Special machines:Applications of Synchros

Applications of Synchros

The synchros are extensively used in servomechanism for torque transmission, error detection and for adding and subtracting rotary angles. We will consider these applications one by one.

(a) Torque Transmission

Synchros are used to transmit torque over a long distance without the use of a rigid mechanical connection. Fig. 39.18 represents an arrangement for maintaining alignment of two distantly-located shafts. The arrangement requires a control transmitter (CX) and a control receiver (CR) which acts as a torque receiver. As CX is rotated by an angle a, CR also rotates through the same angle a. As shown, the stator windings of the two synchros are connected together and their rotors are connected to the same single-phase ac supply.

Working. Let us suppose that CX rotor is displaced by an angle a and switch S W1 is closed to energize the rotor winding. The rotor winding flux will induce an unbalanced set of three single-phase voltages (in time phase with the rotor voltage) in the CX stator phase windings which will circulate currents in the CR stator windings. These currents produce the CR stator flux field whose axis is fixed by the angle a. If the CR rotor winding is now energized by closing switch S W2, its flux field will interact with the flux field of the stator winding and thereby produce a torque. This torque will rotate the freely-moving CR rotor to a position which exactly corresponds with the CT rotor i.e. it will be displaced by the same angle a as shown in Fig. 39.18. It should be noted that if the two rotors are in the same relative positions, the stator voltages in the two synchros will be exactly equal and opposite. Hence, there will be no current flow in the two stator windings and so no torque will be produced and the system will achieve equilibrium. If now, the transmitter rotor angle changes to a new value, then new set of voltages would be induced in the transmitter stator windings which will again drive currents through the receiver stator windings. Hence, necessary torque will be produced which will turn the CR rotor through an angle corresponding to that of the CT rotor. That is why the transmitter rotor is called the master and the receiver rotor as the slave, because it follows its master. It is worth noting that this master-slave relationship is reversible because when the receiver rotor is displaced through a certain angle, it causes the transmitter rotor to turn through the same angle.

(b) Error Detection

Synchros are also used for error detection in a servo control system. In this case, a command in the form of a mechanical displacement of the CX rotor is converted to an electrical voltage which appears at the CT rotor winding terminals which can be further amplified by an amplifier.

For this purpose, we require a CX synchro and a CT synchro as shown in Fig. 39.19. Only the CX rotor is energized from the single-phase ac voltage supply which produces an alternating air-gap flux field. This time-varying flux field induces voltages in the stator windings whose values for a = 30º are as indicated in the Fig. 39.19. The CX stator voltages supply magnetizing currents in the CT stator

windings which, in turn, create an alternating flux field in their own air-gap. The values of the CT stator phase currents are such that the air-gap flux produced by them induces voltages that are equal and opposite to those existing in the CX stator. Hence, the direction of the resultant flux produced by the CX stator phase currents is forced to take a position which is exactly identical to that of the rotor axis of the CT.

If the CT rotor is assumed to be held fast in its electrical zero position as shown in Fig.39.19, then the rms voltage induced in the rotor is given by E = Emax sin a, where Emax is the maximum voltage induced by the CT air-gap flux when coupling with the rotor windings is maximum and a is the displacement angle of the CT rotor.

In general, the value of the rms voltage induced in the CT rotor winding when the displacement of the CX rotor is ax and that of the CT rotor is aT is given by E = Emax sin (ax – aT)