STATOR WINDINGS SHELL LOWER END CAP SHAFT BEARING COILS CORE LEADS TO SLIP RINGS SLIP RINGS STATOR
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1.9 CONTROL SYNCHRO
The basic control synchro system has two units; a synchro control transmitter (CX) and a synchro control transformer (CT) connected as shown in Figure
11.
Control Synchro Figure 11
1.9.1 PRINCIPLE OF OPERATION
The CX synchro is similar to that used in the torque synchro system. The control transformer has a stator, which in design and appearance resemble the synchro units already discussed but with high impedance coils to limit the alternating currents through the coils. Further differences in the CT are that the rotor winding has its coils wound so that no torque is produced between it and the stator magnetic fields and the rotor is not energized by the supply voltage applied to the rotor of the control synchro.
The CT rotor acts as an inductive winding for determining the phase and magnitude of error signal voltages. The signals, after amplification, are fed to a two-phase motor, which is mechanically coupled to the CT rotor. A control synchro system is at electrical zero when the rotor of the CT is at 90° with respect to the CX rotor. This is the situation as shown in Figure 10 above.
INPUT SHAFT S1 S1 S3 S2 S3 S2 A.C. SUPPLY
M
SERVO MOTOR A.C. SUPPLY CX CTELECTRONIC FUNDAMENTALS SERVOMECHANISM PAGE 13 of 23
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If the input shaft is rotated and the CX rotor is disturbed, voltages are induced in the CX stator and currents flow down the transmission lines to the stator windings S1, S2 and S3 of the CT. A magnetic flux is produced, depending on
the amount of displacement of the CX rotor and the orientation of its
displacement. This flux links with the rotor of CT, inducing a voltage into it, again depending on the amount, or rate of displacement, and its orientation. The voltage, or error voltage, representing the electrical difference between the rotors of CX and CT, is then amplified and passed to the control phase of a two-phase motor. The ac reference phase supply is fixed. The motor now rotates.
Its direction depends on the phase of the error signal, as can be seen from Figure 12.
Phase Error Signal Figure 12
As it rotates, the motor drives the rotor of CT in such a direction as to reduce the error voltage to zero and the new position is reached. By using the error signal amplified by a servo amplifier, a servomotor can be driven to move a control surface as in Figure 11.
APPLIED VOLTAGE
CLOCKWISE ROTATION VOLTAGE IN-PHASE
ANTI-CLOCKWISE ROTATION VOLTAGE OUT-OF-PHASE
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1.10 DIFFERENTIAL SYNCHRO
There are two types of differential synchro system: ♦ Torque.
♦ Control.
In each, a special type of synchro is inserted between the synchros of the basic torque or control systems. It is called a ‘differential synchro’ and differs from the basic synchros in that it has a three-phase stator and rotor. In a torque differential system it is abbreviated to TDX and in a control differential system, CDX. The inclusion of this synchro between a torque transmitter and receiver or control transmitter and transformer permits an additional input to be algebraically added to, or subtracted from, the system. The layout of a differential synchro and its circuit symbol are shown at Figure 13.
Differential Synchro Figure 13 S2 S1 S3 R2 R1 R3 ROTOR STATOR CIRCUIT SYMBOL S1 S2 S3 R1 R2 R3
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Figure 14 shows the construction of a differential synchro
Differential Synchro Construction Figure 14 STATOR ASSEMBLY ROTOR ASSEMBLY SKEW CUT TO ENABLE SMOOTHER RUNNING STATOR CONNECTIONS STATOR WINDINGS ROTOR COILS
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1.11 TORQUE DIFFERENTIAL SYNCHRO
Figure 15 shows a differential synchro system set up for the SUBTRACTION of two inputs.
Torque Differential Synchro Figure 15
Note that the rotors of the normal transmitter TX and receiver TR are supplied in parallel with the single-phase ac supply. The stator windings of the TX are connected to the stator windings of the TDX and its three rotor windings are connected to the three-stator windings of the TR. The rotor of the TDX is not energized by the ac supply.
The circuit is such that one input shaft turns the TX rotor and the second input shaft drives the TDX rotor. The TDX receives an electrical signal
corresponding to a particular angular position of the TX rotor, which it modifies by an amount corresponding to the angular position of its own rotor. This modified signal appears at the TDX output and is transmitted to the receiver, where it produces an angular flux, which is the difference of the rotor angles of the two transmitters TX and TDX.
If the TDX rotor is locked in one position, the TX/TR chain acts as a normal torque synchro system with a transformer placed between TX and TR.
INPUT
SHAFT 60º SHAFT 15ºINPUT OUTPUTSHAFT
θ1 – θ2 TX TDX TR 60º 45º 15º 60º 45º
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1.12 CONTROL DIFFERENTIAL SYNCHRO
Figure 16 illustrates a control differential synchro system.
Control Differential Synchro Figure 16
As with the straight control synchro system, the ac supply is only applied to the transmitter rotor. The transformer rotor produces an error signal, which after amplification is applied to a motor, causing the CT rotor to move. Apart from these differences the action of the control differential transmitter is the same as for the torque differential synchro system.
Torque differential synchros have been used to combine a direction finding loop reading and a compass reading, in navigation systems, to give a true bearing.
Control differential synchros, combined with servomotors, are used for moving much heavier loads such as radar scanners where the subtraction or addition of two inputs may be necessary.
INPUT
SHAFTθ1 SHAFTINPUTθ
2 OUTPUT SHAFT θ1 – θ2 CX CDX CT ERROR SIGNAL
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1.13 RESOLVER SYNCHRO
This type of synchro is used to convert voltages, which represent the
CARTESIAN co-ordinates of a point, into POLAR co-ordinates and vice versa.
1.13.1 POLAR AND CARTESIAN CO-ORDINATES
A vector, representing an alternating voltage, can be defined in terms of ‘r’ and the angle it makes with the X-axis: angle (θ). These are the polar co-ordinates of the vector written as r/θ. Figure 17 shows the vector diagram for Polar and Cartesian co-ordinates.
Polar & Cartesian Co-ordinates Figure 17
θ
r
X
Y
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1.13.2 RESOLVER SYNCHRO OPERATION
The resolver synchro consists of a stator and rotor, each having two windings arranged in phase quadrature as shown in Figure 18.
Resolver Synchro Figure 18
Figure 16b represents the resolver differently for ease of explanation. The resolver has two coils, R1 R2 and R3 R4 at right angles to each other and
attached to an input shaft. The stator consists of two coils S1 S2 and S3 S4,
also placed at right angles to each other.
INPUT SHAFT S2 S1 S3 S4 R2 R1 R4 R3 ROTOR STATOR R1 R2 R3 R4 S1 S4 S3 S2
a
b
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1.13.3 CONVERSION FROM POLAR TO CARTESIAN CO-ORDINATES
For this purpose, one of the resolver coils is short-circuited, say R3 R4, and
the other, R1 R2, has an alternating voltage applied to it. The magnitude of
this voltage (r) and the angle (θ) through which both rotor coils are turned, represent the polar co-ordinates r/θ. Figure 19 shows a resolver synchro to carry out this function.
Polar to Cartesian Co-ordinates Figure 19
Consider firstly that the rotor shaft position is such that the R1 R2 coil magnetic
field links completely with the stator winding S1 S2, i.e. the coils are aligned.
The maximum voltage will therefore be induced in coil S1 S2. Since the stator
coil S3 S4 is at right angle to stator coil S1 S2, there will be no voltage
developed across it due to R1 R2 coil's magnetic field. When the shaft is
rotated at constant speed through 90°, the rotor coil R1 R2 is now in phase
quadrature to stator S1 S2, which has zero volts induced in it. However, R1 R2
rotor coil is now aligned with stator coil S3 S4 and this now has maximum
voltage induced in it. As the shaft continues to rotate, a cosine voltage wave