Magazine article American Cinematographer

Cinema Workshop

Magazine article American Cinematographer

Cinema Workshop

Article excerpt

SYNCHRONOUS MOTORS

Historically, the synchronous motor is probably the most important of all the types of camera drive systems. From the beginning of sound pictures to the present time, the synchronous motor has been at the heart of double-system sound production, both in the studio and on location. Although its popularity on location has waned in recent years (due to the introduction of the crystal servo motors), it still remains one of the most used, least complex and least expensive of all studio double-system sound techniques.

Before discussing the synchronous motor, we must first take a look at the electric power that comes out of the wall socket. Power in this country is usually 117-volt 60-cycle A.C. The important point here is not so much the voltage, but the 60-cycle A.C. The A.C. stands for "alternating current" as illustrated in FIGURE 1a. Here it can be seen that the power sinusoidally changes polarity 120 times a second. The synchronous motor is constructed with special windings that respond to these changes in polarity.

For example, an electric wall clock is probably the most familiar of all devices powered by a synchronous motor. The motor in the clock is designed in such a way that for every 60 cycles of A.C. power, the second hand moves one second (or the second hand will make one revolution for every 3600 cycles). Take a look at FIGURE 1b. Hopefully this rendering of several cycles of A.C. power will suggest a "gear" the peaks of power corresponding to the teeth of a gear. This is precisely what is actually occurring. The A.C. cycles essentially drive the synchronous motor much like one gear meshes and thus drives another. For a given number of cycles, the motor will be driven the specified number of revolutions.

The motion picture drives in this country are designed so that for every 60 cycles, the motor will turn the camera 24 frames exactly. Remember, the motor is essentially "geared" to the A.C. current- for every 60 cycles, the camera moves 24 frames.

The main theory of shooting double-system with synchronous motors should now be clear. Any number of cameras with synchronous drives and any number of sprocketed recorders with synchronous drives may be employed, and all units will be in sync with one another (assuming they are all plugged into a common power source). Because they are all powered (and thus "gear driven") by a common A.C. signal, they are all essentially "geared" to each other. If the speed of the 60-cycle signal should deviate, all the cameras and recorders will likewise deviate the identical amount. …

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