A variable reluctance (VR) motor is a brushless electric motor having a rotor and multiple pairs of inductive stator windings. Torque, in the direction of a point of minimum reluctance, is exerted upon the rotor in an amount proportional to the amount of current flowing through a pair of stator windings. Rotor torque, speed, and rotational direction are controlled in a VR motor by controlling the amount of current flowing through selected pairs of stator windings.
VR motor control circuits which are constructed according to the prior art control the current flow through selected pairs of stator windings by alternating between a charge state and a discharge state. In the charge state, a voltage is impressed across the selected pair of windings until a desired current level is reached or slightly exceeded. A reverse voltage is then applied across the pair of stator windings to discharge the current flowing through the windings. In order to maintain a desired current through the windings, such a prior art control circuit must alternate between the charge and discharge states. The result is that the actual current is the average of the triangular current ripple which is created. This winding current ripple is highly disadvantageous because variations in rotor torque result and electromagnetic and acoustic noise is emitted.
In the illustrated preferred embodiment of the present invention a three state inductive load driver is used to control the current flow through each pair of stator windings in a VR motor. In a charge state the driver impresses a positive voltage across a pair of windings until a desired current level is reached. In a discharge state a reverse voltage is impressed across the windings until the current level has discharged to a desired level. An idle state, in which the windings are shorted, exists between the charge and discharge states. Since the current through a shorted ideal inductance is constant, the winding current during the idle state is constant except for decay due to minor resistive losses. Thus, a desired current level can be achieved and maintained with only a minimum of current ripple. Further, circuit complexity is minimized and the circuit operates at a low voltage and a high efficiency level.