The nameplate is the transformer's official identity and its contract with the operator: every value on it is a manufacturer commitment and a limit that must be respected. An engineer who reads the nameplate well answers half the questions in the field before opening any catalog.
The most important nameplate items
| Item | What it means | Why it matters to you |
|---|---|---|
| Rated power (kVA) | The continuous apparent power allowed | The basis for current calculations and selecting protection devices |
| Rated voltages (HV/LV) | The voltage on each side at no-load and the nominal tap position | Compatibility with the network and loads |
| Frequency (Hz) | The design frequency, 50 or 60 | Operating a transformer at a frequency lower than its design is a real problem |
| Connection group (e.g. Dyn11) | Winding connections and the angular displacement | An essential condition for paralleling — see Vector Group |
| Short-circuit impedance Z% | The transformer's relative impedance | Short-circuit calculations and load sharing in parallel operation |
| Cooling system | Such as ONAN/ONAF and the capacity of each mode | The actual available capacity depending on cooling status |
| Tap changer positions | The taps and the voltage at each tap | Reference for voltage adjustment and the TTR test |
| Weights and oil | Total weight, oil weight, and oil volume | Transport, lifting, foundations, and emergency planning |
The rule for working with the nameplate
- The nameplate is a binding reference: any test or operating decision is compared against its values, not memory or estimation.
- Photograph the nameplate and document it in the transformer's file from the moment of receipt; many old nameplates corrode and lose their data.
- For a step-by-step reading guide, see How to Read a Transformer Nameplate.
Sample answer: The rated power in kVA (the basis for current and protection calculations), the rated voltages on both sides (compatibility with the network), the connection group such as Dyn11 (a paralleling requirement and understanding the angular displacement), the short-circuit impedance Z% (short-circuit current calculations and load sharing), and the cooling system such as ONAN/ONAF (the actual capacity available depending on cooling). Frequency, weight, and tap changer position data are also included.
Treating the transformer's power rating as a single fixed number. Many transformers have more than one rating depending on the cooling mode (ONAN/ONAF), and the power actually available depends on the status of the fans and pumps at the moment of operation.
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