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Control transformer

A variation of the synchro transmitter is the control transformer. It has three equally spaced stator windings like a TX. Its rotor is wound with more turns than a transmitter or receiver to make it more sensitive at detecting a null as it is rotated, typically, by a servo system. The CT (Control Transformer) rotor output is zero when it is oriented at a angle right angle to the stator magnetic field vector. Unlike a TX or RX, the CT neither transmits nor receives torque. It is simply a sensitive angular position detector.

Control transformer (CT) detects servo null.

In Figure above, the shaft of the TX is set to the desired position of the radar antenna. The servo system will cause the servo motor to drive the antenna to the commanded position. The CT compares the commanded to actual position and signals the servo amplifier to drive the motor until that commanded angle is achieved.

Servo uses CT to sense antenna position null

When the control transformer rotor detects a null at 90^o to the axis of the stator field, there is no rotor output. Any rotor displacement produces an AC error voltage proportional to displacement. A servo (Figure above) seeks to minimize the error between a commanded and measured variable due to negative feedback. The control transformer compares the shaft angle to the the stator magnetic field angle, sent by the TX stator. When it measures a minimum, or null, the servo has driven the antenna and control transformer rotor to the commanded position. There is no error between measured and commanded position, no CT, control transformer, output to be amplified. The servo motor, a 2-phase motor, stops rotating. However, any CT detected error drives the amplifier which drives the motor until the error is minimized. This corresponds to the servo system having driven the antenna coupled CT to match the angle commanded by the TX.

The servo motor may drive a reduction gear train and be large compared to the TX and CT synchros. However, the poor efficiency of AC servo motors limits them to smaller loads. They are also difficult to control since they are constant speed devices. However, they can be controlled to some extent by varying the voltage to one phase with line voltage on the other phase. Heavy loads are more efficiently driven by large DC servo motors.

Airborne applications use 400Hz components-- TX, CT, and servo motor. Size and weight of the AC magnetic components is inversely proportional to frequency. Therefore, use of 400 Hz components for aircraft applications, like moving control surfaces, saves size and weight.

Resolver

A resolver (Figure below) has two stator winding placed at 90^o to each other, and a single rotor winding driven by alternating current. A resolver is used for polar to rectangular conversion. An angle input at the rotor shaft produces rectangular co-ordinates sinθ and cosθ proportional voltages on the stator windings.

Resolver converts shaft angle to sine and cosine of angle.

For example, a black-box within a radar encodes the distance to a target as a sine wave proportional voltage V, with the bearing angle as a shaft angle. Convert to X and Y co-ordinates. The sine wave is fed to the rotor of a resolver. The bearing angle shaft is coupled to the resolver shaft. The coordinates (X, Y) are available on the resolver stator coils:

X=V(cos(∠bearing))

Y=V(sin(∠bearing))

The Cartesian coordinates (X, Y) may be plotted on a map display.

A TX (torque transmitter) may be adapted for service as a resolver. (Figure below)

Scott-T converts 3-φ to 2-φ enabling TX to perform resolver function.

It is possible to derive resolver-like quadrature angular components from a synchro transmitter by using a Scott-T transformer. The three TX outputs, 3-phases, are processed by a Scott-T transformer into a pair of quadrature components.

There is also a linear version of the resolver known as an inductosyn. The rotary version of the inductosyn has a finer resolution than a resolver.

Summary: Selsyn (synchro) motors

• A synchro, also known as a selsyn, is a rotary transformer used to transmit shaft torque.
• A TX, torque transmitter, accepts a torque input at its shaft for transmission on three-phase electrical outputs.
• An RX, torque receiver, accepts a three-phase electrical representation of an angular input for conversion to a torque output at its shaft. Thus, TX transmits a torque form an input shaft to a remote RX output shaft.
• A TDX, torque differential transmitter, sums an electrical angle input with a shaft angle input producing an electrical angle output
• A TDR, torque differential receiver, sums two electrical angle inputs producing a shaft angle output
• A CT, control transformer, detects a null when the rotor is positioned at a right angle to the stator angle input. A CT is typically a component of a servo-- feedback system.
• A Resolver outputs a quadrature sinθ and cosine(theta) representation of the shaft angle input instead of a three-phase output.
• The three-phase output of a TX is converted to a resolver style output by a Scott-T transformer.