Influence of unequal air-gap thickness in servo valve torque motors

The current paper considers servo valve torque motors having four air-gaps. An ideal for these types of torque motors is to have four equal air-gaps at free state. However, real servo valve torque motors have unequal air-gaps due to production errors. Because the influence of the inequality of air-gaps is not clear, servo valve manufacturers have made great efforts to establish the equality of the air-gaps. The current paper gives a prospect on how an imbalance of air-gaps changes the flux densities in the air-gaps and the resultant torque characteristic. An important result of the paper is that some of the air-gap imbalance induces a torque change of magnitude of second- or higher-powers of the imbalance. The imbalance of the air-gaps due to different pole heights results in an offset proportional to the magnitude of the imbalance. These results can be used for setting a tolerance limit for air-gap thickness.