Control Transformers

Shop control transformers used to supply lower-voltage power to industrial control circuits, machinery and automation equipment. Common connected loads include contactors, motor starters, relays, solenoids, pilot devices and other electromagnetic control components.

Compare control transformers by primary and secondary voltage, VA rating, frequency, winding material, construction and enclosure. Selection should account for both the continuous control-circuit load and the temporary inrush demand created when electromagnetic devices energize.

How to Select a Control Transformer

A control transformer supplies the voltage required by a machine or industrial control circuit. These transformers commonly power contactor coils, motor starters, relays, solenoids, timers, pilot lights and other control components.

Control-transformer sizing must account for two different demands: the continuous sealed load after the connected devices are energized and the temporary inrush load required to pull in contactors, relays and solenoids. A transformer that supports the steady-state load may still be undersized if it cannot provide sufficient voltage during the initial inrush period.

Important Selection Factors

• Primary voltage: Match the available supply and verify the required primary tap or winding connection.
• Secondary voltage: Confirm the voltage required by the complete control circuit.
• Continuous VA: Add the sealed or steady-state VA requirements of all simultaneously energized loads.
• Inrush VA: Account for the temporary demand of contactors, relays, solenoids and starters during pickup.
• Frequency: Verify operation at the available 50 Hz, 60 Hz or combined frequency rating.
• Construction: Choose open-style or encapsulated construction according to the installation environment.
• Winding material: Review copper or aluminum winding construction when required by the application or specification.
• Protection and accessories: Determine whether fuse blocks, finger-safe covers, terminal covers or mounting accessories are required.

Open-Style vs. Encapsulated Control Transformers

Open-style transformers are commonly installed inside control panels or enclosures that provide the necessary environmental protection. Encapsulated transformers enclose the windings and core in a protective compound and may be preferred where moisture, dust or contaminants are a concern. The complete installation must still meet the environmental and enclosure requirements of the application.

Replacement Checklist

• Manufacturer and complete transformer part number
• Primary voltage and existing tap connection
• Required secondary voltage
• Transformer VA rating
• Connected sealed and inrush loads
• Frequency
• Open-style or encapsulated construction
• Mounting dimensions and terminal arrangement
• Photographs of the nameplate and wiring diagram