The design principle of a Control Synchro is to minimise errors in the output signal due to current loading, magnetic non-linearity and temperature rise, using high impedance windings and special attention to the magnetic circuits. The Control Transformer which provides the error signal to a servo amplifier, can be considered a ‘null’ detector and it is most often used in this way. However, the ‘null’ is never zero due to residual voltages. This is due to stray couplings within the laminated stator that result in an in-phase voltage, a quadrature voltage, both at fundamental frequency, plus several harmonics. These residual voltage levels are quoted in the performance data tables for each unit.