Control Transformers
What is a Control Transformer?
Control Transformers are useful where the available voltage must be changed to accommodate the voltage required by the load. For many electrical circuits, the National Electrical Code (NEC) requires a separately derived neutral secondary connection provided by Delta-Wye connected transformers.
Transformers are manufactured in a variety of choices to meet many applications. Dry-type transformers are offered
encapsulated, ventilated or non-ventilated, 600 Volt Class, isolation type, single and three phase, through 500 kVA. Indoor and outdoor models an option.
General purpose transformers can be located close to the load. No vaults are required for installation and no long, expensive feeder lines are needed. Common applications include inductive and resistive loads such as motors, lighting and heating.
For proper transformer selection, three characteristics of the load circuit must be determined in addition to the minimum voltage required to operate the circuit. These are total steady state (sealed) VA, total inrush VA, and inrush load power factor.
Sealed VA - Total steady state, sealed VA is the volt-amperes that the transformer must deliver to the load circuit for an extended period of time.
Inrush VA - Total inrush VA is the volt-amperes that the transformer must deliver upon initial energization of the control circuit. Energization of electromagnetic devices takes 30-50 milliseconds. During this inrush period the electromagnetic control devices draw many times normal current - 3-10 times normal is typical.
Inrush load power factor is difficult to determine without detailed vector analysis of all the load components. Generally such an analysis is not feasible, therefore, a safe assumption is 40% power factor. Until recently, 20% PF was commonly used for transformer calculations, however, tests conducted on major brands of control devices indicate that 40% PF is a safer default assumption.
Selection Steps for Industrial Control Transformers:
- Determine the supply and load voltages. The supply voltage is the available voltage to the control transformer. The load voltage is the operating voltage of the devices that will be connected to the transformer output.
- Calculate the total sealed VA by adding the VA requirements of all components that will be energized together (timers, contactors, relays, solenoids, pilot lamps, etc.). Sealed VA data is available from the control device manufacturer. >p>
- Add the inrush VA of all components that will be energized together. Be sure to include the sealed VA of components that don't have an inrush, (lamps, timers, etc.) as they present a load to the transformer during maximum inrush.
- Calculate selection inrush VA in one of the following two ways:
When selecting your transformer, check to be sure that the nameplate VA rating exceeds the sealed VA of the control circuit calculated in Step 1. If it does not, select a larger transformer VA that exceeds the circuit sealed VA.
- Selection inrush VA = Square root of [(VA sealed)2 + (VA inrush)2]
Alternative Method
- VA sealed + VA inrush = Selection inrush
Method B will result in a slightly oversized transformer
By following the above procedure, the secondary voltage delivered by the transformer will be 90% of the nameplate secondary voltage under maximum inrush conditions at rated input voltage.
Typical applications include:
- Hospitals
- Office Buildings
- Industrial Plants
- Schools
- Commercial Buildings
- Shopping Centers
- Apartment Buildings
- High Rise Buildings
- Institutional Buildings
Control transformers are available in a variety of primary and secondary voltage combinations from 120/240 primary voltage, 24 secondary voltage at 50/60Hz through 480 primary voltage, 240 secondary voltage at 50/60Hz up to 5000 VA rating.