There is a need for efficient electrical machines that have high torque capability over a large speed range and the ability to control machine speed, in particular for electrical drives for vehicles, such as electric or hybrid vehicles, or other electric generation applications which require high torque at zero and low speed.
For purposes of providing traction power, such as in electric vehicles, it is desirable to have an electric motor with a high constant power speed ratio (CPSR). Referring to FIG. 1, torque and power as a function of speed is shown for an electric motor. At low speed, high torque is available with such torque assisting with launch. As Nmin is reached, the motor's maximum power is accessed and no more power is available as speed is further increased. Recalling that P=2*Π*T*N; as power, P, is constant, as speed, N, is increased, torque, T, reduces. CPSR is the maximum speed at which rated power can be delivered (Nmax) divided by the lowest speed at which maximum power is available (Nmin). Nmin is also the highest speed at which rated maximum torque can be delivered. The maximum speed (Nmax) is limited primarily by a limit on back EMF voltage, and also by damage to the rotor or other inherent limitations of the motor. For example shown in FIG. 1, the CPSR is a factor of two.
It is desirable to have a CPSR of four or more for automotive applications. Although it is possible to achieve that with induction motors, motors with field coils, or switched reluctance motor technologies, permanent magnet motors are preferred due to their higher power density and higher efficiency. Permanent magnet (PM) motors, however, do not inherently have CPSRs in such a high range. A significant amount of effort is being expended in determining cost-effective, lightweight, and efficient solutions to address the limited CPSR of PM motors.
One alternative is to provide a transmission between the electric motor and the final drive. However, transmissions are heavy, costly, and must be controlled, either by the operator or by a controller. Another alternative is to electrically adjust the field strength of the electric motor if it has electrically excited field windings. This approach is not available to motors with permanent magnet fields.
Another approach to is to weaken the magnetic field, thus increasing the motor speed for a given back EMF or applied voltage. For any given motor, torque produced is proportional to current multiplied by magnetic field strength, while RPM is proportional to voltage/field strength. So for a given power (voltage*current) in, a motor makes a certain amount of mechanical power, (T*N). If the magnetic field is weaker, the motor makes the same power but at higher speed and lower torque.
In an electric motor, there is an air gap between the rotor and the stator. The motor is usually designed to have as small an air gap as practical. The field strength can be weakened, however, by increasing that air gap. Such a system has been employed in axial flux motors, in which the rotor and the stator are substantially disk shaped. The displacement between the two disks can be increased to reduce the field strength. In a radial flux motor, the rotor may be centrally located with the stator arranged outside the rotor circumferentially displaced from the rotor. If the rotor, for example, is displaced along the axis of rotation, the effective field strength of the radial flux motor is reduced. The mechanisms that adjust the relative positions of the rotor and stator are relatively expensive and yield a more cumbersome motor. In alternatives in which a portion of the windings are switched off or the relative positions of the rotor and stator are adjusted, an electronic controller commands the adjustments based on input signals. Such controllers can be costly.