Technical Article

Transformer Nameplate Details and Sound Levels

July 24, 2021 by Alex Roderick

In this article, you’ll learn about transformer nameplate data and different sound levels in a transformer due to the phenomenon called magnetostriction.


A transformer, like all other electrical equipment, is designed and manufactured to function in an electrical system at a specified voltage, frequency, load, etc. The nameplate data is carefully checked to ensure that the transformer is installed and maintained according to the data (see Figure 1). The nameplate should never be defaced or removed. The five sections of a nameplate include general information, the physical terminal arrangement, a schematic diagram, a vector reference, and a voltage ratio table.

Figure 1. A nameplate describes the operating parameters of a transformer. Image Courtesy of ABB


General Information

This section includes information on transformer serial number, type, rating, nominal voltages, impedance, tap changer range, oil capacity, weights, etc.


Physical Terminal Arrangement 

Normally as a block drawing, the physical locations of all terminals are given with reference to some obvious feature of outside construction. This enables all connections to be positively identified.


Schematic Diagram

A complete winding schematic diagram shows all internal connections, coil tap numbers, internal selector switches, and main terminal markings. The combined use of the physical terminal arrangement drawing and the schematic diagram enables the user to determine exactly how each bushing is connected internally. Also included on the schematic diagram are all auxiliary components such as current and potential transformers showing their actual electrical positions and ratios.


Vector Reference

On all 3-phase units, primary and secondary voltage reactors are drawn to indicate the phase angle between the primary and secondary voltages. The vector diagrams are labeled with the terminal designations used on the schematic diagram.


Voltage Ratio Table

One or more tables are given for transformers with a tap changer. The table lists the tap indicator number, the actual coil numbers, and how they are connected. The voltage ratio is given for each tap position and, in some instances, the full-load current at that tap position. On many nameplates, to avoid the use of voltage ratios, which would mean small decimal numbers, the nominal voltage of the winding without the tap is taken, and the voltage required to produce this nominal value is listed for each tap. Both methods give the same information.


Sound Levels

All transformers hum when energized. The hum is due to a property of magnetic materials called magnetostriction. A changing magnetic field causes small changes in the size of the metal parts in the direction of the magnetic field. The change in size results in vibrations within the laminated steel core structure. 

The vibrations generate a humming sound. The hum has a fundamental frequency of twice the applied frequency. The sound volume is determined by the transformer design, construction characteristics, and the methods used in the installation. Toroidal transformers typically have the lowest volume of all transformer types. Electrical maintenance personnel should monitor transformer sound levels. Any noticeable change in sound level could be the result of loosening clamp hardware, vibration isolators, or over-excitation and should be investigated.

The level of sound produced by a transformer is measured in decibels (dB). A decibel (dB) is the unit used to measure the intensity of sound. The decibel number represents a ratio of the level of sound to a reference level (usually 0 dB). The noises in and around most indoor locations (ambient sound level) normally mask transformer sounds. Tables are available for the average ambient sound levels in areas where transformer noise could be a problem (see Table 1).

Decibels Example Loudness
180 Rocket Engine Deafening
160 Jet Engine  
150 Explosion  
140 Loud Rock Music  
130 Air Raid Siren Very loud
120 Thunder  
110 Chain Saw  
100 Subway  
90 Heavy Truck Traffic  
80 Vacuum Cleaner Loud
70 Busy Street  
60 Hair Dryer Moderate
50 Normal Conversation  
40 Running Refrigerator  
30 Quiet Conversation Faint
20 Quiet Living Room  
10 Whisper Very Faint
0 Intolerably Quiet  
Table 1. The ambient sound level must be considered when selecting a transformer.

The sound level produced by a transformer is normally not a problem when transformers are installed in substations, in vaults, or outdoors. There are specific critical locations where sound is an important factor. Test procedures have been established so that transformer manufacturers can publish the sound level ratings of their transformers.

Sound levels are based on the kVA rating of the transformer (see Table 2). For example, if a 15 kVA distribution transformer with a 50 dB rating is installed in a factory that has an ambient sound level of 85dB, the sound of the transformer would not be heard above the ambient sound. If this same transformer were installed in an apartment building where the ambient sound level is 30dB, the transformer sound would be noticeable and would be considered objectionable.

kVA Rating Sound Level (dB)
3-9 40
10-50 45
51-150 50
151-300 55
301-500 60
501-700 62
701-1000 64
1001-1500 65
1501-2000 66
2001-3000 68
Table 2. The transformer sound level is based on the kVA rating.

In other areas such as schools, churches, and hospitals where the ambient sound level is very low, special precautions must be taken to select a transformer with a low sound rating and to locate and install the transformer to keep the sound level at a minimum.


Sound levels are measured 1′ from the outside edge of the transformer.


Featured image used courtesy of ABB