An important consideration in selecting a D/S-R converter is to make sure it has sufficient drive capability for the intended load. Drive capability is specified as VA (Volt Amperes) since actual CT’s or CDX’s (Control Transformers Or Control Differential Transmitters) have an Inductive impedance. The key parameter needed to calculate required VA for the Synchro/CDX or Resolver load device is Zso. For a 3-wire Synchro, or a CDX, Zso is the equivalent impedance with two of the stator inputs tied together and measured to the third stator lead. The Zso for a given CT/RT/CDX is specified by the manufacturer. For resolver RT’s, Zso is the impedance of one of the two input stator windings.

To calculate the VA required for a Synchro CT or CDX load:

VA= ¾(VL-L)² / |Zso| Where VL-L is the D/S Line to Line output voltage and where |Zso| is the absolute value of Zso.

An example of a very common military CT is 11CT4e, Zso = 700 +j4900 and |Zso| is 4,950.

Therefore, required VA to drive an 11ct4e Synchro receiver is: VA= ¾(90)² / 4950 = .75(8100) / 4950 = 1.23.

A CDX output is usually connected to a CT. The total load on the D/S would be the sum of the CDX VA requirement and the CT VA requirement, each calculated based on their respective Zso values.

For Resolver RT loads, the calculation of VA is VA= (VL-L)² / |Zso|.

For a typical 11.8V L-L, size 08, RT, |Zso|=173, and VA = (11.8)² / 173 = 0.804 If the required VA is greater than what is available from the selected D/S, you can utilize a Synchro or Resolver Boost Amplifier. A technique in lieu of using a Boost Amplifier is a method whereby the load can be "tuned" using AC capacitors. This technique and calculations for capacitor values is detailed in the NAI Synchro Handbook Chapter 4, pages 32 & 33. The handbook is available on the NAI website, under "Application Notes" Synchro Handbook Chapter 4, pages 32 & 33.