Special machines:Types of Synchros

Types of Synchros

There are many types of synchros but the four basic types used for position and error-voltage applications are as under :

(i) Control Transmitter (denoted by CX) – earlier called generator (ii) Control Receiver (CR) – earlier called motor (iii) Control-Transformer (CT) and (iv) Control Differential (CD). It may be further subdivided into control differential transmitter (CDX) and control differential receiver (CDR).

All of these synchros are single-phase units except the control differential which is of three-phase construction.

(a) Constructional Features

1. Control Transmitter

Its constructional details are shown in Fig. 39.15 (a). It has a three-phase stator winding similar to that of a three-phase synchronous generator. The rotor is of the projecting-pole type using dumbell construction and has a single-phase winding. When a single-phase ac voltage is applied to the rotor through a pair of slip rings, it produces an alternating flux field along the axes of the rotor. This alternating flux induces three unbalanced single phase/voltage in the three stator windings by transformer action. If the rotor is aligned with the axis of the stator winding 2, flux linkage of this stator winding is maximum and this rotor position is defined as the electrical zero. In Fig. 39.15 (b), the rotor axis is displaced from the electrical zero by an angle displaced 120º apart.

(b) Control Receiver (CR)

Its construction is essentially the same as that of the control transmitter shown in Fig. 39.15 (a). It has three stator windings and a single-phase salient-pole rotor. However, unlike a C X, a CR has a mechanical viscous damper on the shaft which permits CR rotor to respond without overshooting its mark. In normal use, both the rotor and stator windings are excited with single-phase currents. When the field of the rotor conductors interacts with the field of the stator conductors, a torque is developed which produces rotation.

(c) Control Transformer (CT)

As shown in Fig. 39.15 (b) its stator has a three-phase winding whereas the cylindrical rotor has a single-phase winding. In this case, the electrical zero is defined as that position of the rotor that makes the flux linkage with winding 2 of the stator zero. This rotor position has been shown in Fig. 39.15 (b) and is different from that of a control transmitter.

(d) Control Differential (CD)

The differential synchro has a balanced three-phase distributed winding in both the stator and the rotor. Moreover, it has a cylindrical rotor as shown in Fig. 39.16 (a). Although three-phase windings are involved, it must be kept in mind that these units deal solely with single-phase voltages. The three winding voltages are not polyphase voltages. Normally, the three-phase voltages are identical in magnitude but are separated in phase by 120º. In synchros, these voltages are in phase but differ in magnitude because of their physical orientation.

(e) Voltage Relations

Consider the control transmitter shown in Fig. 39.17. Suppose that its rotor winding is excited by a single-phase sinusoidal ac voltage of rms value Er and that rotor is held fast in its displaced position from the electrical zero. If K = stator turns / rotor turns, the rms voltage induced in the stator winding

is E = KEr. However, if we assume K = 1, then E = Er.

Example 39.5. The rotor of a control transmitter (C X) is excited by a single-phase ac voltage of rms value 20 V. Find the value of E1s, E2s and E3s for rotor angle a = + 40º and – 40º. Assume the stator/rotor turn ratio as unity. Also, find the values of terminal voltages when a = + 30º.