A control transformer is a transformer used to supply control-circuit voltage for devices such as coils, relays, and indicators. In industrial panels it is selected by the control burden it has to support, including steady-state load and inrush demand.
Difficulty: BeginnerPosted: 2026-03-15
Quick answer
A control transformer is a transformer used to supply control-circuit voltage for devices such as coils, relays, and indicators.
This matters when a control circuit needs a lower control voltage, when a starter bucket needs transformer power for coils and auxiliaries, or when a replacement has to match both voltage and VA burden.
Plain-language definition
A control transformer supplies the control side of a circuit rather than the main load power. In practical panel work that often means feeding coils, relays, indicators, or similar control devices at a lower control voltage.
Because it supports a control burden, it is selected by voltage and VA demand, not just by the idea of stepping voltage up or down.
Where control transformers are commonly used
Motor starters and contactor control circuits
Industrial control panels with low-voltage control devices
Retrofits where the control circuit must match existing voltage expectations
Applications with coils, indicators, and similar control burdens
Common confusion with similar terms
A control transformer is not sized the same way as a power transformer feeding the main load. The control burden and largest inrush event matter to the selection.
That is also why control transformer pages often discuss VA sizing, coil burden, and protection together.
Common mistakes
Treating the transformer like a generic voltage change without checking VA burden.
Ignoring inrush demand from coils and similar control loads.
Picking a transformer without reviewing available standard VA sizes and protection.
Important note
Before replacing a control transformer, verify primary voltage, secondary voltage, burden, inrush demand, and the manufacturer protection guidance for the exact application.
FAQ
Is a control transformer selected by voltage only?
No. The control burden and inrush demand also have to be checked.
Why does VA rating matter on a control transformer?
Because the transformer has to support the control-circuit load, including the largest inrush event it will see.
The information in this article is provided for general educational and reference purposes. Industrial equipment
selection, installation, and operation should always be verified against manufacturer documentation, applicable
electrical codes, and the requirements of the specific application.
Strike Industrial does not design electrical systems and cannot evaluate every operating condition. Before
installing or modifying industrial equipment, consult qualified personnel such as a licensed electrician, controls
engineer, or equipment manufacturer when appropriate.
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