If synchros are to work properly in a system, they must be connected and aligned correctly with respect to each other and to the other devices with which they are used. The reference point for alignment of all synchro units is ELECTRICAL ZERO. The mechanical reference point for the units connected to the synchros depends upon the particular application of the synchro system. Whatever the application, the electrical and mechanical reference points must be aligned with each other. The mechanical position is usually set first, and then the synchro device is aligned to electrical zero.
There are various methods for zeroing synchros. Some of the more common zeroing methods are the voltmeter, the electrical-lock, and the synchro-tester methods. The method used depends upon the facilities and tools available and how the synchros are connected in the system. Also, the method for zeroing a unit whose rotor or stator is not free to turn may differ from the procedure for zeroing a similar unit whose rotor or stator is free to turn.
The most accurate method of zeroing a synchro is the ac voltmeter method. The procedure and the test circuit configuration for this method vary somewhat, depending upon which type of synchro is to be zeroed. Transmitters and receivers, differentials, and control transformers each require different test circuit configurations.
Regardless of the synchro to be zeroed, there are two major steps in each procedure. The first step is the coarse or approximate setting. The second step is the fine setting. The coarse setting ensures the device is zeroed on the 0° position rather than the 180° position. Many synchro units are marked in such a manner that the coarse setting may be approximated physically by aligning two marks on the synchro. On standard synchros, this setting is indicated by an arrow stamped on the frame and a line marked on the shaft, as shown in figure 1-37. The fine setting is where the synchro is precisely set on 0°.
Figure 1-37. - Coarse electrical zero markings.
For the ac voltmeter method to be as accurate as possible, an electronic or precision voltmeter having 0- to 250-volt and a 0- to 5-volt ranges should be used. On the low scale this meter should also be able to measure voltages as low as 0.1 volt.
Zeroing Transmitters and Receivers (Voltmeter Method)
Since the TX, CX, and TR are functionally and physically similar, they can be zeroed in the same manner. For the TX and CX to be properly zeroed, electrical zero voltages (S2 = 52V; S1 and S3 = 26V) must exist across the stator winding when the rotor of the transmitter is set to 0° or its mechanical reference position. The synchro receiver (TR) is properly zeroed when the device it actuates assumes its zero or mechanical reference position while electrical zero voltages (S2 = 52V; S1 and S3 = 26V) exist across its stator windings. The following is a step-by-step procedure used to zero the TX, CX, and TR.
Carefully set the unit (antenna, gun mount, director, etc.) whose position the CX or TX transmits, accurately on 0° or on its reference position. In the case of the TR, deenergize the circuit and disconnect the stator leads before setting its rotor on zero or to its reference position. The rotor may need to be secured in this position; taping the dial to the frame is usually sufficient. Deenergize the synchro circuit and disconnect the stator leads. NOTE: Many synchro systems are energized by individual switches. Therefore, be sure that the synchro power is off before working on the connections.Set the voltmeter to its 0- to 250-volt scale and connect it into the circuit as shown in view A of figure 1-38.
Figure 1-38A. - Zeroing a transmitter or receiver by the voltmeter method.
Energize the synchro circuit and turn the stator until the meter reads about 37 volts (15 volts for a 26-volt synchro). Should the voltmeter read approximately 193 volts (115 volts + 78 volts = 193 volts), the rotor is at 180°. Turn it through a half revolution to bring it back to 0°. If you cannot obtain the desired 37 (or 15) volts, use the lowest reading you can take with the meter. This is the coarse setting and places the synchro approximately on electrical zero. Deenergize the synchro circuit and connect the meter as shown in view B. Start with a high scale on the meter and work down to the 0- to 5-volt scale to protect the meter movement.
Figure 1-38B. - Zeroing a transmitter or receiver by the voltmeter method.
Reenergize the synchro circuit and adjust the stator for a zero or minimum voltage reading. This is the fine electrical zero position of the synchro.
When you have reconnected a TX and a TR into a system after zeroing them, you can perform a simple check on the system to see if they are accurately on electrical zero. First, place the transmitter on 0°. When the system reaches the point of correspondence, the induced voltages in the S1 and S3 stator windings of both synchros should be equal. Since the terminals of S1 and S3 are at equal potential, a jumper between these terminals at the TR should not affect the circuit. If, however, the TR rotor moves when you connect a jumper, there is a slight voltage difference between S1 and S3. This voltage difference indicates that the transmitter is not on electrical zero. If this is the case, recheck the transmitter for electrical zero.
Zeroing Differential Synchros (Voltmeter Method) A differential synchro is zeroed when it can be inserted into a system without introducing any change. If a differential synchro requires zeroing, use the following voltmeter method:
Carefully and accurately set the unit whose position the CDX or TDX transmits on zero or on its reference position. In the case of the TDR, deenergize the circuit and disconnect all leads before setting its rotor to 0° or to its reference position. You may need to secure the rotor in this position; taping the dial to the frame is usually sufficient. Deenergize the circuit and disconnect all leads on the differential except leads S2 and S3 when you use the 78-volt (10.2 volts for 26-volt units) supply from the transmitting unit to zero the differential. Set the voltmeter to its 0- to 250-volt scale and connect it as shown in view A of figure 1-39. If the 78-volts is not available from the transmitter or from an auto transformer, you may use a 115-volt source instead. If you use 115 volts instead of 78 volts, do not leave the synchro connected for more than 2 minutes or it will overheat and may become permanently damaged.
Figure 1-39A. - Zeroing differential synchros by the voltmeter method.
Figure 1-39B. - Zeroing differential synchros by the voltmeter method.
Energize the circuit, unclamp the differential's stator, and turn it until the meter reads minimum. The differential is now approximately on electrical zero. Deenergize the circuit and reconnect it as shown in view B. Reenergize the circuit. Start with a high scale on the meter and work down to the 0- to 5-volt scale to protect the meter movement. At the same time, turn the differential's stator until you obtain a zero or minimum voltage reading. Clamp the differential stator in position, ensuring the voltage reading does not change. This is the fine electrical zero position of the differential.
Zeroing a Control Transformer (Voltmeter Method)
Two conditions must exist for a control transformer (CT) to be on electrical zero. First, its rotor voltage must be minimum when electrical zero voltages (S2 = 52 volts; S1 and S3 = 26 volts) are applied to its stator. Second, turning the shaft of the CT slightly counterclockwise should produce a voltage across its rotor in phase with the rotor voltage of the CX or TX supplying excitation to its stator. To zero a CT (using 78 volts from its transmitter) by the voltmeter method, use the following procedure:
Set the mechanism that drives the CT rotor to zero or to its reference position. Also, set the transmitter (CX or TX) that is connected to the CT to zero or its reference position. Check to ensure there is zero volts between S1 and S3 and 78 volts between S2 and S3. If you cannot obtain these voltages, you will need to rezero the transmitter. NOTE: If you cannot use the 78 volts from the transmitter circuit and, an auto transformer is not available, you may use a 115-volt source. If you use 115 volts instead of 78 volts, do not energize the CT for more than 2 minutes because it will overheat and may become permanently damaged. Deenergize the circuit and connect the circuit as shown in view A of figure 1-40. To obtain the 78 volts required to zero the CT, leave the S1 lead on, disconnect the S3 lead on the CT, and put the S2 lead (from the CX) on S3. This is necessary since 78 volts exists only between S1 and S2 or S2 and S3 on a properly zeroed CX. Now energize the circuit and turn the stator of the CT to obtain a minimum reading on the 250-volt scale. This is the coarse or approximate zero setting of the CT.
Figure 1-40A. - Zeroing a control transformer by the voltmeter method.
Figure 1-40B. - Zeroing a control transformer by the voltmeter method.
Deenergize the circuit, reconnect the S1, S2, and S3 leads back to their original positions, and then connect the circuit as shown in view B. Reenergize the circuit. Start with a high scale on the meter and work down to the 0- to 5-volts scale to protect the meter movement. At the same time, turn the stator of the CT to obtain a zero or minimum reading on the meter. Clamp down the CT stator, ensuring the reading does not change. This is the fine electrical zero position of the CT.
Zeroing Multispeed Synchro Systems.
If multispeed synchro systems are used to accurately transmit data, the synchros within the systems must be zeroed together. This is necessary because these synchros require a common electrical zero to function properly in the system.
First, establish the zero or reference position for the unit whose position the system transmits. Then, zero the most significant synchro in the system first, working down to the least significant. For example, zero the coarse synchro, then the medium synchro, and finally the fine synchro. When you zero those synchros, consider each synchro as an individual unit and zero it accordingly.
Q.62 When is a synchro receiver (TR) properly zeroed?