  # Introduction to Test EquipmentNavy Electricity and Electronics Training Series (NEETS)Chapter 1:  Pages 41-1 through 1-50

NEETS   Module 15 - Principles of Synchros, Servos, and Gyros

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

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.

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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?

MULTISPEED SYNCHRO SYSTEMS

The data to be transmitted is another important factor that we must consider when we discuss the accuracy of a synchro system. If this data covers a wide range of values, the basic synchro system is unable to detect any small changes in the data. When this happens, the accuracy of the system decreases. Because of this difficulty, multispeed synchro systems were developed. They handle this type of data very effectively and still maintain a high degree of accuracy.

Multispeed synchro systems use more than one speed of data transmission. The speed of data transmission is, simply, the number of times a synchro transmitter rotor must turn to transmit a full range of values. For example, a system in which the rotors of synchro devices turn in unison with their input and output shafts is commonly called a 1-speed data transmission system. In this system, the transmitter's rotor is geared so that 1 revolution of the rotor corresponds to 1 revolution of the input. Until now, the discussion of synchro systems has dealt exclusively with this 1-speed system.

In a 36-speed data transmission system, the rotor of the synchro transmitter is geared to turn through 36 revolutions for 1 revolution of its input. Units transmitting data at one speed are frequently called 1- speed synchros; a unit transmitting data at 36-speed would be a 36-speed synchro, and so forth.

It is quite common in synchro systems to transmit the same data at two different speeds. For example, ship's course information is usually transmitted to other locations on a ship at 1-speed and 36- speed. A system in which data is transmitted at two different speeds is called a dual- or double-speed system. Sometimes a dual-speed system will be referred to by the speeds involved, for example a 1- and 36-speed system.

In summary, the speed of data transmission is referred to as 1-speed, 2-speed, 36-speed, or some other definite numerical ratio. To indicate the number of different speeds at which data is transmitted, refer to the system as being a single-speed, dual-speed, or tri-speed synchro system.

SINGLE-SPEED SYNCHRO SYSTEM

If the data to be transmitted covers only a small range of values, a single-speed system is normally accurate enough. However, in applications where the data covers a wide range of values and the accuracy of the system is most important, the 1-speed system is not adequate enough and must be replaced by a

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more suitable system. Increasing the speed of a single-speed system from 1-speed to 36-speed provides greater accuracy, but the self-synchronous feature of the 1-speed system is lost. If primary power is interrupted in a 36-speed system and the transmitter is turned before power is reapplied, the synchros could realign themselves in an erroneous position. The number of positions in which the transmitter and receiver rotors can correspond is the same as the transmission speed. Thus, in the 36-speed system, there are 35 incorrect positions and only 1 correct position of correspondence.

For accurate transmission of data over a wide range of values without the loss of self-synchronous operation, multispeed synchro systems must be used. Multispeed synchro systems use more than one speed of data transmission and, therefore, require more than one output shaft.

DUAL-SPEED SYNCHRO SYSTEM

A basic dual-speed synchro system consists of two transmitters and two receivers, as shown in figure 1-34. One transmitter receives the external input to the system and, through a network of gears, passes the effects of the external input to the second transmitter. The gear ratio between these two transmitters determines the two specific speeds the system will use to transmit the input data. The two speeds of this system are often referred to as fast and slow, high and low, or more often as fine and coarse. Figure 1-34. - Dual-speed synchro system.

If, for example, the gear ratio between the two transmitters is 36 to 1, 1 revolution of the rotor of the first transmitter causes 36 revolutions of the rotor of the second transmitter. The first transmitter-the one that accepts the external input-can be called the coarse transmitter, and the second one can be called the fine transmitter. Representative speeds include 1 and 36, 2 and 36, and 2 and 72.

The output of each transmitter is passed through standard synchro connections to a receiver. One receiver receives the coarse signal and the other one receives the fine signal. The two receivers may or may not be connected by a network of gears similar to the network between the two transmitters. In some dual-speed applications, a double receiver is used instead of two individual receivers.

The double receiver (fig. 1-35) consists of a coarse and a fine receiver enclosed in a common housing. It has a two-shaft output one inside the other. The coarse and fine receivers may be likened to the hour and minute hands of a clock. The coarse receiver corresponds to the hour hand, and the fine

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receiver corresponds to the minute hand. This double receiver has the advantage of requiring less space than two single receivers. However, it also has a disadvantage - when one receiver goes bad, both must be replaced. Figure 1-35. - Cutaway view of a double receiver.

In the dual-speed synchro system, data is transmitted by the coarse transmitter, while the system is far out of correspondence and then is shifted to the fine transmitter as the system approaches correspondence. This shifting from coarse to fine control is done automatically when the fine error signal exceeds the coarse error signal. The fine synchro transmitter then drives the system to the point of correspondence.

When the dual-speed synchro system includes control transformers, there is always the possibility of a 180º error being present in the system. The reason is the rotor of the CT is not energized and its error- voltage output is zero both at its proper position and also at a point 180º away from that position. To prevent the CT from locking 180º out of phase with the rest of the system, a low voltage is applied across the rotor terminals of the coarse CT as shown in figure 1-36.

1-44 Figure 1-36. - Dual-speed synchro system using a stickoff transformer.

This voltage is normally about 2.5 volts and is commonly termed "stickoff" voltage. It is obtained from the secondary of a small transformer. The voltage induced in the secondary of the transformer shifts the 0º position of the coarse CT To reestablish a new 0º position, the stator of the coarse CT must be turned through an angle that induces an opposing 2.5 volts in the rotor to cancel the stickoff voltage. Therefore, at 0º the two voltages cancel and no input exists to drive the servo amplifier. Should the rotor of the CT stop at 180º, the same 2.5 volts would be induced in the rotor. However, it would be in phase with the stickoff voltage and no cancellation would occur. The end result is an error signal at 180º that drives the dual-speed synchro system out of any false synchronizations.

TRI-SPEED SYNCHRO SYSTEM

The advent of long-range missiles and high-speed aircraft has brought about the need for accurately transmitting very large quantities. This is best done by a tri-speed synchro system, which transmits data at three different speeds. These speeds are sometimes referred to as coarse, medium or intermediate, and fine. A typical weapons systems, for example, might transmit range in miles, thousands of yards, and hundreds of yards. By providing this range in three different scales, greater accuracy is obtained than would be possible with a dual-speed system. Representative speeds for tri-speed systems include 1, 36, and 180; 1, 36, and 360; and 1, 18, and 648.

Q-53.   What is the name given to the synchro system that transmits data at two different speeds?

Q-54.   What is the main reason for using a multispeed synchro system instead of a single-speed synchro system?

Q-55.   In a dual-speed synchro system what determines the two specific speeds at which the data will be transmitted?

Q-56.   What type of synchro system is used to transmit very large quantities?

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Q-57.   What is the disadvantage of using a double receiver instead of two individual receivers? Q-58.   What is the purpose of "stickoff voltage"?

ZEROING SYNCHROS

If synchros are to work properly in a system, they must be connected and aligned correctly with respect to each other and to the other devices with which they are used. The reference point for alignment of all synchro units is ELECTRICAL ZERO. The mechanical reference point for the units connected to the synchros depends upon the particular application of the synchro system. Whatever the application, the electrical and mechanical reference points must be aligned with each other. The mechanical position is usually set first, and then the synchro device is aligned to electrical zero.

There are various methods for zeroing synchros. Some of the more common zeroing methods are the voltmeter, the electrical-lock, and the synchro-tester methods. The method used depends upon the facilities and tools available and how the synchros are connected in the system. Also, the method for zeroing a unit whose rotor or stator is not free to turn may differ from the procedure for zeroing a similar unit whose rotor or stator is free to turn.

VOLTMETER METHOD

The most accurate method of zeroing a synchro is the ac voltmeter method. The procedure and the test circuit configuration for this method vary somewhat, depending upon which type of synchro is to be zeroed. Transmitters and receivers, differentials, and control transformers each require different test circuit configurations.

Regardless of the synchro to be zeroed, there are two major steps in each procedure. The first step is the coarse or approximate setting. The second step is the fine setting. The coarse setting ensures the device is zeroed on the 0º position rather than the 180º position. Many synchro units are marked in such a manner that the coarse setting may be approximated physically by aligning two marks on the synchro. On standard synchros, this setting is indicated by an arrow stamped on the frame and a line marked on the shaft, as shown in figure 1-37. The fine setting is where the synchro is precisely set on 0º.

1-46 Figure 1-37. - Coarse electrical zero markings.

For the ac voltmeter method to be as accurate as possible, an electronic or precision voltmeter having 0- to 250-volt and a 0- to 5-volt ranges should be used. On the low scale this meter should also be able to measure voltages as low as 0.1 volt.

Q-59.   What is the reference point for alignment of all synchro units?

Q-60.   What is the most accurate method of zeroing a synchro?

Q-61.   What is the purpose of the coarse setting of a synchro?

Zeroing Transmitters and Receivers (Voltmeter Method)

Since the TX, CX, and TR are functionally and physically similar, they can be zeroed in the same manner. For the TX and CX to be properly zeroed, electrical zero voltages (S2 = 52V; S1 and S3 = 26V) must exist across the stator winding when the rotor of the transmitter is set to 0º or its mechanical reference position. The synchro receiver (TR) is properly zeroed when the device it actuates assumes its zero or mechanical reference position while electrical zero voltages (S2 = 52V; S1 and S3 = 26V) exist across its stator windings. The following is a step-by-step procedure used to zero the TX, CX, and TR.

1.  Carefully set the unit (antenna, gun mount, director, etc.) whose position the CX or TX transmits, accurately on 0º or on its reference position. In the case of the TR, deenergize the circuit and disconnect the stator leads before setting its rotor on zero or to its reference position. The rotor may need to be secured in this position; taping the dial to the frame is usually sufficient.

2.  Deenergize the synchro circuit and disconnect the stator leads. NOTE: Many synchro systems are energized by individual switches. Therefore, be sure that the synchro power is off before working on the connections.Set the voltmeter to its 0- to 250-volt scale and connect it into the circuit as shown in view A of figure 1-38.

1-47 Figure 1-38A. - Zeroing a transmitter or receiver by the voltmeter method.

3.  Energize the synchro circuit and turn the stator until the meter reads about 37 volts (15 volts for a 26-volt synchro). Should the voltmeter read approximately 193 volts (115 volts + 78 volts = 193 volts), the rotor is at 180º. Turn it through a half revolution to bring it back to 0º. If you cannot obtain the desired 37 (or 15) volts, use the lowest reading you can take with the meter. This is the coarse setting and places the synchro approximately on electrical zero.

4.  Deenergize the synchro circuit and connect the meter as shown in view B. Start with a high scale on the meter and work down to the 0- to 5-volt scale to protect the meter movement. Figure 1-38B. - Zeroing a transmitter or receiver by the voltmeter method.

5.  Reenergize the synchro circuit and adjust the stator for a zero or minimum voltage reading. This is the fine electrical zero position of the synchro.

When you have reconnected a TX and a TR into a system after zeroing them, you can perform a simple check on the system to see if they are accurately on electrical zero. First, place the transmitter on 0º. When the system reaches the point of correspondence, the induced voltages in the S1 and S3 stator
windings of both synchros should be equal. Since the terminals of S1 and S3 are at equal potential, a

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jumper between these terminals at the TR should not affect the circuit. If, however, the TR rotor moves when you connect a jumper, there is a slight voltage difference between S1 and S3. This voltage difference indicates that the transmitter is not on electrical zero. If this is the case, recheck the transmitter for electrical zero.

Zeroing Differential Synchros (Voltmeter Method)

A differential synchro is zeroed when it can be inserted into a system without introducing any change. If a differential synchro requires zeroing, use the following voltmeter method:

1.  Carefully and accurately set the unit whose position the CDX or TDX transmits on zero or on its reference position. In the case of the TDR, deenergize the circuit and disconnect all leads before setting its rotor to 0º or to its reference position. You may need to secure the rotor in this position; taping the dial to the frame is usually sufficient.

2.  Deenergize the circuit and disconnect all leads on the differential except leads S2 and S3 when you use the 78-volt (10.2 volts for 26-volt units) supply from the transmitting unit to zero the differential. Set the voltmeter to its 0- to 250-volt scale and connect it as shown in view A of figure 1-39. If the 78-volts is not available from the transmitter or from an auto transformer, you may use a 115-volt source instead. If you use 115 volts instead of 78 volts, do not leave the synchro connected for more than 2 minutes or it will overheat and may become permanently damaged. Figure 1-39A. - Zeroing differential synchros by the voltmeter method.

1-49 Figure 1-39B. - Zeroing differential synchros by the voltmeter method.

3.   Energize the circuit, unclamp the differential's stator, and turn it until the meter reads minimum. The differential is now approximately on electrical zero. Deenergize the circuit and reconnect it as shown in view B.

4.   Reenergize the circuit. Start with a high scale on the meter and work down to the 0- to 5-volt scale to protect the meter movement. At the same time, turn the differential's stator until you obtain a zero or minimum voltage reading. Clamp the differential stator in position, ensuring the voltage reading does not change. This is the fine electrical zero position of the differential.

Zeroing a Control Transformer (Voltmeter Method)

Two conditions must exist for a control transformer (CT) to be on electrical zero. First, its rotor voltage must be minimum when electrical zero voltages (S2 = 52 volts; S1 and S3 = 26 volts) are applied to its stator. Second, turning the shaft of the CT slightly counterclockwise should produce a voltage across its rotor in phase with the rotor voltage of the CX or TX supplying excitation to its stator. To zero a CT (using 78 volts from its transmitter) by the voltmeter method, use the following procedure:

1.  Set the mechanism that drives the CT rotor to zero or to its reference position. Also, set the transmitter (CX or TX) that is connected to the CT to zero or its reference position.

2.  Check to ensure there is zero volts between S1 and S3 and 78 volts between S2 and S3. If you cannot obtain these voltages, you will need to rezero the transmitter. NOTE: If you cannot use the 78 volts from the transmitter circuit and, an auto transformer is not available, you may use a 115- volt source. If you use 115 volts instead of 78 volts, do not energize the CT for more than 2
minutes because it will overheat and may become permanently damaged.

3.  Deenergize the circuit and connect the circuit as shown in view A of figure 1-40. To obtain the 78 volts required to zero the CT, leave the S1 lead on, disconnect the S3 lead on the CT, and put the S2 lead (from the CX) on S3. This is necessary since 78 volts exists only between S1 and S2 or S2 and S3 on a properly zeroed CX. Now energize the circuit and turn the stator of the CT to obtain a minimum reading on the 250-volt scale. This is the coarse or approximate zero setting of the CT.

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