Transformer
Transfers electrical energy between circuits by magnetic coupling, stepping voltage up or down.
What it is
A transformer is two (or more) coils of wire — the primary and the secondary — wound on a shared magnetic core. There's no electrical connection between the windings; energy passes across the gap as a changing magnetic field.
Because it relies on a changing field, a transformer only works on AC. Apply DC and you get a brief pulse, then nothing but a hot winding.
How it works
Alternating current in the primary creates an alternating magnetic flux in the core. That flux links the secondary winding and induces a voltage in it.
The voltage ratio follows the turns ratio: the secondary voltage relates to the primary voltage as the number of secondary turns relates to primary turns. More secondary turns step the voltage up; fewer step it down.
Power in roughly equals power out (minus small losses), so if voltage steps down, available current steps up in proportion — and vice versa.
Where it's used
Control transformers drop mains down to a safer control voltage (for coils, PLCs, relays). Isolating transformers separate a circuit from the supply for safety. Distribution transformers step network voltages down for premises.
On site, a failing or undersized control transformer is a common cause of control voltage that sags under load — coils that chatter or won't pull in.
Safety first
Transformer outputs can be live even when the input looks dead, and vice versa. Prove dead on the side you're working on. Control transformers run hot when overloaded.
Isolate, lock out / tag out, and prove dead before working unless a live test is specifically required, authorised, and carried out under proper supervision. Always follow local regulations, your site procedures, and the equipment manufacturer's documentation.
Related faults
No control voltage in the panel
Nothing in the control circuit will operate — contactors won't pull in, indicators are dead, the PLC may be off. The control voltage that should be there simply isn't.
Contactor chattering or buzzing instead of holding in
The contactor rapidly clicks/buzzes, pulls in and drops out repeatedly, or hums loudly without seating cleanly. Often comes with arcing noise and heat.
Contactor coil overheating / burning smell
The contactor coil runs hot, discolours, or gives off a burning smell, and may eventually fail. It might still operate for now but won't last.
Related definitions
Contactor
An electrically-operated switch that uses a coil to make or break a load circuit, usually three-phase power.
Control vs power circuits
Low-power control logic decides what happens; the power circuit carries the load — kept separate for safety and clarity.
AC vs DC
Alternating current reverses direction many times a second; direct current flows one way. Why it matters on site.