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ASSOCIATE AIEE Τ OVERCOME inher ent difficulties in im pulse telemeters and to elimi nate sources of mechanical maintenance problems, yet without requiring unusual or undesirable types of carriercurrent channels, the new frequency -type telemeter was developed. In this system the frequency of the transmitter (modulating) signal varies in proportion to the magnitude of the measured quantity, over the range from 6 to 27 cycles per second. Frequency was chosen as the quantity to be recognized by this system be cause it is entirely independent of the transmission channel characteristics which are variable in nature. This range in frequency was selected to provide the most desirable balance between fast time of response, minimum carrier band width requirements, and mini mum instrument damping. The use of a sinusoidal transmitter signal, instead of the more common square wave impulse signal, permits better utilization of the carrier-current equipment of the type employing audio tone modulation of the carrier. Instantaneous response without overshoot, absence of contacts and relays,and presence of a minimum of moving parts are among the outstanding characteristics of this system. THE TRANSMITTERThe telewattmeter of Figure 1 will illustrate the prin ciples of the telemeter transmitter. The primary de tector is a watt-hour meter of standard construction. It is connected to the circuit to be measured in the usual manner, and produces a shaft rotation at a speed which is proportional to the input watts. (Similar detectors can be used to measure vars, a-c volts, or a-c amperes.)A voltage whose frequency varies in proportion to the measured quantity is generated by means of a photo electric pickup actuated by the rotation of the watthour meter shaft. A disk having a serrated edge is

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I F ALL TIE LINES were of ample capacity and stations under automatic control were well distributed geographically, there would be no need for tie line control except where required by contract interchange schedules. With the large scale advent of system interconnections and the necessity for more efficient operation, the tendency is to use frequency control biased by tie line load rather than frequency control only.The most common type of frequency control responds to instantaneous frequency difference with corrective impulses readjusting the prime mover speed level setting at a rate at least approximately proportional to the deviations from standard frequency. The frequency-responsive device for this type of equipment is usually an a-c bridge circuit or sometimes a pendulum-type clock in which corrective signals can be sent out for each swing of the pendulum.To obtain the advantage of smoother and less rapid prime mover gate or valve operation under automatic FREQUENCY COMPARATOR STANDARD FREQUENCY SOURCE HSV UNDER-VOLTACE| AND FREO. RELAYS DIFFERENTIAL ΗΠΖΉ UNDER-VOLTACEJ ANO FREO. RELAYS hP BUS SELECTOR 115 V 3» .

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