Brushless dc servomotors
Brushless DC motors resemble a dc shunt motor turned inside out. Permanent magnets, located on the rotor, or a wound rotor excited by dc voltage through slip rings, requires that the flux created by the current carrying conductors in the stator rotate around the inside of the stator in order to achieve motor action. The rotating field is obtained by placing three stator windings around the interior of the stator punching. The windings are then interconnected so that introducing a three-phase excitation voltage to the three stator windings (which are separated by 120 electrical degrees) produces a rotating magnetic field. This construction speeds heat dissipation and reduces rotor inertia. The permanent magnet poles on the rotor are attracted to the rotating poles of the opposite magnetic polarity in the stator creating torque. As in the dc shunt motor, torque is proportional to the strength of the permanent magnetic field and the field created by the current carrying conductors. The magnetic field in the stator rotates at a speed proportional to the frequency of the applied voltage and the number of poles.
The rotor rotates in synchronism with the rotating field, thus the name “synchronous motor” is often used to designate motors of this design. More recently, this motor design has been called an electrically commutated motor (ECM) due to its similarity to the dc shunt motor. In the dc shunt motor, the flux generated by the current carrying winding (rotor) is mechanically commutated to stay in position with respect to the field flux. In the synchronous motor, the flux of the
current carrying winding rotates with respect to the stator; but, like the dc motor, the current carrying flux stays in position with respect to the field flux that rotates with the rotor. The major difference is that the synchronous motor maintains position by electrical commutation, rather than mechanical commutation.
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