International projectionist (Jan-Dec 1945)

the armature will approach the rated value, not the ohmic value. The voltage that results from this induced current is called the back or counter electromotive force. This counter voltage will also oppose the line voltage. The net voltage acting on the armature is equal to Eline — Ecounter. Where la is the armature current and R» is the armature resistance, the armature current by Ohm's law is I. = Eline — Ecounter Ra The above equation may be transposed so that the counter electromotive force can be found from values that can easily be measured: Ecounter = Eline LR». Shunt Motors Motors are identified by the manner in which the field coil is connected to the armature coil. Permanent magnets are not suitable for commercial motors because of their varying characteristics, therefore field coils are commonly used for this purpose. When the field coil is connected in parallel (shunt) with the armature the motor is called a shunt motor. Figure 4 shows the hookup diagram for a shunt motor. The rheostat in series with the field is used for speed control. Of all the motors in use today, the shunt motor is best suited for speed control. Shunt motors usually are started at light loads. This is not due to the fact that these motors have a low starting torque, but rather because the starting boxes that are used with such motors are designed to start at light loads only and would quickly overheat under other conditions. Shunt motors are used where constant speed is desired, and also where it is necessary to vary the speed. The rheostat is used to set the motor to the desired speed, and the speed will then remain substantially constant even if the load should vary. These motors will be found driving blowers, spinning frames, machine shop drives, etc. Series Motors In a series motor the field is connected in series with the armature. Series motors may be started with heavy loads but the speed will rapidly drop as series field armature the load is increased. The load should never be removed from a series motor. If the load were removed the field current would drop to a very low value. Under these conditions the motor has a tendency to race. The resulting speed is certain to become so great that the motor will wreck itself. The connections of the series motor are shown in Figure 5. These motors are used where the speed or load varies over wide ranges. Such an application is found in the street car. When a street car is going uphill the speed of the series motor automatically drops. This fact is very important because it proves that the speed of the series motor decreases with increasing load and, therefore, excessive overloads are prevented. If a shunt motor were used for street car work it would constantly be taking excessive overloads. For example, if the car were moving uphill the motor would try to run as fast as when going downhill, because shunt motors try to maintain constant speed. Compound Motors If a series field is added to a shunt motor it becomes a compound motor (Figure 6) and has some of the desirable features of both the shunt and the series motors. If the series winding is series field armature rheostat vww FIGURE 5. Series motor. FIGURE 6. Compound motor. connected so that its magnetic field aids that of the shunt winding, the connection is called cumulative-compound. Cumulative-compound motors have a definite no-load speed, and, therefore, do not run away when the load is removed. These motors are principally used for driving machines which are subject to sudden applications of heavy loads. Rolling mill machinery, shears, punches, cranes, and elevators use such motors. When the series field is wound so that it opposes the shunt winding the motor is called a differential-compound motor. As the load is increased the current through the series field is increased. Since the series field opposes the shunt field the total resultant field will decrease; when the field of a motor decreases the speed increases. The speed of a differential-compound motor tends to increase with increasing loads. The diagram of a compound motor is shown in Figure 6. S.M.P.E. CONFERENCE TO BE HELD IN HOLLYWOOD MAY 14-18 The 57th semi-annual technical conference of the Society of Motion Picture Engineers will be held at the Hollywood-Roosevelt Hotel in Hollywood, Calif., May 14-18, according to an announcement by the group. It is pointed out that members and others contemplating the presentation of papers can assist the Papers Committee by mailing complete manuscripts not later than April 20 to the West or East Coast chairmen of the Papers Committee, or to the Society's New York office. Committee chairmen are: Pacific Coast section, H. W. Moyse; Papers, C. R. Daily; Publicity, E. 0. Blackburn, West Coast, and Julius Haber, East Coast; Registration and Information, W. C. Kunzmann; Reception and Local Arrangements, Emery Huse; Luncheon and Dinner Dance, L. L. Ryder; Hotel and Transportation, C. W. Handley; Projection Programs, 35-mm, R. H. McCullough, assisted by officers and members of I. A. T. S. E., Locals 150 and 165, and 16-mm, H. W. Remershied. Eastern and Mid-Western members who plan to attend the conference are cautioned to check railroad and Pullman accommodations, and make hotel reservations at least 30 days prior to leaving for the West Coast. MOTIOGRAPH SALES SHOW ITS WAR WORK EXPANSION Motiograph's manufacturing activities last year were much larger than previously, the company announces, with sales more than six times the 1941 figure. The company's three plants still are working at full capacity on production of war items and a limited amount of sound and projection equipment. A backlog of orders amounting to more than $4,500,000 is being held, not including substantial orders for sound and projection equipment for post-war delivery. The company's post-war projector model now is going through rigid tests under actual theatre conditions, with Motiograph claiming the projector has more new innovations of design than ever has appeared on a projector mechanism, yet its design and construction is extremely simple. FORRESTAL GETS EXECUTIVE POST WITH ANSCO DIVISION • George W. Burpee, president of General Aniline and Film Corporation, announces the appointment of James Forrestal as assistant to E. A. Williford, vice-president in charge of the Ansco Division. Mr. Forrestal, in addition to his new duties, will continue in charge of the Ansco marketing division. He joined the organization in 1933 as general sales manager. APRIL QUESTIONS 1. From which material is the commutator made? 2. Which motor is best suited for speed control? 3. Which type of motor may wreck itself if its load is removed? 4. Which type of motor is best suited for trolley cars? The correct answers to these questions will appear in the next issue. 20 INTERNATIONAL PROJECTIONIST