1-35
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?
Q-41. What are the three types of control synchros?
Q-42. How do the CX and CDX differ from the TX and TDX?
Q-43. What three things prevent a CT rotor from turning when voltages are applied to its stator
windings?
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 0º. Notice, that at 0º, 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 0º, 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.
