corrective maintenance for communication and electronic equipment: Rotation frequencies of some mechanical devices must be determined; the output frequency of electric power generators is checked when the engine is started and during preventive maintenance routines; carrier equipment that operates in the audio-frequency range must be adjusted to operate at the correct frequencies; and radio transmitters must be accurately tuned to the assigned frequencies to provide reliable communications and to avoid interfering with radio circuits operating on other frequencies.">
Frequency measurements are an essential part of preventive and corrective maintenance for communication and electronic equipment: Rotation frequencies of some mechanical devices must be determined; the output frequency of electric power generators is checked when the engine is started and during preventive maintenance routines; carrier equipment that operates in the audio-frequency range must be adjusted to operate at the correct frequencies; and radio transmitters must be accurately tuned to the assigned frequencies to provide reliable communications and to avoid interfering with radio circuits operating on other frequencies. These are only a few of the applications for making frequency measurements.
Frequency-measuring equipment and devices, particularly those used to determine radio frequencies, constitute a distinct class of test equipment, because of the important and critical nature of such measurements. The requirement of precise calibration is extremely important in all frequency-measuring work. To provide accurate measurements, every type of frequency-measuring device must be calibrated against some frequency standard.
Of considerable importance in the measurements of frequency or wavelength are the standards against which frequency-measuring devices are compared and calibrated. Frequency standards belong to two general categories: primary and secondary standards. The PRIMARY FREQUENCY STANDARD maintained by the U.S. National Bureau of Standards has long-term stability and an accuracy of 1 part in 1012, using an atomic clock. A SECONDARY FREQUENCY STANDARD is a highly stable and accurate standard that has been calibrated against the primary standard. Secondary standards are maintained by calibration laboratories that service your test equipment.
The National Bureau of Standards provides time and frequency standards from station WWV at Fort Collins, Colorado, and from station WWVH at Kekaha, Kauai, Hawaii. The following technical radio services are given continuously by these stations:
The UTC scale uses the ATOMIC SECOND as a time interval. UT1 is based on the earth's uniform rate of rotation. Since the earth's rotation is not precisely uniform, UT1 is an adjustable interval.
To ensure reliable coverage of the United States and extensive coverage of other parts of the world, radio stations WWV and WWVH provide the primary standard radio frequencies listed in table 3-1. The transmission of WWV and WWVH are interrupted for 5 minutes of each hour. The silent period begins at 15 minutes past the hour for station WWVH and 45 minutes past the hour for station WWV. These silent periods are provided to eliminate errors caused by interference.
Table 3-1. - NBS Frequency Standards and Time Transmission
Two primary standard audio-frequency tones (440 Hz and 600 Hz) are broadcast on all WWV and WWVH carrier frequencies. In the absence of a message, a 500-Hz tone is broadcast during the message interval. The 440-Hz signal that denotes the 1-hour mark is the standard musical pitch, A above middle C. The 600-Hz tone provides a frequency standard for checking the 60-Hz power-line frequency.
The standard time pulse marking interval of 1 second consists of five cycles of a 1,000-Hz tone at WWV and six cycles of a 1,200-Hz tone at WWVH. These marker pulses are heard as clock ticks. Intervals of 1 minute are marked by a 0.8-second, 100-Hz tone for WWV and a 0.8-second, 1,200-Hz tone for WWVH. Each hour is marked by a 0.8-second, 1,500-Hz tone on both stations. Universal Time Coordinated (UTC) is announced on WWVH between the 45 and 52.5 seconds of each minute and on WWV between the 52.5 and 60 seconds of each minute.
An announcement of radio propagation conditions (geophysical alert) for the North Atlantic area is broadcast by station WWV in voice at 18 minutes after each hour. For example, these short-term announcements might state, "The radio propagation quality forecast at ... (normal, unsettled, disturbed)." The propagation format is repeated phonetically and in numerical code to ensure clarity. The letter designations N, U, and W, signifying "normal," unsettled," and "disturbed," respectively, classify the radio propagation conditions at the time of the broadcast. The digits from 1 to 9 indicate the expected radio propagation conditions during the next 6 hours; refer to table 3-2 for code interpretations. The National Bureau of Standards forecasts are based on information obtained from a worldwide network of geophysical and solar observations.
Table 3-2. - NBS Radio Propagation Coding
MECHANICAL ROTATION AND VIBRATION METHODS
There are many instances when you are very much concerned with the question of rotational or vibratory speeds. Knowledge of rotational speeds is necessary where the output of a direct current generator has fallen below a minimum desired output or where the speed of a motor (such as the motor in a teletypewriter or radar antenna) must be maintained at a constant value. There are many instruments that you can use for this purpose, such as tuning forks, stroboscopes, vibrating-reed meters, and electromechanical counters. The oscilloscope and the frequency counter are two of the other devices which may be used, but their use may require the employment of accessory equipment.
Tuning Fork Methods
A tuning fork is generally used in conjunction with the measurement of the rotational speed of a teletypewriter or facsimile motor but is not limited to this application. However, you must remember that the tuning fork can be used at only one frequency, the frequency of vibration for which it was manufactured, and therefore cannot be used on variable-speed motors. To use the tuning fork, you direct a source of light upon the point to be observed. In the case of a teletypewriter, a black-and-white segmented target is painted on the outer circumference of the motor governor. Radial spokes in a flywheel could be used equally well. Permit the motor to reach operational speed under normal load conditions; otherwise, the motor will slow down considerably when the normal load is applied. Strike the tuning fork against the side of your hand to set it into vibration. Then observe the target through the slots in the plates attached to the tines of the fork. The correct speed is obtained when the segments of the target appear to be stationary. If the segments seem to move backward, apparently against the known motor rotational direction, the speed is too low. If the segments seem to move forward, the speed is too high. There is also the possibility that the target segments will appear to jump back and forth or to disappear suddenly. Such erratic action is often because of governor malfunctioning. The correct speed adjustment is reached when the targets appear to be stationary.
When using a stroboscope to measure the speed of rotating or reciprocating mechanisms, hold the instrument so that the light from the stroboscope lamp falls directly on the part to be observed. If the part is uniform, or symmetrical, place an identification mark with chalk or a grease pencil on the portion to be observed. This method provides a positive means of identification, because if only one reference mark is observed during measurement, you can be sure that either the fundamental synchronization or a submultiple thereof has been obtained. If the approximate speed of rotation is known, the stroboscope controls may be set to the appropriate positions prior to actual measurement. The main frequency control that determines the rate of the flashing light is then varied until the reference mark on the moving part appears to be standing still. The calibrated scale of the stroboscope will then show the speed directly in revolutions per minute (rpm).
If you have no idea of the speed of the moving part, it is best to start the measurement procedure at the highest frequency that the stroboscope can deliver. The flashing rate of the stroboscope can then be gradually reduced until a single stationary image of the reference mark is obtained. This is the point of fundamental synchronism that corresponds to the speed of the moving part. Do not continue to reduce the flashing rate of the instrument beyond this point without a valid reason for doing so. If you do continue the reduction, a stationary image will still be observed, but the stroboscope will indicate a submultiple of the true rotational speed; thus, a measurement error will be introduced.
Stroboscopes generally have a high- and low-range switch. The typical low range is from 600 to 3,600 rpm, and the upper range is from 3,600 to 15,000 rpm; there is a slight overlap in ranges to ensure reliable frequency coverage. In view of the limitation imposed by flasher tube life, the stroboscope should always be operated at a flashing rate that is as low as possible, consistent with the rotational speed of the observed part. If you should be required to operate this instrument over a long period of time, use a submultiple of the fundamental synchronous speed. The pattern will remain just as stationary, and the tube life will be greatly extended. In addition, the quality of the light is better at the lower ranges than at the upper end of the scale. Sometimes you will encounter a rotating or vibrating device that is moving faster (or slower) than the measuring range of the stroboscope will accommodate. Although such speeds can still be measured, you must use the multiple or submultiple synchronism points.
There are two methods of measuring high speeds. The first method is to obtain a single stationary image of the rotating object at a subharmonic speed relationship and to record that value as A. Then obtain a second single stationary image at the next lower subharmonic speed relationship, and record this value as B. The unknown speed may then be computed from the following formula:
For example, assume reading A was 4,000 rpm and reading B was 3,500 rpm. The computation would be as follows:
The second method is used where the value of A X B becomes progressively smaller. The A reading is obtained as in the previous example (for the sake of easier computation, suppose that the A reading is still 4,000 rpm). Then obtain another submultiple reading for B, keeping in mind the number of times a stationary single image was observed. If a stationary single image was observed seven different times and the final B reading was 2,000 rpm, the calculation would become as follows:
At speeds lower than the lowest range of the stroboscope, multiple images will be observed. For example, assume a dial reading of 900 rpm was obtained when two stationary images were observed. Then dividing the rpm by the number of images will give the unknown shaft speed, as shown below:
Exercise caution in using a stroboscope. The illusion of stopped motion is very convincing. Do not attempt to touch the moving equipment.
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