Tachogenerator Interface Board

This is a small, inexpensive interface board that fits in the (optional) expansion connector of either Pro-120, VTX or earlier NCC controllers and is used with a tachogenerator to sense true motor speed to use in a closed loop control system.

It was originally developed for Dutch distributor - where it is used in an agricultural seedling planter - the space between the plantlets must be carefully controlled, hence the use of a tachogenerator to accurately control the speed of the machine.

The board should be ideal for most Robot Wars contestants: fitting tachogenerator feedback rectifies most of their known problems, including motor matching (inability to steer straight) and a tendency to bounce when load changes suddenly. However - the system will require setting up as a system, so does require some understanding.

The board available is physically matched to the Pro and NCC series controllers, but it is also a useful addition to other controllers.

Two versions of the board exist: one has a 9 way connector for the Pro series, the other has a 6 way, for the NCC series controllers. Note that the NCC tacho board will work electically with the VTX but is too large to fit into the boxed VTX.

DESCRIPTION:

Model AT-4 V is combination of Tachogenerator and indicator in one piece for direct mounting on machines. The indicator is mounted on the tachogenerator through a suitable rubber mount so that machine vibration are not transmitted to the movement of the indicator. Tachogenerator front part can be made specially to suit replacement of imported speed indicators.

APPLICATIONS:

· Direct mounting type
speed indicator.

· Local Indicator.

On the JT8D engine the inlet gearbox drives the N1 tachogenerator.

AC Drives

Transdrive are specialists in Variable Speed Drives (often known as AC Drives or Inverters) they are most often used to control standard ‘squirrel cage’ motors which are widely used as the workhorse of industry.

Standard inverters (using simple open loop control) are used in the majority of VSD applications such as pumps, fans and mechanical handling devices.

High performance drives with superior speed holding and dynamic response (often called "Flux Vector" drives) are available for more demanding applications such as Machine Tool Applications, chemical and plastic processing, web handling and hoisting where torque control dynamics and speed holding are more critical.

HITACHI AC drive

SELSYN AND SYNCHRO DEVICES
A device called a Selsyn was developed about 1925.
This comprised of a system whereby a generator and a motor so connected by wire that angular rotation or position in the generator is reproduced simultaneously in the motor.
The generator and receiver are also called, a transmitter and receiver.
About 1942 or 43 the term, synchro, became the general term, replacing the word, selsyn.
Synchros are special a-c motors employed for transmitting angular position and rotation over a distance to far for mechanical shafting to do the job.
Of course in the USN synchros are usually thought of as the devices that transmit information about gun fire control systems, in the civilian world they do a host of other things.
1. Remote indication of position, Such as indicating the position of generator rheostats, water-wheel governors, water-reservoir levels gates or valves, and turntables.
2. Remote signaling systems, such as signaling from switchboard to generator room, steel-mill furnace to blower room, and marine signals between engine room and bridge.
3. Automatic or remote position control, such as system frequency, synchronizing of incoming generators, water-wheel governors, gates or valves, color screens on lights in theaters, and motor drives.
4. Operation of two machines so as to maintain a definite time-position relation or a definite speed relation, such as lift bridges, hoists, kiln drives, elevators, unit printing presses, and conveyors.
Synchros are known by various trade names such as Selsyn, Synchrotie, Autosyn, and Telegon. Units are available in single-phase and three-phase types. The poly-phase units conform in appearance and general characteristics to a three-phase wound-rotor induction motor. The single-phase units have a three-phase wound secondary and a single-phase primary. Some single-phase units are constructed with the primary located on the stationary member of the machine and others with the primary on the movable member. Depending upon the torque developed by the machine, synchros are classified as indicating synchros or power synchros.
Power synchros are always of three-phase construction. Most of the indicating synchros are constructed single-phase. The indicating synchros are manufactured in a general-purpose type and a high-accuracy type. The general-purpose type will indicate angular position within an accuracy of plus or minus 5 deg, and the high-accuracy type provides an accuracy of indication within plus or minus 1 degree.
A synchro system requires the use of at least two synchro machines. The connections for two single-phase units are, shown in Fig. 1 and those for two three-phase units in Fig. 2. With both the single-phase and the poly-phase units, if the rotors of the two machines are in corresponding positions, the voltages of the two secondaries at each instant of time will be equal in magnitude and opposite in direction. Therefore, no current will flow in the rotor circuits, no torque will be developed in either machine, and the machines will be in equilibrium. When the two rotors are not in corresponding positions, the voltages of the two secondaries will not neutralize each other and a current will be produced in the two secondaries. This current will produce torque in the machines, which will act upon the rotors, tending to move them into corresponding positions.
In signaling or indicating applications, the synchro located at the sending end is called the transmitter and the synchro at the receiving end is called the receiver. If desired, a number of receivers can be connected in parallel to a single transmitter. The torque excited upon each receiver will be reduced from that available when only one receiver is used.
When it is desired to have the position taken by the receiver differ from that of the transmitter, a three-phase synchro is introduced between the transmitter and the receiver. This intermediate synchro is called a differential synchro. With such a system, the receiver will take up a position that will be either the sum or the difference of the angles applied to the transmitter and the differential. Conversely, if two synchros are connected through a differential synchro and each is turned through an angle, the differential synchro will indicate the difference between the two angles, shown in Figure 4.
The fundamental synchro circuit generally used for control purposes is shown in Fig.3. When the positions of the rotors coincide, a maximum voltage will be induced in stator No. 2. When the rotors are, displaced 90 electrical degrees from each other, no resultant voltage will be produced between the terminals of rotor No. 2. This is true, since the axis of the winding rotor No 2 will be located 90 electrical degrees from the axis of the, field produced in machine 2. The system is adjusted so that, when the desired condition exists, the rotors will be 90 electrical degrees displaced from each other. This is the equilibrium position. The circuit therefore functions to produce an error or voltage in the stator of machine 2 whenever there is a displacement of the shafts from the equilibrium position. The relative polarity of the error voltage depends upon the direction of the displacement. The error voltage can be employed to instigate the functioning of other devices, which will operate to correct the condition causing the displacement. These correction devices will function until the synchros are brought into the equilibrium position, when absence of voltage in rotor No. 2 will stop their functioning.
In the military all windings of the rotors and stators are connected and terminated as “Like Wound” devices.
“Like Wound” means that the numbered wire ends are terminated and numbered from the windings so that the coils in all machines are in identical positions.
This is done so that if replacement is necessary there is no need to worry about the wiring being connected wrong or backwards.
The waveform produced by the new machine will be in or of the same polarity and phase angle as the machine being replaced.
Only mechanical re-alignment of the machine is necessary.
FIG. 1 Circuit for two single-phase synchros.
FIG. 2 Circuit for two three-phase synchros.
FIG. 3 Fundamental synchro control circuit.
FIG. 4 Circuit for two single-phase synchros and a differential synchro.
FIG. 5 Schematic of a single-phase transmitter or receiver synchro.
FIG. 6 Schematic of a three-phase transmitter or receiver or a differential synchro.

SELSYN AND SYNCHRO DEVICES ACCURACY
Power synchros are always of three-phase construction.
Most of the indicating synchros are constructed single-phase.
The indicating synchros are and were manufactured in a general-purpose type and a high-accuracy type.
The general-purpose type will indicate angular position within an accuracy of plus or minus 5 deg.
The high-accuracy type provides an accuracy of indication within plus or minus 1 degree.
To improve this accuracy two synchros are geared together.
You then have a coarse synchro, which measures the entire 360 degree circle it is measuring.
The second synchro, that is geared to the coarse synchro, is the fine synchro.
The normal gearing between the synchros is a ratio of 36 to 1.
So the fine synchro will rotate 36 times for each revolution of the coarse synchro.
This means that one revolution of the fine synchro is equal to 10 degrees.
The accuracy is then 36 times more accurate than using one coarse synchro alone.
In a few cases the gearing between the synchros is a ratio of 72 to 1.
So the fine synchro will rotate 72 times for each revolution of the coarse synchro.
This means that one revolution of the fine synchro is equal to 5 degrees.
The accuracy is then 72 times more accurate than using one coarse synchro alone.
72 to 1 gear ratio.
A fine synchro at this ratio will make one complete revolution every 5 degrees of the coarse synchro for a ratio of 5/72.
Divide 72 into 5 equals .0694444 possible degree of error in this system.
36 to 1 gear ratio.
A fine synchro at this ratio will make one complete revolution every 10 degrees of the coarse synchro for a ratio of 10/36.
Divide 36 into 10 equals .2777777 possible degree of error in this system.
Which of course is just over 1/4 of one degree.

SELSYN (SYNCHRO) MOTOR

Normally, the rotor windings of a wound rotor induction motor are shorted out after starting. During starting, resistance may be placed in series with the rotor windings to limit starting current. If these windings are connected to a common starting resistance, the two rotors will remain synchronized during starting. (Figure below) This is usefull for printing presses and draw bridges, where two motors need to be synchronized during starting. Once started, and the rotors are shorted, the synchronizing torque is absent. The higher the resistance during starting, the higher the synchronizing torque for a pair of motors. If the starting resistors are removed, but the rotors still paralleled, there is no starting torque. However there is a substantial synchronizing torque. This is a selsyn, which is an abbreviation for “self synchronous”.

Starting wound rotor induction motors from common resistors.

The rotors may be stationary. If one rotor is moved through an angle θ, the other selsyn shaft will move through an angle θ. If drag is applied to one selsyn, this will be felt when attempting to rotate the other shaft. While multi-horsepower (multi-kilowatt) selsyns exist, the main appplication is small units of a few watts for instrumentation applications-- remote position indication.

Selsyns without starting resistance.

Normally, the rotor windings of a wound rotor induction motor are shorted out after starting. During starting, resistance may be placed in series with the rotor windings to limit starting current. If these windings are connected to a common starting resistance, the two rotors will remain synchronized during starting. (Figure below) This is usefull for printing presses and draw bridges, where two motors need to be synchronized during starting. Once started, and the rotors are shorted, the synchronizing torque is absent. The higher the resistance during starting, the higher the synchronizing torque for a pair of motors. If the starting resistors are removed, but the rotors still paralleled, there is no starting torque. However there is a substantial synchronizing torque. This is a selsyn, which is an abbreviation for “self synchronous”.

Starting wound rotor induction motors from common resistors.

The rotors may be stationary. If one rotor is moved through an angle θ, the other selsyn shaft will move through an angle θ. If drag is applied to one selsyn, this will be felt when attempting to rotate the other shaft. While multi-horsepower (multi-kilowatt) selsyns exist, the main appplication is small units of a few watts for instrumentation applications-- remote position indication.

Selsyns without starting resistance.

Instrumentation selsyns have no use for starting resistors. (Figure above) They are not intended to be self rotating. Since the rotors are not shorted out nor resistor loaded, no starting torque is developed. However, manual rotation of one shaft will produce an unbalance in the rotor currents until the parallel unit's shaft follows. Note that a common source of three phase power is applied to both stators. Though we show three phase rotors above, a single phase powered rotor is sufficient as shown in Figure below.

Transmitter - receiver

Small instrumentation selsyns, also known as sychros, use single phase paralleled, AC energized rotors, retaining the 3-phase paralleled stators, which are not externally energized. (Figure below) Synchros function as rotary transformers. If the rotors of both the torque transmitter (TX) andtorque receiver (RX) are at the same angle, the phases of the induced stator voltages will be identical for both, and no current will flow. Should one rotor be displaced from the other, the stator phase voltages will differ between transmitter and receiver. Stator current will flow developing torque. The receiver shaft is electrically slaved to the transmitter shaft. Either the transmitter or receiver shaft may be rotated to turn the opposite unit.

Synchros have single phase powered rotors.

Synchro stators are wound with 3-phase windings brought out to external terminals. The single rotor winding of a torque transmitter or receiver is brough out by brushed slip rings. Synchro transmitters and receivers are electrically identical. However, a synchro receiver has inertial damping built in. A synchro torque transmitter may be substituted for a torque receiver.

Remote position sensing is the main synchro application. (Figure below) For example, a synchro transmitter coupled to a radar antenna indicates antenna position on an indicator in a control room. A synchro transmitter coupled to a weather vane indicates wind direction at a remote console. Synchros are available for use with 240 Vac 50 Hz, 115 Vac 60 Hz, 115 Vac 400 Hz, and 26 Vac 400 Hz power.

Synchro application: remote position indication.