Special machines:Printed-circuit (Disc) DC Motor

Printed-circuit (Disc) DC Motor

(a) Constructional Details

It is a low-voltage dc motor which has its armature (rotor) winding and commutator printed on a thin disk of non-magnetic insulating material. This disk-shaped armature contains no iron and etched- copper conductors are printed on its both sides. It uses permanent magnets to produce the necessary magnetic field. The magnetic circuit is completed through the flux-return plate which also supports the brushes. Fig. 39.13 shows an 8-pole motor having wave-wound armature. Brushes mounted in an axial direction bear directly on the inner parts of the armature conductors which thus serve as a commutator.

Since the number of armature conductors is very large, the torque produced is uniform even at low speeds. Typical sizes of these motors are in the fractional and subfractional horsepower ranges. In many applications, acceleration from zero to a few thousand rpm can be obtained within 10 ms.

(b) Speed Control

The speed can be controlled by varying either the applied armature voltage or current. Because of their high efficiency, fan cooling is not required in many applications. The motor brushes require periodic inspection and replacement. The rotor disk which carries the conductors and commutator, being very thin, has a limited life. Hence, it requires replacing after some time.

(c) Main Features

The main features of this motor are (i) very low-inertia (ii) high overload current capability (iii) linear speed-torque characteristic (iv) smooth torque down to near-zero speed (v) very suitable for direct- drive control applications (vi) high torque/ inertia ratio.

(d) Advantages

(i) High efficiency (ii) Simplified armature construction (iii) Being of low- voltage design, produces minimum of radio and TV interference.

(e) Disadvantages

(i) Restricted to low voltages only

(ii) Short armature life (iii) Suited for intermittent duty cycle only because motor overheats in a very short time since there is no iron to absorb excess heat (v) liable to burn out if stalled or operated with the wrong supply voltage.

(f ) Applications

These low-inertia motors have been developed specifically to provide high performance characteristics when used in direct-drive control applications. Examples are :

(i) high speed paper tape readers (ii) oscillographs (iii) X –Y recorders (iv) layer winders (v) point-to-point tool positioners i.e. as positioning servomotors (vi) with in-built optical position encoder, it competes with stepping motor (vii) in high rating is being manufactured for heavy-duty drives such as lawn mowers and battery-driven vehicles etc.