control transformer is illustrated in figure 1-25. The stator windings of the CX are connected to the stator windings of the CT and both synchros are shown on 0°. Notice, that at 0°, the CT rotor is perpendicular to its S2 winding. This is contrary to what we have learned so far about synchros, but it is just another peculiarity of the CT. When the rotor of the CX is on 0°, the rotor's magnetic field points straight up as shown (the black arrow).">
CONTROL SYNCHRO SYSTEM OPERATION
A control synchro system consisting of a control transmitter and a control transformer is illustrated in figure 1-25. The stator windings of the CX are connected to the stator windings of the CT and both synchros are shown on 0°. Notice, that at 0°, the CT rotor is perpendicular to its S2 winding. This is contrary to what we have learned so far about synchros, but it is just another peculiarity of the CT. When the rotor of the CX is on 0°, the rotor's magnetic field points straight up as shown (the black arrow). The voltages induced in the CX stator windings, as a result of this field, are impressed on the CT stator windings through the three leads connecting the S1, S2, and S3 terminals. Exciting currents proportional to these voltages flow in the CT stator windings and establish a magnetic field in the CT in the same direction (white arrow) as the magnetic field (black arrow) in the CX. Observe that the rotor of the CT is perpendicular to the stator magnetic field and, therefore, the induced voltage in the rotor is zero, as indicated by the straight line on the oscilloscope presentation.
Figure 1-25. - CX-CT system operation with rotor in correspondence.
When the CT rotor is rotated 90°, as shown in figure 1-26, the rotor is parallel to the resultant stator field Maximum magnetic coupling occurs between the rotor and stator fields at this point. As a result of this coupling, the stator windings induce a maximum of 55 volts into the rotor winding. The phase of this voltage depends upon the direction in which the CT rotor is turned. The rotor of the CT is wound so that clockwise rotation of the stator magnetic field induces a voltage across the rotor which is proportional to the amount of rotation and in phase with the ac supply voltage. Counterclockwise rotation of the stator magnetic field produces a voltage that is still proportional to the amount of rotation, but 180° out of phase with the supply voltage. Keep in mind that the clockwise rotation of the CT stator magnetic field is the same as the counterclockwise rotation of the CT rotor. This phase relationship between the ac supply voltage and the CT output voltage becomes more apparent in figure 1-27.
Figure 1-26. - CX-CT system operation with the CX rotor at 0° and the CT rotor at 90°.
Figure 1-27A. - Control synchro system operation.
Figure 1-27B. - Control synchro system operation.
Figure 1-27C. - Control synchro system operation.
When the rotor of the CX in view A of figure 1-27 is turned 60° clockwise, the magnetic field in the CX (black arrow) and the magnetic field in the CT (white arrow) also rotate 60° clockwise. This action induces a voltage in the CT rotor that is in phase with the ac supply, as indicated by the oscilloscope presentation. If the rotor of the CX in view B is turned 60° in a counterclockwise direction from its 0° position, the magnetic field (white arrow) in the CT also rotates counterclockwise through the same number of degrees as the CX. Since the magnetic field in the stator of the CT cuts through the rotor in the opposite direction, the induced voltage in the rotor is now out of phase with the ac supply to the CX, as shown in the oscilloscope presentation.
At times it is necessary, because the CT is used to control servo systems, to have the CT output reduced to zero volts to prevent any further movement of a load. To accomplish this, it is necessary to turn the rotor of the CT through the same number of degrees and in the same direction as the rotor of the CX. This places the CT rotor perpendicular to its own stator field and reduces its output to zero volts as illustrated in view C.
The CT output voltage discussed throughout this section is commonly referred to as an ERROR SIGNAL. This is because the voltage represents the amount and direction that the CX and CT rotors are out of correspondence. It is this error signal that eventually is used in moving the load in a typical servo system.
Now that we have covered the basic operation of the control synchro system, let us see how this system works with a servo system to move heavy equipment. Figure 1-28 shows a block diagram of a typical servo system that uses a control synchro system. Assume the shaft of the CX in this system is turned by some mechanical input. This causes an error signal to be generated by the CT because the CX and the CT rotors are now out of correspondence. The error signal is amplified by the servoamplifier and applied to the servomotor. The servomotor turns the load, and through a mechanical linkage called RESPONSE, also turns the rotor of the CT. The servomotor turns the rotor of the CT so that it is once again in correspondence with the rotor of the CX, the error signal drops to zero volts, and the system comes to a stop.
Figure 1-28. - A positioning servo system using a control synchro system.
Q.44 When a CT is on electrical zero, what is the relationship between its rotor and
the S2 winding?
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