High speed electric motors

Details High speed electric motors High speed electric motors

1999-09-20

2001-02-27

Ro, Bentsu (Department: 2837)

Electricity: motive power systems

Switched reluctance motor commutation control

C318S438000

Reexamination Certificate

active

06194852

ABSTRACT:

FIELD OF THE INVENTION
This invention relates to improvements in high speed electric motors and relates particularly to a high speed, brushless DC motor.
The invention will be described with particular reference to its application as a motor for a refrigerant, air or gas compressor, although it will be understood that the principles of the invention may be applied to motors used in other applications.
BACKGROUND OF THE INVENTION
The design and construction of a high speed, brushless DC motor for use as a compressor motor gives rise to a number of problems.
Such a motor must produce sufficient power to be effective as a compressor motor over a range of loads. To minimize the combined volume of the motor and compressor it is desirable to design a high speed, low torque motor of high efficiency.
High speed, brushless DC motors are known. In this connection. reference can be made to the text “Direct Current Machines”, by Say and Taylor, 2nd Edition (Pitman International), and particularly sections 4.8, 5.10 and 11.4. Such motors are generally of a lower power output than that required for a refrigerant compressor motor. One form of such a motor uses a rare earth magnet (in particular, a grade of Neodymium-Iron-Boron [NdFeB]). This material, however, and other rare earth magnetic material, generally has a low tensile strength and is somewhat brittle. This has restricted its use to relatively lower powered high speed electric motors which are unsuitable for compressor motors.
In a refrigeration compressor application using the new refrigerants now available, such as R134A and other CFC refrigerants which are considered environmentally safe, it is not possible to use lubricants which are chemically incompatible with the refrigerant. This may exclude the use of conventional bearings in the compressor and hence the desire to be able to use non-contact bearings like magnetic or foil bearing. Such bearings would also, therefore, be used in the driving motor.
It is an object of the present invention to design a high speed electric motor which has a relatively high power output and a very high power to volume ratio.
It is also an object of the invention to design an electric motor able to be used for a refrigerant compressor, the rotor preferably being supported by oilless or non-contact bearings, such as magnetic bearings, foil bearings or the like but also able to be used with standard bearings in different applications.
It is also desirable to provide an electric motor for use as a compressor motor and the control of which is able to be effected by relatively low cost control circuitry.
It is also desirable to provide an electric motor the rotor of which incorporates a rare earth magnet in a structure which provides the necessary mechanical strength.
SUMMARY OF THE INVENTION
According to one aspect of the invention there is provided a high speed, brushless, DC motor having a rotor comprising a solid rare earth magnet core magnetized diametrically, said rotor including a sleeve surrounding said magnet core to radially constrain the core during high speed rotation thereof, the sleeve extending axially in both directions from the core to form a hollow shaft for the rotor, the sleeve being formed of non-magnetic, low electrical conductivity material.
Using the rotor sleeve to form a hollow motor shaft minimizes rotor weight and hence load on the bearings, whilst still providing the desired shaft stiffness and offering a cost effective construction method.
The motor of the invention uses the solid, rare earth magnet core on the rotor for maximum efficiency and lowest loss. The motor is preferably designed to run between operating speeds of 20,000 rpm and 50,000 rpm, which reduces the volume of the motor and compressor. Such high rotational speeds, however, place severe mechanical demands on the rotor which must withstand the high centrifugal forces generated. The rotor must also be sufficiently stiff that its resonant bending frequency exceeds the maximum rotating frequency. The rotor must also be as light as possible to minimize loading on the rotor bearings.
In a preferred embodiment of the invention for use with a refrigerant compressor, the rotor is supported on active magnetic bearings which locate the rotor both axially and radially. However, other forms of normal mechanical bearings or other types of contactless bearings may be used with the motor of the invention in other environments.
The rotor sleeve is preferably formed of a non-magnetic, high strength metal, such as Inconel, which has a relatively high Young's modulus and is able to provide the necessary bending stiffness required for the rotor shaft.
In the preferred form of the invention, the rare earth magnet core is maintained under compressive load (at rest) by pre-stressing the sleeve. Such pre-stressing may be generated by assembling the core and the sleeve with a substantial heat differential, such as by heating the sleeve. The outer diameter of the magnet core and the inner diameter of the sleeve are formed to provide an interference fit of a precise magnitude. When the sleeve is shrunk onto the core the relative dimensional change, both radially and axially, to effect an axial and radial compression of the magnet core. As the rotor rotates, the centrifugal forces generated are counteracted by the tension in the sleeve so that, at maximum rotational speed, minimal, if any, tension occurs in the magnet core. Thus, the compressive forces on the magnet core exerted by the sleeve ensure that the material of the core, which has a low tensile strength, does not crack, move, distort or the like as a result of the rotational forces on the rotor.
According to another aspect of the present invention, there is provided a high speed, brushless, DC motor having a rotor comprising a solid, rare earth magnet core surrounded by a constraining sleeve. and control circuitry including an electronic inverter of a predetermined rating, the control circuitry including means to maintain substantially constant power through the range of motor speeds from a base speed to a maximum speed while maintaining a power factor close to 1, by varying the advance angle of the supplied armature current relative to the back-emf thereby effecting flux-weakening throughout the constant power speed range. The currents and the power factors at the extremes of this range are designed to be equal, with the current lagging the voltage at the base speed and leading it at the maximum speed.
The optimal advance of the current angle at a given speed and torque will depend on the relative motor material cost and inverter cost (or value of increased output for a given inverter rating) as well as the relative thermal significance of iron and copper loss. If the current advance at base speed is set to equal, or approximately equal, the power factor angle at the base speed, the resulting power factor (and inverter utilization) exceeds that which can be achieved if no flux-weakening is used at the base speed.
This method of control minimizes the costs of the electronic control circuitry through use of lower cost components by optimizing the motor design around the ratio of magnet flux to armature flux. Thus, by designing the motor for flux-weakening at the base speed and through to the maximum speed, a lower inverter rating and iron loss is achieved, albeit at the expense of increased copper loss.
In one particular form of motor of this invention, at maximum power, the current advance must be around 15°-20° at the design base speed, i.e. approximately 32,000 rpm, and around 50°-55° at the design maximum speed, i.e. approximately 48,000 rpm. The motor is designed to have the required ratio of armature flux at peak power to magnet flux, and the use of a rotor which does not contain iron minimizes the inductance and minimizes the motor size for the required flux ratio.
In order that the invention is more readily understood, one particular embodiment thereof will now be described with reference to the accompanying drawings:


REFERENCES:
patent: 4638200 (1987-01

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