operated at different speeds.
A further known type of motor is the universal motor which is schematically illustrated in FIG. 3. Here both the rotor and the stator having a winding arrangement. Such a universal motor can be operated with either DC power or AC power, and generally provides a compact and high powered motor. To some extent, a universal motor can be viewed as combining the advantages of both induction motors and permanent magnet motors. However, the difficulties associated with coupling the required waveforms to the rotor winding arrangements have significant drawbacks.
More detail of the arrangement of a universal motor is schematically shown in FIG. 4. The rotor 10, configured to rotate on axle 15, has a set of windings 20 arranged around its circumference. The stator (not explicitly illustrated) surrounds the rotor 10 within which a set of stator coils (windings) 30 are arranged to correspond to the rotor windings 20. A power supply 35 supplies electrical power to both a stator coil control unit 40 and a rotor coil control unit 45. The stator coil control unit 40 generates a suitable stator waveform from the supplied power and provides this to the stator coils 30. However, powering the rotor windings 20 is not as simple as powering the stator windings 30 due to the rotation of the rotor. Accordingly, typical universal motor designs seek to take advantage of the rotation of the rotor by providing a commutator 50 which couples the externally supplied power to the rotor coils 20. The commutator comprises a set of contacts 55 mounted around the circumference of the axle of the rotor which intermittently come into contact with contacts 60. The set of contacts 55 are electrically coupled to the respective different windings 20 which make up the rotor winding arrangement. Hence, the rotation of the rotor sequentially couples different rotor coils 20 to the rotor coil control 45 enabling a sequence of waveforms to be applied to the rotor coils 20. However commutators are relatively primitive mechanical connectors which tend to be both noisy and inefficient.
An alternative known approach to providing the required electrical connections to the rotor is to arrange these in the form of slip rings. However, due to the number of separate electrical connections that are typically required, and the corresponding number of slip rings which must then also be provided, considerable manufacturing complexity is associated with a rotor comprising numerous slip rings and also with its shaft with a corresponding number of concentric power channels (typically layered on the shaft).
Accordingly it would be desirable to provide an electric motor apparatus configured as a universal motor, thus benefiting from the above described advantages of universal motors, but without the above-described disadvantages of coupling the motor windings to a external power source via noisy and inefficient commutators or using numerous slip rings.