1-39
SYNCHRO CAPACITORS
As we stated earlier, the speed and accuracy of data transmission are most important. With the use of
more complex synchros, like the differential and the control transformer, the accuracy of the synchro
systems may be affected. The following discussion will deal with how complex synchros affect the
accuracy of synchro systems and what can be done to keep this accuracy as high as possible. Synchro
capacitors play a major role in maintaining a high degree of accuracy in synchro systems.
When a torque transmitter is connected to a torque receiver (TX-TR), very little, if any, current flows
in the stators when the rotors are in correspondence. This is because the voltages induced in the TR
windings almost exactly balance out the voltages induced in the TX windings. As a result, the TR is very
sensitive to small changes in the position of the TX rotor, causing the TR to follow the TX with a high
degree of accuracy.
When a synchro system contains differential synchros (TDX or CDX), the stator currents at
correspondence are greater than they are in a single TX-TR system. The reason is the step-up turns ratio
between the stator and rotor in the differential synchro.
In a synchro system that uses a CT, stator current at correspondence is also greater than in a TX-TR
system. In this case, however, this reason is that the CT rotor is not energized and as a result no voltage is
induced in the stator to oppose the voltage in the transmitter stator. The overall effect of this increase in
stator current is to reduce the accuracy of the system. To maintain high accuracy in a synchro system
containing either differential units or CTs, the stator currents must be kept to a minimum. This is done by
connecting synchro capacitors in the circuit.
To understand the operation of a synchro capacitor and how it reduces current drain on the
transmitter requires a recollection of the voltage and current relationships in inductive and capacitive
circuits. As you learned in module 2 of this series, current lags voltage by 90º in a purely inductive
circuit. You also know that an ideal inductor is impossible to make because there is always resistance
present. Therefore, an inductor has a combination of inductive reactance and resistance. Since current and
voltage are always in phase in a resistive circuit and 90º out of phase in an inductive circuit, we can say
that there are two currents in an inductor-the loss current, which is the resistive (in-phase) current, and the
magnetizing current, which is the inductive (out-of-phase) current. It is this magnetizing current that we
would like to eliminate in the stator coils of the TDX, CDX, and CT because it makes up most of the line
current.
Keeping in mind that current leads voltage by 90º in a capacitive circuit, let's see what happens to
magnetizing current when a capacitor is added to the circuit.
Suppose a capacitor is hooked up across one of the stator coils of a TDX and its capacitance is
adjusted so that its reactance equals the reactance of the coil. Since the two reactances are equal, the
current they draw from the line must also be equal. However, these currents are going to be 180º out of
phase, because the current in the coil lags the line voltage, while the capacitor's current leads it. Since the
two currents are equal in magnitude but opposite in phase, they cancel. The total line current is reduced
by this effect and, if a capacitor is placed across each coil in the TDX, the line current decreases even
further. This, in effect, increases torque in synchro systems near the point of correspondence and,
therefore, increases overall system accuracy.
Connecting capacitors across individual stator windings is impractical because it requires that the
stator winding's common connection be outside the synchro. Since this is not done with synchros, another
method has been devised to connect up the capacitors which works just as well. This method is shown in
figure 1-29.
