TROUBLESHOOTING SYNCHRO SYSTEMS

One of your duties in the Navy is to keep the synchro systems in your equipment in good working order. Therefore, it is essential that you become familiar with the details of synchro maintenance and repair.

First, let's consider some of the more common problem areas you should avoid when working with synchros. As with any piece of electrical or electronic equipment, if it works - leave it alone. Do not attempt to zero a synchro system that is already zeroed just because you want to practice. More often than not, the system will end up more out of alignment than it was before you attempted to rezero it. Do not attempt to take a synchro apart, even if it is defective. A synchro is a piece of precision equipment that requires special equipment and techniques for disassembly. Disassembly should be done only by qualified technicians in authorized repair shops. A synchro, unlike an electric motor, does not require periodic lubrication. Therefore, never attempt to lubricate a synchro. Synchros also require careful handling. Never force a synchro into place, never use pliers on the threaded shaft, and never force a gear or dial onto the shaft. Finally, never connect equipment that is not related to the synchro system to the primary excitation bus. This will cause the system to show all the symptoms of a shorted rotor when the equipment is turned on; but, the system will check out good when the equipment is off.

Trouble in a synchro system that has been in operation for some time is usually one of two types. First, the interconnecting synchro wiring often passes through a number of switches; at these points opens, shorts, or grounds may occur. You will be expected to trace down these troubles with an ohmmeter. You can find an open easily by checking for continuity between two points. Similarly, you can find a ground by checking the resistance between the suspected point and ground. A reading of zero ohms means that the point in question is grounded. Secondly, the synchro itself may become defective, due to opens and shorts in the windings, bad bearings, worn slip rings, or dirty brushes. You can do nothing about these defects except replace the synchro.

Troubles in new and modified synchro systems are most often because of (1) improper wiring and (2) misalignment caused by synchros not being zeroed. You are responsible for finding and correcting these troubles. You can check for improper wiring with an ohmmeter by making a point-to-point continuity and resistance check. You can correct misalignment of a synchro system by rezeroing the entire system.

TROUBLE INDICATORS

When trouble occurs in an electronic installation that contains a large number of synchro systems, it may be very difficult to isolate the trouble to one particular system. Since it is vital that maintenance personnel locate the point of trouble and fix it in as short a time as possible, indicators, which aid in locating the trouble quickly, are included in the equipment. These indicators are usually signal lights, mounted on a central control board and connected to the different synchro systems. When trouble occurs in a synchro system, the signal light connected to it may either light or flash. Maintenance personnel identify the defective system by reading the name or number adjacent to the light.

Signal lights indicate either overload conditions or blown fuses. Overload indicators are usually placed in the stator circuit of a torque synchro system because the stator circuit gives a better indication of mechanical loading than does the current in the rotor circuit. One version of this type of indicator, as shown in figure 1-44, consists of a neon lamp connected across the stator leads of a synchro system by two transformers. The primaries, consisting of a few turns of heavy wire, are in series with two of the stator leads; the secondaries, consisting of many turns of fine wire, are in series with the lamp. The turns ratios are designed so that when excess current flows through the stator windings, the neon lamp lights. For example, when the difference in rotor positions exceeds about 18, the lamp lights, indicating that the load on the motor shaft is excessive.

Figure 1-44. - Overload stator current indicator.

Blown fuse indicators are front panel lights which light when a protective fuse in series with the rotor blows. Figure 1-45 shows a typical blown fuse indicator. If excessive current flows in the rotor windings of this circuit because of a short or severe mechanical overload, one of the fuses will blow and the neon lamp across the fuse will light.

Figure 1-45. - Simple blown fuse indicator.

Another type of blown fuse indicator uses a small transformer having two identical primaries and a secondary connected, as shown in figure 1-46. With both fuses closed, equal currents flow through the primaries. This induces mutually canceling voltages in the secondary. If a fuse blows, the induced voltage from just one primary is present in the secondary, and the lamp lights.

Figure 1-46. - Blown fuse indicator requiring only one lamp.

SYMPTOMS AND CAUSES

To help the technician further isolate synchro problems, many manufacturers furnish tables of trouble symptoms and probable causes with their equipment. These tables are a valuable aid in isolating trouble areas quickly. Tables 1-2 through 1-7 summarize, for a simple TX-TR system, some typical trouble symptoms and their probable causes. Keep in mind, if two or more receivers are connected to one transmitter, similar symptoms occur. However, if all the receivers act up, the trouble is usually in the transmitter or main bus. If the trouble appears in one receiver only, check the unit and its connections. The angles shown in these tables do not apply to systems using differentials, or to systems whose units are incorrectly zeroed.

Table 1-2. - General Symptoms

Preliminary Actions: Be sure TR is not jammed physically. Turn TX slowly in one direction and observe TR

Table 1-3. - Open or Shorted Rotor

Preliminary Action: Set TX to 0 and turn rotor smoothly counterclockwise.

Table 1-4. - Shorted Stator

Table 1-5. - Open Stator

Table 1-6. - Wrong Stator Connections, Rotor Wiring Correct

Table 1-7. - Wrong Stator and/or Reversed Rotor Connections

In a control system, the trouble may be slightly more difficult to isolate. However, the existence of trouble is readily indicated when the system does not properly respond to an input order. For control systems, it is easier to locate the trouble by using a synchro tester or by checking the operating voltages.

VOLTAGE TESTING

Another good way to isolate the trouble in an operating synchro system is to use known operating voltages as References for faulty operation. Since the proper operation of a system is indicated by specific rotor and stator voltages, an ac voltmeter can be used to locate the trouble. When an ac voltmeter is connected between any two stator leads, the voltage should vary from 0 to 90 volts (0 to 11.8 volts for 26-volt systems) as the transmitter rotates. The zero and maximum voltage values should occur at the following headings:

Meter Connected Between  Zero Voltage Headings Maximum Voltage Headings S1 and S2 120, 300 30, 210 S2 and S3  60, 240 150, 330  S1 and S3 0, 180  90, 270

The rotor voltage should remain constant at all times, either 115 volts or 26 volts. In a system where the units are close enough to permit checking, the voltage between the R1 and R2 terminal of any unit energized by the primary ac source and the corresponding R1 or R2 terminal of any other unit energized by the primary ac source should be zero. When the excitation voltage (115 volts or 26 volts) is above or below the nominal value, the maximum stator voltages will also be above or below normal.

SYNCHRO TESTERS

Synchro testers, as stated earlier, are used primarily for quickly locating a defective synchro. These testers are capable of functioning as either transmitter or receiver.

When a transmitter is suspected of being defective, a synchro tester is usually substituted in its place to simulate its actions. When the tester is used in this manner, a braking arrangement on the tester applies the necessary friction to hold its shaft in different positions so you can determine whether the transmitter is good or bad. When using the tester as a transmitter, it is usually a good idea to use only one receiver so as not to overload the tester. If the tester is connected in place of a TR or used to check the output of a transmitter, the brake is released, allowing the rotor to turn and indicate the transmitter's position. By observing the tester's response to the transmitted signal, you can determine if the TR is defective or if the transmitter's output is incorrect.

Q.71 What should you do with a synchro that has a bad set of bearings?
Q.72 Name two types of trouble you would expect to find in a newly installed synchro system.
Q.73 What type of indicator is usually placed in the stator circuit of a torque synchro system?
Q.74 What is the most probable cause of trouble in a synchro system that has all of its receivers reading incorrectly?
Q.75 If an ac voltmeter is connected between the S2 and S3 windings on a TX, at what two rotor positions should the voltmeter read maximum voltage?
Q.76 What precaution should you take when substituting a synchro tester in a circuit for a transmitter?

You should now have a good working knowledge of synchro systems. For further study and assistance in applying this knowledge to synchro troubleshooting and alignment, consult the following References: