1. Field of the Invention
This invention relates to switched reluctance motors, and, more particularly, to a method and a circuit for controlling a switched reluctance motor whereby acoustic noise generated by the motor is reduced.
2. Disclosure of Related Art
A conventional switched reluctance motor (SRM) includes a stator having a plurality of pairs of diametrically opposed stator poles and a rotor having a plurality of pairs of diametrically opposed rotor poles. Windings or coils are typically disposed about the stator poles and the windings around any two diametrically opposed stator poles may be connected in series or in parallel to define one motor phase of the multiphase SRM. The windings associated with a motor phase may be referred to as a phase coil. By generating current through the phase coil, magnetic fields are established about the stator poles and a torque is produced that attracts a pair of rotor poles into alignment with the stator poles. The current in the phase coils is generated in a predetermined sequence in order to produce a constant torque on the rotor. The period during which current is provided to the phase coil--and the rotor poles are brought into alignment with the stator poles--is known as the "active stage" or conduction interval of the motor phase. At a certain point--either as the rotor poles become aligned with the stator poles or at some point prior thereto--it becomes desirable to commutate the current in the phase coil to prevent a negative or braking torque from acting on the rotor poles. Once this "commutation point" is reached, current is no longer generated in the phase coil and the current is allowed to dissipate from the phase coil. The period during which current is allowed to dissipate from the phase coil is known as the "inactive stage" of the motor phase.
Conventional SRMs generate a high and undesirable level of acoustic noise in comparison to other types of motors. One source of this acoustic noise is the deformation of the stator and the rotor that occurs when the phase coils are energized and deenergized. In particular, when the phase coils are energized, the magnetic forces that are generated tend to "ovalize" the rotor and the stator. When the phase coils are deenergized, the rotor and the stator return to their original shape and relatively high levels of acoustic noise are generated.
One conventional method for reducing acoustic noise has been to slow the dissipation of current in each phase coil during the inactive stage of the motor phase so as to produce a less abrupt change in magnetic force, and therefore, the shape of the SRM. This method has previously been implemented in SRMs having a two-switch per phase topology (i.e., one switch connected to either end of the phase coil) by keeping one of the switches closed for a period of time during the inactive stage of the motor phase. This produces a two-stage decay in which the current in the phase coil is dissipated slowly through the control circuit during the first stage and the current is then returned quickly to the power supply during the second stage when the remaining closed switch is opened. The two-switch per phase topology is a relatively expensive switching topology, however. Moreover, as described in commonly assigned U.S. Pat. No. 5,742,139, the entire disclosure of which is incorporated herein by reference, this method has not adequately reduced the acoustic noise generated in switched reluctance motors because it fails to reduce acoustic vibrations resulting from the interaction of multiple motor phases.
There is thus a need for a method and circuit for controlling a switched reluctance motor that will minimize or eliminate one or more of the above-mentioned deficiencies.