Shown below is a schematic of a Three Phase Motor Starter. The area highlighted in yellow is the part of the schematic which contains the control transformer.
The Control Transformer is powered by two of the three phases. Here it receives power from phases A and B.
This is a single phase transformer and lowers the voltage to a more common value which is useful when adding lights, timers or remote switches not rated for higher voltages.
Transformers have a primary side and a secondary side.
The primary side is the higher voltage side and the secondary side is the lower voltage side.
Control transformers are rated in volt amps (VA).
The control transformer primary side voltage must match the incoming line voltage and the secondary side must match the load voltage.
The VA rating must be greater than the VA rating of the load.
If lights and timers are used the VA rating must be greater than the total load.
The Secondary Fuse protects the control circuit and transformer from damage, including fire damage.
The VA rating of the transformer is used to properly select the fuses.
The fuse must be smaller than the VA rating of the transformer.
Calculate the fuse size by first determining the size of the transformer.
The VA rating is the math formula used in determining the amperage at a given voltage.
A secondary voltage of 120 volts from a 100VA transformer will produce .83 amps...
e.g., 100 VA ÷ 120 Volts = .83 Amps.
The VA rating is divided by the voltage.
The result is the transformer's output amperage.
The Secondary Fuse size for a 100VA transformer must be 8/10 Amp or smaller.
A fast acting fuse is best. Refer to the NEC.
The primary fusing protects the control transformer from damage, including fire damage.
The VA rating of the transformer is used to properly select the fuses.
The fuses must be matched to the VA rating of the transformer.
The VA rating is the math formula used in determining the amperage at a given voltage.
A primary voltage of 480 volts for a 100VA transformer will draw .21 amps...
e.g., 100 VA ÷ 480 Volts = .21 Amps.
The VA rating is divided by the voltage.
The result is the transformer's input amperage.
The Primary Fuse size for a 100VA transformer must be .21 Amp or 150% larger.
Oversizing Primary Fuses is necessary due to the initial load when power is first applied (the initial load is also referred to as "inrush").
A timed delay fuse is best. Refer to the NEC.