What a transformer does
A transformer changes the voltage of an alternating current supply. It does this without moving parts, using two or more coils wound around a shared magnetic core. Alternating current in one coil, called the primary, creates a changing magnetic field in the core. That field induces voltage in the other coil, the secondary. The ratio of turns in the two coils sets the ratio of the voltages.
If the secondary has fewer turns than the primary, the voltage steps down. If it has more, it steps up. Energy is not created: when voltage goes down, the available current goes up, and the reverse. A real transformer loses a small fraction of energy as heat.
Transformers work only with alternating current. This is a main reason that power systems are built on it.
Why plants use them
Power is transmitted at high voltage because it reduces current and losses over distance. Equipment, however, runs at much lower voltages. Plants therefore use transformers to bring supply voltage down to a distribution level, and then often again to the voltage that machines, lighting and controls need.
Transformers also serve other purposes:
- Isolation. The primary and secondary are electrically separate, which limits how some disturbances and faults pass through and can support safety measures.
- Matching voltage. Equipment designed for a different voltage than the local supply can be fed through a transformer.
- Grounding reference. The secondary winding can create a new, separately grounded system, which is part of grounding systems design.
- Control power. Small transformers supply low voltage for controls and instruments.
Common types
Power and distribution transformers. The larger units in substations and electrical rooms. They step medium voltage down to the main plant distribution level.
Dry-type transformers. Cooled by air rather than liquid. They are common indoors because they avoid the fire and spill concerns of liquid fill. They are generally limited to smaller or moderate sizes compared with liquid-filled units, and can be bulkier for a given rating.
Liquid-filled transformers. The core and windings sit in an insulating liquid that also carries heat to radiators or fins. They are widely used outdoors and for larger ratings. The liquid needs monitoring, and sites usually have containment and fire safety provisions appropriate to the fluid used.
Pad-mounted and substation transformers. Enclosed units installed on a concrete pad outdoors, designed to be safe from casual contact.
Control and isolation transformers. Small units inside panels and machines.
Autotransformers. Use a single winding with a tap and are used in some starting and voltage-adjusting applications. They do not provide electrical isolation.
Instrument transformers. Current and voltage transformers that scale quantities for metering and protection, usually found in switchgear.
Specialty types. Rectifier transformers for large DC loads, furnace transformers and drive isolation transformers are designed for demanding duty, including harmonics.
Reading a nameplate
The nameplate carries the key information: the kVA rating, primary and secondary voltages, the winding connection type, the impedance, the cooling class and the temperature rise. A three-phase transformer may connect its windings in delta or in wye arrangements, and the combination affects things such as grounding options and how harmonics behave. Reading the nameplate and the manufacturer’s manual correctly is the starting point for any decision about installing or loading a transformer.
Taps
Many transformers have taps on a winding that allow a small adjustment to the output voltage, for instance to compensate for a supply that runs consistently high or low. Changing taps is usually done with the transformer de-energized, and only by qualified people.
Heat and loading
Heat is what ages a transformer. The insulation around the windings degrades faster at higher temperatures, so the practical service life depends on how hot the windings run. Factors include:
- How heavily the transformer is loaded, and for how long.
- Ambient temperature and ventilation in the room.
- Whether the load produces harmonics, which add heating.
- Dust or blocked air paths on dry types, and condition of the liquid and cooling surfaces on liquid-filled ones.
Short periods of overload may be allowed by the manufacturer’s data, but continuous overload shortens life. Many transformers have temperature monitoring and alarms.
Where transformers sit in the plant
Transformers usually feed switchgear and electrical panels, and they supply motor control centers and other loads further downstream. Drives sometimes need special transformers because variable speed drives produce harmonics.
Inspection and maintenance
Within preventive maintenance, typical tasks include checking for unusual noise, odor, leaks, rust or discoloration, keeping clearances and ventilation clear, cleaning dry types, checking connections and sampling and testing the liquid in liquid-filled units, as the manufacturer recommends. The humming sound that transformers make is normal, but a change in it may be worth investigating.
Safety
A transformer can stay energized on the secondary side from back-feed, and can hold dangerous energy even when it seems quiet. Work should only be done by qualified personnel under lockout tagout procedures, with the manufacturer’s instructions and local regulations in hand. Selection and sizing are engineering decisions and should not be improvised.