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230   Electric Drives and Electromechanical Systems


             of being relatively expensive and complex, if only a limited range of motion is required.
             The requirement for a limited range of movement can be found in many applications,
             including the operation of air or hydraulic servo-valves and oscillating mirrors.
             In addition, their inherent reliability of operation makes a limited-angle torque motors
             an ideal solution for applications where limited actuation is critical, for example
             in spacecraft latches, where the only previous solution was to use pyrotechnics.
                The basic construction of a limited-angle torque motor is shown in Fig. 9.2. While
             they are broadly similar to brushless d.c. motors, the limited-angle torque motor is a
             single-phase device, which eliminate the need for the commutation logic and the three-
             phase power bridge that are found in multiphase machines. The torque motor’s winding
             can be wound in conventional slots or as a toroid over a slotless stator. The rotor in a
             limited-angle torquer incorporates one or more magnets. The slot-wound limited-angle
             torque motor has a number of advantages over toroidally wound motors; in particular
             they have better thermal dissipation and a higher torque constant. However, because of
             the presence of slots, the output torque ripple and hysteresis losses are greater.
             The torque ripple can be considered to be zero with toroidally wound motors due to the
             non-varying reluctance path and the large air gap. In addition, the slot-wound
             limited-angle torque motor exhibits a higher motor constant, K m , than the
             corresponding toroidally wound motor, due to the larger number of conductors that are
             exposed to the magnetic field.
                Cogging is essentially zero in toroidally wound limited-angle torque motor, a result of
             a non-varying reluctance path and relatively large air gap. Toroidally wound armatures,
             moreover, are typically moulded onto the stator, which protects the windings from
             damage and holds them in place.


























             FIG. 9.2 Internal construction of two types of limited angle torque motors. (A) Slotted Armature. (B) Toroidal
             armature.
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