RADAR CIRCUIT ANALYSIS 13-14 THE DIFFERENTIAL SElSYN In a mechanical system, a differential con- 52 nects three shafts together in such a way that the amount that one shaft turns is equal to the dif- zsv ference between the amounts that the other two turn. The differential selsyn is named according to the two functions it performs. If the shaft of the differential selsyn serves to indicate the differ- ence in shafts positions of two other selsyns, the other two selsyns are generators and the differ- ential selsyn is a motor. If the differential selsyn shaft position is to be subtracted from that of a 53 selsyn generator (or vice versa) and the differ- ence to be indicated by the rotor position of a i----OV---o-i motor, the differential selsyn is then a generator. ~---OV--~~_-4 ~ The stator of the differential generator or motor is very similar to the stator of the ordi- nary selsyn. It consists of three sets of coils 0 wound in slots and spaced 120 apart around the Differential Transformer Action at 0° Position inside of the field structure. The rotor, however, differs considerably from that of an ordinary The selsyn generator just above is connected selsyn. It is cylindrical in shape and has three sets to a differential selsyn in which the rotor leads of coils wound in slots and equally spaced around are open. Both shafts are in the 0° position. Since the circumference. Connections to the external the stator coils of the differential selsyn connect circuits are made through three brushes riding on to the stator coils of the generator, the voltages three slip rings on the rotor shaft. AE in the are equal in magnitude and are in phase. Since ordinary selsyn, the motor has an oscillation the differential is a 1:1 transformer, the induced damper and the generator does not. There is no voltages are equal in magnitude and are in phase connection to the llOv AC as in the case of the with the stator voltages. When the generator ordinary selsyn. The three leads to the rotor are shaft is turned to some other position, the volt- designated Rl, R2, and R3. Electrical zero is the ages in the stator coils of the differential and, position in which rotor coil R2 is aligned with consequently, those induced in the rotor coils stator coil 82, R1 with 81, and R3 with 83. will be the same as those in the stator coils of the Electrically, the differential selsyn acts as a 1:1 generator. Thus the rotor voltages of the differ-- transformer with the stator coilsthe primary and ential are as shown in the rotor position diagram the rotor coils the secondary. Due to the air gap, on page 13-10, as the generator shaft is rotated there must be a few more turns on the stator with the differential shaft remaining in the 0° coils than on the rotor; hence, the transformation position. A motor with its stator windings con- is not a 1:1 ratio when the rotor coils are con- nected to the rotor windings of the differential nected as the primary. will have the correct voltages applied causing it BALL 8EA~lNG Rofor of Differenfial Selsyn RADAR CIRCUIT ANALYSIS 13·15 generator rotor is in the 345° position, the field axis is 15° clockwise from the axis of the S2 52 secondary coil, the voltage induced in it equals 52 cos (-15°). Likewise the voltages induced in Sl and S3 equal 52 cos (-255°) and 52 cos (-135°). These voltages set up a field in the motor which equals the vector sum of the three fields of the stator coils, resulting in a field with an axis at the 345° position. The rotor of a motor accordingly moves to the 345° position. Now in the differential selsyn, the composite primary field has reached the 0° position while the secondary (rotor) coils have rotated 15°. This 53 51 makes the field axis 15° clockwise from the axis of the R2 coil. The induced voltages then have the values previously stated and shown in the OV ----4'"1 transformer action illustration to the left. If the motor had its stator coils connected to the terminals R1, R2, R3 of the differential, the Transformer Action at 15° Position voltages applied to it would cause the rotor of the motor to move to the 345° position. to assume the same position as the generator Conversely, if the rotor of the differential were rotor. This means, that with the differential turned to the 345° position, the motor would rotor in the 0° position the system functions as turn to the 15° position. If the generator shaft it would if the differential were out of the circuit position is at 0°, the voltages induced in the and connections were directly from generator to rotor of the differential selsyn are the same motor. as if the rotor of the differential were at 0° When the generator rotor is in the 0° position, and the rotor of the generator were rotated in and the rotor of the differential is turned to 15° the opposite direction. as shown directly above, the voltages induced Thus, whenever the shafts of both generator in the rotor coilswill be as indicated. The voltages and differential rotate, the resultant field and between terminals R1-R2, R2-R3 and R3-R1 are induced voltages vary with the difference of the respectively 63v, 87v, and 24v and the R2-R3 angular position of the shafts. A motor connected voltage is in phase with the R1-R2 voltage across to the rotor coils of the differential will assume a the rotor of the generator. The other voltages are shaft position equal to the difference of the angu- 180° out of phase. These voltages correspond to lar positions of the other two shafts. The mathe- the voltages induced in the stator windings of a matical proof, a quite long one, is beyond the generator in which the rotor is at the 345° posi- scope of this manual. Briefly, it involves setting tion. To understand this action, consider the up equations for the voltages induced in each relative angle of the inducing magnetic field axis of the rotor coils of the differential by each of and the axis of the secondary coils. When a the sta tor coils and adding the induced voltages DIFFERENTIAL GENERATOR 60· GENERATOR 180· MOTOR 1200 Selsyn System with Differential Generator RAQAR CIRCUIT ANALYSIS 13-16 in pairs. A qualitative explanation of what causes the motor shaft to turn to the indicated position is fairly simple to understand. For example, take the case illustrated at the bottom of page 13-15. The generator rotor is at the 180° position. Hence the voltages induced in 81, 82, 83 are 115V 26v, 52v, 26v, respectively in the phases in- DIRECT CONNECTIONS dicated. All the voltages 81, 82, 83 of the dif- ferential are identical. These voltages set up a magnetic field in the differential which has the same position, 180°, for its axis as the field in the generator. If the axis of the rotor of the differ- ential generator were aligned with the 0° posi- tion, the induced voltages would be identical to il5V the stator voltages. But since the rotor coil axis DIFFERENTIAL REVERSED is shifted to the 60° position, the axis is only 120° from that of the magnetic field. Therefore the voltages induced in R1, R2, R3 are 52 cos 120°, 52 cos 240°, 52 cos 0° volts respectively. The vol.tagesfrom R1 to R2, from R2 to R3 and from R3 to R1 are therefore Ov, 78v and 78v with ___ R~l~rR~' ~R' R, R2-R3 being 180°out of phase with R1-R2 of the 1i5V generator. When these voltages are applied to "A" REVERSED the motor, it will cause its rotor to turn to the position such that the voltages induced in its stator coils by the rotor just balance these voltages. This happens when the rotor is at the 1200 position, the difference between 1800 and 60°. R, R, By reversing pairs of leads either between the 115V generator and the differential or between the "S"R~RSED differential and the motor, you can make any Various Connec.ron for Selsyn System one of the shafts assume a position equal to the sum or difference of the angular positions of the required, the use of a differential is avoided if at other shafts. The proper connections for various all possible. types of operation are shown in the illustration to the right. The Control Transformer A differential selsyn cannot be a 1:1 trans- The uses of selsyn are to cause two shafts to former in both directions. Therefore the perfect rotate in synchronization and to produce an balance of voltages indicated is not strictly error voltage which indicates the difference in accurate. The differential selsyn is designed such position of two shafts. The systems discussed to that the voltages induced in the rotor coils are this point are for synchronization purposes. For equal to the voltages applied to the stator wind- the purpose of producing error voltages, there is ings provided the shaft is at 0°. When a differen- a type of selsyn known as the control transformer tial is connected between a generator and a which is used in conjunction with a generator. motor, some current flows in the generator's Both the generator and the transformer have stator circuit, reducing the output voltages due their shafts -connected to loads.