Elimination of asymmetry zero-drift errors in magnetic servo amplifiers

When applying magnetic amplifiers to position-indicating synchro-transmission systems, achievement of an extremely small zero drift is of paramount importance. It is well known that zero-drift errors caused by actual deviations from perfect symmetry of the saturable-reactor and rectifier components of magnetic-amplifier push-pull circuits may be reduced to certain specified values by using proper testing procedures for grading and matching of these components. However, limitation of such errors to sufficiently small values still represents a difficult problem in the design and production of high-performance magnetic servo amplifiers. Application of 2-phase induction motors in connection with special circuit arrangements offers the possible elimination of asymmetry zero-drift errors by supplying the magnetic-amplifier circuit from a magnetic frequency multiplier producing an exact multiple of the frequency of the a-c line feeding directly both the line phase of the motor and the synchro generator of the error-detecting system. In this case, even with nonsymmetrical characteristics of the saturable-reactor and rectifier components, operation of the motor under no-signal conditions will be inherently independent of changes in magnitude and frequency of the a-c line voltage, changes in ambient temperature, etc., with regard to the magnetic amplifier. Application of a higher frequency carrier for supplying the magnetic servo amplifier offers other advantages: its operation becomes more efficient and its speed of response is correspondingly increased. Actually, the possibility of using smaller sized saturable-reactor elements may compensate somewhat for the necessity of additional components of the magnetic frequency multiplier.