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TX-TDR-TX System Operation (Subtraction) As we previously explained, the differential receiver differs chiefly from the differential transmitter in its application. The TDX in each of the previous examples combined its own input with the signal from a synchro transmitter (TX) and transmitted the sum or difference to a synchro receiver (TR). The synchro receiver then provided the system's mechanical output. When the differential receiver (TDR) is used, the TDR itself provides the system's mechanical output. This output is usually the sum or difference of the electrical signals received from two synchro transmitters. Figure 1-22 shows a system consisting of two TXs (No. 1 and No. 2) and a TDR connected for subtraction. Figure 1 - 22. - TX-TDR-TX system operation (subtraction).
In this figure the signal from TX No. 1 rotates the resultant TDR stator field 75 counterclockwise. In a similar manner, the signal from TX No. 2 rotates the resultant TDR rotor field counterclockwise 30. Since the two resultant fields are not rotated by equal amounts, a torque is exerted on the rotor to bring the two fields into alignment. This torque causes the rotor to turn to 45, the point at which the two fields are aligned. To bring the two fields into alignment, the TDR rotor need turn only through an angle equal to the difference between the signals supplied by the two TXs. TX-TDR-TX System Operation (Addition) To set up the previous system for addition, it is necessary to reverse only the R1 and R3 leads between the TDR rotor and TX No. 2. With these connections reversed, the system operates as shown in figure 1-23. Figure 1-23. - TX-TDR-TX system operation (addition).
Assume the TDR rotor is initially at 0. TX No. 1 is turned to 75, and TX rotor No. 2 is turned to 30. The TDR stator field still rotates counterclockwise 75, but because R1 and R3 on the TDR rotor are reversed, its rotor field rotates 30 clockwise. The angular displacement of the two fields then, with respect to each other, is the sum of the signals transmitted by the two TXs. The magnetic force pulling the TDR rotor field into alignment with that of the stator turns the TDR rotor to 105. Therefore, the system solves the equation 75 + 30 = 105. Q.38 What connections in a TDR system are reversed to set up the system for addition? CONTROL SYNCHRO SYSTEMS It should be clear to you from our discussion of torque synchro systems that, since they produce a relatively small mechanical output, they are suitable only for very light loads. Even when the torque system is moderately loaded, it is never entirely accurate because the receiver rotor requires a slight amount of torque to overcome its static friction. When large amounts of power and a higher degree of accuracy are required, as in the movement of heavy radar antennas and gun turrets, torque synchro systems give way to the use of CONTROL SYNCHROS. Control synchros by themselves cannot move heavy loads. However, they are used to "control" servo systems, which in turn do the actual movement. Servo systems are covered in depth in the next chapter in this module. There are three types of control synchros: the CONTROL TRANSMITTER (CX), the CONTROL TRANSFORMER (CT), and the CONTROL DIFFERENTIAL TRANSMITTER (CDX). The control transmitter (CX) and the control differential transmitter (CDX) are identical to the TX and the TDX we discussed previously except for higher impedance windings in the CX and CDX. The higher impedance windings are necessary because control systems are based on having an internal voltage provide an output voltage to drive a large load. Torque systems, on the other hand, are based on having an internal current provide the driving torque needed to position an indicator. Since we discussed the theory and operation of the TX and the TDX earlier, we will not discuss their counterparts, the CX and CDX. However, we will cover the third control synchro, the CT, in depth during this discussion. CONTROL TRANSFORMERS A control transformer is just what its name implies-a control synchro device accurately governing some type of power amplifying device used for moving heavy equipment. Figure 1-24 shows a phantom view of a typical CT and its schematic symbols. Figure 1-24. - (A) Phantom view of a typical CT; (B) CT schematic symbols.
The CT compares two signals, the electrical signal applied to its stator and the mechanical signal applied to its rotor. Its output is a difference signal that controls a power amplifying device and thus the movement of heavy equipment. The unit construction and physical characteristics of a control transformer are similar to those of a control transmitter or torque receiver, except that there is no damper and the rotor is a drum or wound rotor rather than a salient-pole rotor. An interesting point about the rotor is that it is never connected to an ac supply and, therefore, induces no voltages in the stator coils. As a result, the CT stator currents are determined solely by the voltages applied to the high-impedance stator windings. The rotor itself is wound so that its position has very little effect on the stator currents. Also, there is never any appreciable current flowing in the rotor because its output voltage is always applied to a high-impedance load. As a result, the CT rotor does not try to follow the magnetic field of its stator and must be turned by some external force. The stator windings of the CT are considered to be the primary windings, and the rotor windings the secondary windings. The output, which is taken off the R1 and R2 rotor leads, is the voltage induced in the rotor windings. The phase and amplitude of the output voltage depend on the angular position of the rotor with respect to the magnetic field of the stator. Q.40 What type of synchro is used in systems requiring large amounts of power and a
high degree of accuracy? |
 
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