Transformer Nameplate Details Explained
Transformer nameplates contain several standard items of information and other
optional information. Transformer nameplate must specify the following parameters:
Volt-Ampere (VA) or kilovolt-amperes (kVA) rating
The voltage rating of both the primary and secondary circuits
The impedance rating of the transformer (normally restricted to 25 kVA or larger)
The required clearances for transformers with ventilated openings
The amount and kind of insulating liquid where used.
On dry-type transformers (no liquid coolant or insulation), the nameplate listing must also
include the class temperature rating of the winding insulation.
Fig.1: Transformer Nameplate
Transformer kVA Rating
The nameplate always indicates the size of the transformer in terms of how much
apparent power (rated kVA) it is designed to deliver to the load on a continuous basis.
By its very nature, the transformer will have more than one rated voltage, depending on
which side we are looking at and how many windings there are on that side.
Transformer Voltage Ratings
The following is a list of some conventions for specifying transformer voltage ratings:
U-W
The dash between the voltages U and W indicates they are on different sides of the
transformer. For example 480—120 tells us the primary winding is rated 480 V and the
secondary is rated 120 V.
U/W
The slash indicates the two voltages are from the same winding and that both voltages
are available;
1. g., 120/240 can indicate a 240 volt winding with a center tap.
U×W
The cross indicates a two-part winding that can be connected in series or parallel to
give higher voltage or current, respectively. Only one voltage is available at a time; e.g.,
120×40 indicates the transformer can operate at 120 V or 240 V, but not both
simultaneously.
U Y/W
The Y indicates a three-phase winding connected in a wye configuration. The first letter
(U) is the line voltage and the second letter (W) is the phase voltage (line to neutral).
Clearly, U=√3 W. Examples include 208Y/ 120 and 480Y/277.
Transformer Rated Frequency
The rated frequency will also be indicated on the nameplate (usually 60 Hz for the
United States). Operating the transformer at a lower frequency will increase the core
flux unless the voltage is reduced, this could cause magnetic saturation of the core and
overheating due to increased hysteresis and eddy current losses.
Maximum Temperature Rise
The maximum allowable temperature rise for the transformer is also shown on the
nameplate and is based on an assumed ambient temperature of 30 oC.
Insulation Class
The insulation class indicates the type of transformer insulation.
Transformer Percentage Impedance
Percent impedance is a representation of the impedance of the windings referred to
one side of the transformer. This number is the percentage of rated voltage that must be
applied to the high side to cause rated current on the low side when the low side is
shorted.
Other items that may be on the nameplate include the number of phases, a Wiring
diagram, and tap-changing information.
Transformer Nameplate Information
Following are the key information which are provided on the transformer nameplate
from the manufacturer.
Serial number Number of phases
Frequency Voltage rating
kVA Rating Temperature Rise
Polarity Percentage Impedance
Connection Diagram Name of Manufacturer
Type of insulating liquid Conductor Material for each Winding
Basic Insulation Level (BIL) Total Weight (kg)
