A control transformer is a device used in electronic circuits to ensure a constant or stable voltage or current with a low volt-amp or low power rating. Both industrial and electronic industries benefit from such feature.
Some of the devices that require a control transformers are solenoids, relays, magnetic coils that move with applied current, and electronically controlled electrical switches. The transformer is expected to maximize the output voltage regulation and inrush capability, especially when the electromagnetic devices are initially energized.
The insulating materials used in transformers are of high quality in order to insulate electrically the turn to turn windings, primary to secondary windings and ground, and layer to layer windings.
The transformers are oven-cured and sealed with VT polyester resins to reduce moisture. In turn, this characteristic offers protection from the environment as the unit has a strong mechanical bond.
Manufacturers of control transformers have a wide variety of choices that consumers can choose from. One manufacturer is the Hammond Power Solutions, which has the Imperator series, open core and coil control, Q series, and MVC series.
The Imperator series are machine tool rated industrial control transformers for high inrush industrial applications such as the relays, magnetic motor starters and solenoids. Its standard voltage group is 11, and VA that vary from 50VA to 500VA.
The Spartan line of industrial open core and coil transformers are perfect for general applications including the light and industrial duty loads. This is also ideal for applications that do not require high inrush as the Spartan series provides efficient and affordable solutions.
The Q series is for general applications at a reasonable price. It is also good for indoor and outdoor applications. The Q series has a single encapsulated phase and have the standard NEMA type 3R enclosure, which are ideal for circuit isolation and industrial lighting, supplication of machine tool circuits, actuation of relays, signal and alarm systems, and bells; and operation of small motors, valves and dampers.
The MVC series are also excellent for general application that needs to adjust a medium voltage supply to match a low voltage. The MVC series are available from 500VA to 5000VA.
In order to determine the correct specifications for the control transformers, there are 3 characteristics of load circuit that should also be identified besides the minimum voltage requirement. This includes the total inrush VA, the total steady-state VA, and inrush load power factor.
The total inrush VA is actually the Volt Amperes that transformers are expected to deliver after the initial energizing of the control circuit is completed. The energizing of electromagnetic devices will take from 30 to 50 milliseconds. During that time, the electromagnetic control devices will draw normal current.
The total steady-state VAs the Volt Amperes that transformers are expected to deliver to the load circuit within an extended period of time. Finally, the inrush load power factor is hard to identify without the complete vector analysis of all load partitions. The safest assumption you can have is 40% power factor.