Page 174 - Electric Drives and Electromechanical Systems
P. 174

168   Electric Drives and Electromechanical Systems






















             FIG. 6.2 The key features of a brushless d.c. motor-drive. The motor’s position feedback is provided by an encoder
             (E1) and is used to determine the switching pattern of the bridge as a function of the demand. Depending on the
             application velocity or position feedback is provided by E2 and provides the demand. The components that
             equate to a brushed d.c. motor are within the dotted line.

             For most motor types, the speed and torque performance are almost identical when
             complemented by high-performance controllers. Hence permanent-magnet brushless
             motors are widely used for power outputs of up to 20 kW; above this power level,
             vector-controlled induction motors are predominating for specialist applications.
             As with all design problems, the final selection of the system is left to the system
             designer, who is able to balance the relative advantages and disadvantages on an
             objective basis.


             6.1 The brushless d.c. motor

             The basic torque and voltage equations of BDCMs closely resemble those of d.c.
             brushed motors. This section presents a simple analysis for determining a motor’s
             characteristics; this also allows an appreciation of its limitations. In this analysis, a
             simple two-pole motor is considered (see Fig. 6.3); the key features of this design are
             the rotor’s magnetic pole arc is 180 degrees and a three-phase stator winding with two
             slots per phase and N turns per slot. The air-gap flux, neglecting any fringing, can be
             considered to be a square wave (see Fig. 6.4). As the rotor is rotated, a voltage is
             induced within the stator windings, and the flux linkage varies linearly as a function of
             the rotor position, with the maximum positive linkage for the winding occurring at
             q ¼ 0 degrees and the maximum negative flux linkage occurring at q ¼ 180 degrees.
                If a single phase is considered, the total flux linkage can be determined by integrating
             the contribution of individual turns, to give a maximum flux linkage of,

                                                j max ¼ NB g prl                          (6.1)
   169   170   171   172   173   174   175   176   177   178   179