International projectionist (Oct 1931-Sept 1933)

24 INTERNATIONAL PROJECTIONIST June 193S ^-DIRECTION OF COIL ROTATION COMMUTATOR Figure 12 — Showing operation of DC generator loop. This type of collector is called a "commutator." As in the case of an AC generator, brushes are mounted so as to make a wiping contact with the split ring and permit current to be drawn from the generator as it rotates. The brushes are mounted so that they change contact from one segment to the other when the loop is at right angles to the lines of force as shown in (b) of Figure 12. It will be remembered from the discussion of the alternatingcurrent generator that, when the loop is in this position, there is no current generated in it. Therefore, shorting of the loop, caused by the brushes making contact with both segments at the same time, will do no damage because there will be no current flowing. When the loop is in the position shown in (a), the current will flow in at the left-hand brush and out at the right-hand brush. When the loop is in position (b) no current will flow because no voltage is generated in the loop. When the loop is in position (c) the current again flows in at the lefthand brush and out at the righthand brush. The current that flows in the loop changes in direction as the loop changes from position (a) to position (c) as it did in the AC generator, but the brush connections also change when the loop is rotated, that is, side "A" of the loop is connected through the commutator to the lefthand brush when the loop is in position (a), and is connected to the righthand brush when the loop is in position (c), so that the net result is that the current always flows in at the lefthand brush and out at the right-hand brush. The current flowing in the load circuit of such an arrangement just described, with only one loop and two segments, is pulsating (continually varying in magnitude), but flowing always in the same direction. In practice, a large number of loops and segments are used so as to give a fairly constant DC voltage. The loops are connected in series in such a way that the generated voltage is the sum of the voltage generated in nearly all of the loops. The brushes are of such size as to short-circuit two or three of the segments, which short-circuits two or three loops. These loops occupy a position as shown in (b) of Figure 12 when they are short-circuited, and very little, if any, voltage is generated in them. However, the current through the loops must change in direction as they pass under the brushes, and an arc will form at the brushes unless they make a firm, even contact. For this reason it is important that the brushes fit the commutator snugly, and that the commutator is kept clean. 'T'HE operation of a motor is the ■"• same as that of a generator except that the current of a motor flows in the opposite direction through the machine from that in which it flows when the machine runs as a generator; that is, the current flows in one direction with respect to the generated voltage in a generator, and in the opposite direction with respect to the generated voltage in a motor. The current flows into the motor through the positive brush and out at the negative brush. The current of a generator flows out of the positive brush and in at the negative brush. The reason for this is the fact that the generated voltage of a motor is always in opposition to the applied voltage, but never quite as great. Therefore, the applied voltage forces a current through the motor against the electrical pressure offered by the generated voltage. When a current is passed through the armature winding of a motor, a magnetic field is created in it which has a north and south pole. These poles are attracted by one pole of the motor field and repelled by the other, causing the armature to rotate. In the case of an alternating current machine the field rotates according to the frequency of the alternating current, and the windings follow this field around. In the case of a DC machine, as the winding rotates the commutator rotates, and the windings through which the current flows is continually changing. The magnetic poles of th^ revolving windings are always kept at a fixed angle with respect to the field poles and a steady rotating force is maintained. As each portion of the armature winding reaches a postion where the field poles produce no turn FIELD RHEOSTAT, ARMATURE '-BRUSHES V. ing effect, the commutator action is such as to disconnect these coils, and to connect others which are in a position where a turning effect may be had. Commercial Motors and Generators Motors and generators, as they are built for use, do not look anything like the illustrations shown in Figures 11 and 12. These illustrations show the action of the machines, but the machines themselves are built in a considerably different form. The field poles of a commercial machine are not "permanent" magnets, but are "electromagnets" consisting of a field-winding on an iron core. The current for the field-winding is supplied from some source of direct current. In the case of a DC generator, this current is usually supplied by the generator itself, and in the case of the DC motor the field current is supplied from the same source as the current that runs the motor. AC motors are more complicated in their action although the same principles are involved. The explanation of the process is very involved and will not be taken up here. Figures 11 and 12, show a loop rotating in a magnetic field. In practice a large number of loops are used and they are mounted on an iron core. This iron core reduces the length of the magnetic field in air, and therefore makes it easier to create a strong magnetic field. The space between the rotating iron core carrying the rotating windings and the poles is called the air gap. This air gap varies with the design and size of the motors. For small machines it is sometimes only a few thousands of an inch, while for large generators the air gap may be several inches wide. The rotating part of motors and generators is called the "rotor." In all DC machines and in some AC machines, it is often called the "armature" of the motor or generator. Strictly speaking, the armature is that part of the machine wherein the magnetic field rotates with respect to the windings. Voltage Control of Generators As stated earlier in this article, the voltage generated in a conductor which is moved through a magnetic field depends upon the number of lines of force cut by the conductor in a second of time. Therefore, if the speed with which the conductor cuts the lines of force is increased, the voltage gener ADJUSTABLE CONTACT BRUSH ELD COIL Figure 13 — Field control of DC motors and generators 'CENTRIFUGAL DEVICE SLIP RING Figure 14 — Wiring diagram illustrating operation of speed control device