Sensing phase current in switched reluctance machines

#This invention relates to sensing phase current in switched reluctance machines.

#The control of a switched reluctance machine can be subdivided into two notional stages. At lower speeds it is possible, within the time in which a rotor passes through its conduction angle with respect to a stator pole, to `chop` control the phase winding current by turning the controlling switch arrangement on and off during the conduction angle. At higher speeds chop control gives way to `single-pulse`, control in which the current is either rising or falling within the conduction angle and there is no opportunity for chopping to take place.

#While torque output is the ultimately controlled parameter, it is particularly important to monitor the phase winding currents in the machine to maintain effective control of the drive. While it is possible to derive a current reading from a direct electrical connection to each phase winding, it is likely to be safer in most situations to use electrically isolated techniques because of the high voltages at which the phase winding operates. To this end, it is known to use devices such as Hall-effect current sensors and flux nulling sensors to achieve a current reading. A suitable flux nulling sensor is the LT200-S manufactured by LEM S.A. of Geneva, Switzerland. It has also been proposed to use a Rogowski coil technique to derive a measure of current, for example in the manner disclosed in British Patent Application No. GB 2259150A.

#The invention can be put into practice in various ways some of which will now be described by way of example with reference to the accompanying drawings in which:

#FIG. 1 is a block diagram of a switched reluctance drive system incorporating one embodiment of the present invention;

#FIG. 2 is a circuit diagram of a power switching circuit incorporating an alternative embodiment of the present invention; and

#Referring to FIG. 1 of the drawings, a switched reluctance machine 1 has a number of phase windings 2 (in this case three) which are individually energized by means of a power switching circuit 3 containing semi-conductor switches and drawing power from (or returning power to) a direct voltage source 4. The direct voltage source may be a battery or be obtained by rectifying an ac mains supply. The semi-conductor switches are turned on and off, depending on the rotor position, according to previously known strategies, by an electronic controller 5. To execute the strategies the electronic controller requires information on the phase currents and the rotor position and speed. In the embodiment shown, a digital processor 6 provides the required information.

#The skilled person will appreciate that torque is produced by the rotor as an output when the switched reluctance machine is controlled to run as a motor. The same machine can be operated as a generator provided the control strategy of the controller is adjusted accordingly. For the sake of simplicity, the invention will be described in relation to a motor, but it applies equally to current sensing in the control of a generator.

#The strategies used to control switched reluctance motors and generators are well known to the skilled person, as evidenced by the article "The Characteristics, Design and Applications of Switched Reluctance Motors and Drives", by Stephenson et al., PCIM 193, Jun. 21-24, 1993. The implementation of such strategies in software routines in the processor 6 will be well known to the skilled person.