Air gap is axial in such motors and the magnetic circuit completes through the ferromagnetic block at one end which also serves as the case of the armature. A schematic of printed d.c. motor is given below (brush and brush-holders are not shown)—
The field is provided by the permanent magnet. The armature winding is obtained on the insulating disc by the printed circuit technique and comprise a large number of half-turns on one face and corresponding number of other half turns on the other face and are interconnected to form the complete winding.
The printed motors are widely used in control system, which include instrument servos, X-Y plotters, magnetic and paper tapes, capstan drives, multicolumn line printers, card readers, machine tools, and antenna drive.
The performance of a printed motor can be analysed by the same equations as those of a separately excited d.c. motor.
However, a printed motor has the following special advantages which makes it ideally suitable for servo applications:
Negiligible electrical time constant because the inductance of winding is negiligibly as conductors are not housed in iron slots.
Very light rotor and low moment of inertia (low mechanical time constant).
Low electromechanical time constant, of the order of 0.001 s to 0.050 s.
Extreme smoothness of the developed electromagnetic torque.
Linear torque-speed characteristics at constant applied voltage.
Wide range of speed control (by controlling the applied voltage across armature terminals) from 1 revolution per day to several thousand rpm.
The maximum output rating of a printed motor is about 2 kW at 110 V, 2500 rpm and it is difficult to obtain suitable permanent magnet for higher ratings.
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