Control power transformers (CPTs) are devices that are imperative in stepping up/down and regulating voltages for industrial control and automation applications. CPTs are particularly effective in stabilizing the control voltage side during overload conditions such as high inrush current on the source voltage side.
Key Terminology
Before the CPT selection process begins, an understanding of a few key terms is required.
Inrush VA: the product of the load voltage (V) multiplied by the current (A) that is required during circuit start-up. It is calculated by adding the inrush VA requirements of all devices (contactors, timers, relays, pilot lights, solenoids, etc.), which will be energized together. Inrush VA requirements are best obtained from the manufacturer of the specific device.
Sealed VA: also referred to as steady state VA, the product of the load voltage (V) multiplied by the current (A) that is required to operate the circuit after initial start-up or under normal operating conditions. It is calculated by adding the sealed VA requirements of all electrical components of the circuit that will be energized at any given time. Sealed VA requirements are best obtained from the manufacturer of the specific device.
Primary voltage: the voltage available from the electrical distribution system and its operational frequency, which is connected to the transformer supply voltage terminals
Secondary voltage: the voltage required for load operation which is connected to the transformer load voltage terminals
Selection Steps
The below questions will help guide a user towards selecting the best suited CPT for a particular application.
- Determine your primary (source) voltage (V)
- Determine your secondary (control) voltage (V)
- Calculate the total amount of VA drawn from your control circuit by determining the Application Inrush VA by using the following industry-accepted formula:
- Application Inrush VA = √(Inrush VA)^2 + (Sealed VA)^2
- Using the Application Inrush VA found in the first step, find the closest value in the Applied Inrush VA column in the table below. There is no scenario where a figure less than the Application Inrush VA should be used.
- After you have identified your Applied inrush VA, select the Transformer VA rating in the same row. Ensure that the Transformer VA rating is greater than or equal to the total sealed requirements. Otherwise, select a CPT with a Transformer VA rating greater than or equal to the total sealed requirements.
CPT Application Inrush VA Selection Chart
Sources: Siemens Speedfax Catalog 2007-2008, Siemens Industrial Control Product Catalog 2017
