Transmitter
The electrical element of the transmitter consists of a resistor wound on a circular former (called the 'toroidal resistor') and tapped at three points 120° apart. Two diametrically-opposed wiper contact arms, one positive and the other negative, are insulated from each other by a slotted arm which engages with a pin actuated by the appropriate mechanical element of the transmitter.
The wiper contact arms are assembled in the form of a bar having rotational freedom about a pivot which carries current to the positive arm. Current to the negative arm is carried via a wiper boss the underside of which is in contact with a ring fitted on the inner side of the terminal moulding. A circlip, fitted at the end of the pivot, holds the complete assembly in place against a spring which gives the required contact pressure on the toroidal resistor.
Receiver
The receiver element consists of a cylindrical two-pole permanent magnet rotor pivoted to rotate within the field of a laminated soft iron stator, carrying a star-connected three-phase distributed winding supplied from the toroidal resistor tappings. A tubular brass housing is fitted inside the stator, and together with its end cover, provides a jewelled bearing support for the rotor spindle. The front end of the spindle projects through the end cover and a dial mounting plate, to carry the pointer. Electrical connection between the transmitting and receiving elements may be either by terminal screws or plug-type connectors. The electrical elements of the receivers are common to all three circuit arrangements of the Desynn system.
Operation
When direct current (DC) is applied to the transmitter contact arms (which are in contact with the toroidal resistor) currents flow in the resistor causing the three tapping points to be at different potentials. For example, with the contact arms in the position shown in Figure 4.3.9 the potential at tapping No 2 is greater than that at No 1 because there is less resistance in the circuit between the positive arm and the No 2 tapping. Thus, currents are caused to flow in the lines between transmitter and receiver, the magnitude and direction of which depend upon the position of the contact arms on the toroidal resistor.
In turn, these currents flow through the coils of the receiver stator and produce a magnetic field about each coil similar to that of a bar magnet; thus either end of a coil may be designated as a Npole or a S-pole, depending on the direction of the current through a coil. The combined fields
extend across the stator gap and cause the permanent-magnet rotor to align itself with their resultant.
A pull-off magnet is fitted to the end plate, its purpose being to act as a power-failure device by exerting an attractive force on the main magnet rotor so as to pull it and the pointer to an off-scale position when current to the stator is interrupted. The strength of this magnet is such that, when the system is in operation, it does not distort the main controlling field.
Figure 4.3.9: Circuit diagram of basic Desynn system
