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 90 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 90 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 90 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 180 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.

Figure 1-29. - The synchro capacitor.

The three delta-connected capacitors, shown in figure 1-29, usually come as a unit mounted in a case with three external connections. The entire unit is called a SYNCHRO CAPACITOR. The synchro capacitor is made in many sizes to meet the requirements of all sizes of standard differentials and control transformers. The synchro capacitor is rated by its total capacity, which is the sum of the individual capacities in the unit.

Figure 1-30 shows how a synchro capacitor affects the operation of a control synchro system. In this figure, the capacitor is placed between the CX and the CT. Two current meters are also placed in the circuit to show the effect the capacitor has on stator current. The meter connected between the capacitor and the CT reads normal stator current, 32 milliamperes (mA). This current would normally flow in the stator of the CX if the synchro capacitor were not connected. The other meter reads 10 mA, which is what is left of the original stator current after the magnetizing current has been canceled by the synchro capacitor. By reducing the current drain on the transmitter, the sensitivity and accuracy of the system increase.

Figure 1-30. - The use of a synchro capacitor with a CT.

Figure 1-31 shows another application of a synchro capacitor; this time in a differential system. in this circuit the capacitor is placed between a TX and a TDX. The meter readings show the same comparison between currents as in the previous paragraph. The only significant difference between this circuit and the one in figure 1-30 is that the differential draws more stator current than the CT.

Figure 1-31. - The use of a synchro capacitor with a TDX.

Some synchro systems contain a differential and a control transformer, as illustrated in figure 1-32. In this figure, there are large stator currents flowing in the CX, since it supplies all the losses as well as the magnetizing current for both synchros. Two meters are placed in the circuit to show the value of stator current for the CDX and CT. Another meter is placed in series with the ac excitation voltage to show the amount of current being drawn from the ac line is 0.9 ampere.

Figure 1-32. - Synchro current in a control synchro system using a CDX and a CT.

Adding synchro capacitors to this system, as shown in figure 1-33, greatly reduces the stator currents and improves the efficiency of the system. Also, notice that the line current is reduced from 0.9 ampere in figure 1-32 to 0.65 ampere in figure 1-33.

Figure 1-33. - The effects of synchro capacitors in a control synchro system using a CDX and a CT.

When a synchro capacitor is used, it is always placed physically close to the differential or control transformer whose current it corrects. This is done to keep the connections as short as possible, because high currents in long leads increase the transmitter load and reduce the accuracy of the system.

We must stress that the synchro capacitor should never be used in a simple transmitter-receiver system. This is because stator currents in this system are zero at correspondence and the addition of a synchro capacitor would only increase the stator current and throw the system out of balance.

Q.48 What is the purpose of the synchro capacitor?
Q.49 What type of synchros usually require the use of synchro capacitors?
Q.50 What type of current is eliminated by synchro capacitors?
Q.51 How are synchro capacitors connected in a circuit?
Q.52 Why are synchro capacitors placed physically close to differentials transmitters and CTs?