Passing now to machines having drum armatures with toothed core, that of most direct interest to our subject is the Immisch electro-motor. It has already been pointed out that the current passing through the coils of any armature tends to develop magnetic polarity in the armature core, and the torque of the armature can be considered to be the effect of the attraction and repulsion between the poles of the field-magnets and those developed in the armature. Since the armature poles depend on the armature current, this explanation is identical with that given previously, where it was stated that the torque is proportional to the product of strength of field and armature current. In the Immisch motor an attempt has been made to avoid to some extent the magnetic induction between like poles in armature and field, and to increase the induction between unlike poles by the device of recessing part of the pole pieces, as shown in Fig. 88. On theoretical grounds no advantage can be expected from this design; and, indeed, it is inapplicable to cases where the motor is required to run in either direction. If this condition has to be fulfilled, the pole pieces are bevelled off on both sides, as shown in the half-section, Fig. 89. The pole pieces are extended in a direction parallel to the armature spindle, as will be seen from the perspective view, Fig. 90. The principle underlying Immisch's motor is that the number of armature coils shall be greater or less by one than the number of field poles. Thus an armature with three coils can be employed in either a two-pole or a four-pole field; an armature of five coils can be employed in a four or six-pole field, and so on. On account of simplicity, however, the usual arrangement is a threecoil armature in combination with a two-pole field. The core of the armature is built up of toothed iron discs, supported on gun-metal arms, and separated from each other by paper insulation, and further divided into groups so as to form three or more air-spaces. Openings are left between the layers of wire crossing each other at the ends, so that air can enter into the space surrounding the spindle; and openings are also left between the winding on the outer surface. As far as the author knows, this is the only drum armature with internal ventilation. The six ends of wire corresponding to the three armature coils are connected to a commutator consisting of two series of three segments each. The two series have an angular displacement with regard to each other, so that the dividing line between two segments in the first series stands opposite the middle of a segment in the second series, and vice versa. Brushes resting on the commutator at opposite points, and overbridging both series, connect two of the coils in parallel circuit, while the third coil is cut out. The two active coils form poles on that part of the armature which is surrounded by the third or inactive coil. Since the latter could, by reason of its position, not contribute anything to the intensity of these poles, the fact of its being cut out does not in any way diminish the torque, but it has the advantage of reducing the resistance of the armature by one-third. By splitting up each brush into two separate brushes, the two active coils can be connected in series, thus rendering the same motor suitable for double the electro-motive force. In many cases this is an advantage, and the change can be effected by means of a simple switch. One of these motors, as tested by the jury at the recent Inventions Exhibition, gave the following results: : The weight of the motor tested was 156 lbs., being at the rate of 62 lbs. per horse-power for full load. The following is a table containing diameter of armature, extreme load, and weight as given by the maker: Armature Extreme 4" 5" 6" 7" 8" 9" 10" 1.5 3.25 6.7 12.5 19 26 35 640 910 1,250 load, h-p. Weight, lbs. 80 156 270 430 Amongst the machines with smooth cylindrical armature cores the " Manchester" dynamo deserves special mention for its compact form. It will be seen from the illustration, Fig. 91, that the magnetic circuit is of the double horse-shoe pattern, the magnetizing coils being placed over that portion of the magnet which in other machines constitutes the yoke. The pole pieces are heavy cast-iron blocks, the lower one being provided with extensions for carrying the bearings of the armature spindle. The magnet cores are wrought-iron cylinders, and their ends are fitted tightly into extensions of the pole pieces. The area of contact between the cast-iron and wrought-iron portions of the magnetic circuit is about twice as large |