Motion Picture News (Oct-Dec 1930)

October 2 5 , 19 3 0 Motion Picture News 69 !THE" Projectionists' Round Table LET us analyze the transformer in a somewhat more thorough manner, but without entering upon the mathematical treatment. Consider the simple transformer shown in Figure 69, repeated again in this lesson. It has two windings, A, the primary and B, the secondary. The usual designations for such windings are P and S respectively and they will be referred to in that manner in further discussion. We assumed in Lesson 21, that the position of the secondary winding with respect to the primary was such that all of the flux pro FiG.69 duced by current flow through the primary coil cut all of the turns of the secondary coil. The medium through which the flux lines had to pass was air. Now, all transformers employed for the transformation of alternating emf and current, with the exception of one type, namely that which operates with an input frequency of hundreds of thousands, if not millions of cycles per second and which is of no interest to us, make use of a magnetic core so that the flux lines have a much easier path and more readily link the primary and the secondary circuits. Thus, all power transformers employ a magnetic core. The exact material depends to a large measure upon the requirements and the quality of the product. The most popular core material used for power transformers is silicon steel. As a general statement relative CORE -/ p ( s 1 j 1 >^ COSE FI& 73 H&7JA to power transformers employed in connection with sound systems and audio transformers employed in sound system, let it be understood that there are definite differences between these two types of devices, although the basic principles are the same. In all respects other than the basic principles, the two should be divorced until both have been fully considered. In this manner shall we avoid the possibility of miscomprehension. We now are considering the power transformer and have no concern as yet with the audio frequency transformer. Transformers designed to transfer electrical energy, referred to as power, from the primary to the secondary circuit are of two general types. When the magnetic path takes the form of a single ring upon which are mounted the primary and secondary windings, be they one or more in number, the transformer is of the core type and of the formation shown in Figure 73. The primary winding is mounted upon one leg of the core and the secondary winding is mounted By John F. Rider upon the opposite leg. The core is of square cross section as shown in Figure 73A. The space within the core is known as the "window." When a number of secondary windings are used, they may be distributed as in Figure 73B. Another type of transformer construction is the shell, illustrated in Figure 74. In this arrangement the secondary is wound atop the primary. There are basic differences between these two forms of construction relating to the length of the magnetic path and the length of the winding, but such differences are of no interest at this time. Let it suffice to say that the core type of transformer is most popular in the sizes employed in talking picture systems. As to the operation of the transformer, the flux generated by the current flowing through the primary threads its way through the core and thereby links with the secondary turns. Naturally, every effort is made to link most of the secondary turns with the flux generated by the primary current. Thus, if we consider the transformer shown in Figure 75, the flux passes through the core in the direction shown by the small arrows. This direction is oc i. L C FIG.73B casioned by the direction of the current flowing through the primary winding. However, all of the flux generated by the primary current does not flow through the core and cut the secondary turns. Some of it leaks across the window as in Figure 75A. Thus, there is introduced a condition which will tend to prevent 100 per cent efficiency in the transformer. Supplementary to this we must recognize the action taking place when the flux traverses the core material. When current is passed through the primary winding, and flux flows through the core, the core material is in a state of magnetization. The direction of the current flow through the primary winding is constant for but a short time, one half cycle, after which period the CORE WINDING J L CORE current reverses and the direction of the magnetizing force likewise reverses. Based upon the assumption that the electromagnetism is due to molecular activity within the core material, this continual agitation of the molecules produces a certain amount of energy dissipation, manifest as heat. This energy is supplied by the primary current, hence there is another factor representing loss and limiting the possibility of 100 per cent electrical efficiency within the transformer. This loss related to the magnetizing action upon the core by an increasing current and rapidly reversing current is referred to as the hysteresis loss. A third form of loss is associated with conductors which may be acted upon by a varying flux, known as eddy-current loss. Wherever a conductor may be acted upon by a varying flux, electric currents are induced in the conductor. Such a conductor is the magnetic core itself and since it has resistance, energy will be dissipated in the form of heat at a rate equal to rr. Hence, we have the third factor which prevents 100 per cent electrical efficiency. What has been said so far is sufficient to show that absolute perfection is not possible in a transformer. Present day design is such, that fairly high percentages are possible, the efficiency increasing with the size of the transformer. The average degree of efficiency in a small transformer / T\--> -j * : *4 in < 1 V vV. , FIG. 74 FIG.7S"A is between 85 per cent and 95 per cent whereas large transformers have been designed with efficiency factors as high as 98 per cent or 99 per cent. Action of Flux Within Transformer. — The flow of current through the primary winding causes the generation of flux which passes through the core in a certain direction and cuts the secondary turns. A voltage is induced across the secondary winding and, if a resistance is connected across the secondary winding, current will flow through the winding and the resistance. Now, the presence of a current through the secondary will naturally create flux around the secondary winding and, because the direction of the current flow through the secondary is the reverse of that through the primary, the direction of this flux will be opposite to that due to the primary current. The secondary current flux will naturally flow through the core and we have the condition This is Lesson 22 in The Rider Series on Sound Projection