When a transformer’s output voltage is too high or too low for the equipment it supplies, a tap changer may help adjust it. A tap changer in a transformer selects different winding connections to change the output voltage. It works with the windings, one of the main transformer components, to change the ratio between input and output voltage.
A transformer tap changer may be adjustable only with the unit de-energized, or it may be built to change taps under load. That difference determines what kind of voltage adjustment is practical.
Transformer taps are connection points along a winding. A transformer tap changer selects between them to change the number of turns in use. This changes the turns ratio between the transformer windings and adjusts the output voltage in steps.
With taps on the primary winding and a constant supply voltage, selecting fewer primary turns raises secondary voltage. Selecting more primary turns lowers it, provided the secondary winding remains unchanged.
The direction of adjustment depends on which winding is tapped and how the connections are arranged. Use the unit's tap table and manufacturer instructions to interpret its settings.
The tap changer type determines whether the transformer must be taken out of service to adjust the voltage.
A DETC must be adjusted with the transformer de-energized, following the manufacturer's instructions and the procedures that apply at your site. You will also see the terms off-circuit, off-load, and no-load tap changer. They point to the same requirement, but disconnecting the load alone does not make a unit safe to adjust.
DETCs suit settings that change infrequently, such as an adjustment during commissioning or after a lasting change in supply voltage.
Arrangements vary by design. The handle or gearbox may sit outside the tank, while the tap selector is inside. Check the unit’s drawings and manufacturer instructions for its layout
An on-load tap changer, also called a load tap changer (LTC or OLTC), changes the winding connection while the transformer keeps supplying the load. Its switching mechanism keeps current flowing as it moves between taps.
The tap changer changes the ratio, while an automatic controller monitors voltage and requests movement according to its configured voltage band.
A tap position identifies one documented winding configuration. It may appear as a number, a letter, a voltage rating, or a percentage above or below nominal. Check the unit's tap table and manufacturer documentation for the available settings, and read them alongside the transformer nameplate ratings.
Be careful with position numbers and plus/minus markings. They are not a universal instruction about secondary voltage. Ranges, increments, numbering, and capacity ratings vary by equipment, and on some designs not every tap carries the same kVA rating.
For this hypothetical example, a two-winding transformer has taps on its primary winding. It is rated to supply 480 V at the 13,200 V tap. Assume the incoming voltage stays at 13,500 V. Calculate the approximate output voltage as: incoming voltage × 480 V ÷ selected primary tap rating.
The primary tap ratings span +5% to -5% of nominal in 2.5% steps. These percentages describe the primary ratings, not the resulting change in secondary voltage. At the stated supply voltage, the 13,530 V tap gives the closest calculated output to 480 V. The calculation excludes impedance drop and loading effects. The position numbers are illustrative.
Tap adjustment may help when the required correction falls within the unit's documented range and operating limits. It does not increase nameplate kVA capacity or repair a fault.
Adjustment may be relevant when:
Broader assessment is needed when:
If voltage problems persist, transformer testing can help check whether the issue is the voltage ratio or an equipment fault. Qualified personnel should carry out the tests and interpret the results.
Some findings justify arranging an assessment. They are investigation triggers, not proof of a specific fault:
When planning transformer maintenance, follow the manufacturer’s guidance. The work needed and its timing also depend on the tap changer’s design, condition, and how often it operates.
Our preventive maintenance plans include testing and field service performed by our team. We start by reviewing the equipment details and reported problem to identify the checks needed and plan the work.
Before contacting us, gather any available equipment details and records, including:
Please do not approach energized equipment or take measurements yourself. Share what is already on file or what qualified personnel can collect safely.
Share the records you have and discuss the equipment issue with our team so we can help explain which checks are needed. If repair or replacement is recommended, we can discuss the options based on your site’s needs and schedule.
It depends on the design. The operating mechanism, such as a handle or gearbox, may be reachable from outside the tank, while selector and switching components often sit inside the tank or a separate compartment. Check the unit's drawings.
For a given kVA, the high-voltage winding carries less current, which can reduce the current-carrying requirements of tap connections and switching contacts. Its larger number of turns can also allow finer percentage adjustments. The arrangement depends on the transformer design, and the high-voltage winding is not always the primary.
No. Some transformers have no voltage adjustment. Others have taps or links that can be reconnected only when the unit is de-energized. Some use a dedicated tap changer. Having winding taps does not necessarily mean a transformer has a tap-changing mechanism.
