Transformer noise is caused by magnetostriction of the core laminations under alternating magnetic flux. This causes the laminations to expand and contract slightly with each cycle, vibrating at twice the supply frequency. Higher flux density from increased voltage raises the noise level. Methods to reduce transformer noise include resiliently mounting the core and coils, using flexible connections, and applying acoustic damping materials. Transformer vibration is also caused by magnetostriction and can be reduced by isolating the transformer from the ground and other structures.

1. TRANSFORMER NOISE AND
VIBRATION
BY RAVEENDRANATH MACHERLA

2. WHAT MAKES A TRANSFORMER NOISE
(HUMMING)?
• ‘NOISE’ IS DEFINED AS UNWANTED SOUND.
• TRANSFORMER NOISE IS PRODUCED BY THE CORE, CAUSED BY A PHENOMENON CALLED
‘MAGNETOSTRICTION’.
• A TRANSFORMER CORE IS MADE UP OF THIN CORE LAMINATIONS (SPECIAL STEEL SHEETS, TO REDUCE EDDY-
CURRENT LOSSES AND TO REDUCE THE CONSEQUENT HEATING EFFECT), VIBRATES BECAUSE OF THE
ALTERNATING MAGNETIC FLUX AND THEREFORE EXPANDS OR CONTRACTS EVER SO SLIGHTLY, WITH EACH
CYCLE OF MAGNETIZATION. THE EXPANSION AND CONTRACTION IS NOT UNIFORM ALL OVER THE SHEET,
AND SO ACROSS ALL SHEETS.
• THESE EXTENSIONS ARE ONLY SMALL DIMENSIONALLY, AND THEREFORE CANNOT BE SEEN BY THE NAKED
EYE. HOEVER, THEY ARE SUFFICIENT TO CAUSE A VIBRATION, AND AS A RESULT NOISE.
• APPLYING MORE VOLTAGE (THAN SYSTEM LEVEL) WILL CAUSE HIGHER FLUX DENSITY WHICH WILL RAISE THE
NOISE LEVEL.
• COOLING SYSTEM (FANS AND PUMPS)

4. WHAT IS
‘MAGNETOSTRICTION’?
IF A PIECE OF MAGNETIC STEEL SHEET IS MAGNETIZED IT WILL
EXTEND ITSELF. WHEN THE MAGNETIZATION IS TAKEN AWAY, IT
GOES BACK TO ITS ORIGINAL CONDITION.
A TRANSFORMER IS MAGNETICALLY EXCITED BY AN ALTERNATING
VOLTAGE AND CURRNET SO THAT IT BECOMES EXTENDED AND
CONTRACTED TWICE DURING A FULL CYCLE OF MAGNETIZATION.

5. TRANSFORMER NOISE FREQUENCY?
• TRANSFORMER HUMMING IS CAUSED BY THE EXTENSION AND CONTRACTION OF THE
CORE LAMINATIONS WHEN MAGNETIZED. UNDER ALTERNATING FLUXES, WE CAN
EXPECT THIS EXTENSION AND CONTRACTION TO TAKE PLACE TWICE DURING A NORMAL
VOLTAGE OR CURRENT CYCLE.
THIS MEANS THAT THE TRANSFORMER IS VIBRATING AT TWICE THE FREQUENCY OF THE
SUPPLY, i.e. FOR 50Hz SUPPLY FREQUENCY, THE NOISE OR VIBRATION IS MOVING AT 100Hz.
THIS IS CALLED FUNDAMENTAL NOISE FREQUENCY.
PRACTICALLY TRANSFORMER NOISE IS MADE UP OF FREQUENCIES OF ODD MULTIPLES OF
THE FUNDAMENTAL KNOWN AS 1ST, 3RD, 5TH AND 7TH HARMONICS.

6. TRANSFORMER NOISE LEVEL VS FLUX DENSITY
• APPLYING A VOLTAGE TO A TRANSFORMER PRODUCES FLUX, OR MAGNETIC LINES OF
FORCE IN THE CORE. THE DEGREE OF FLUX WILL DETERMINE THE AMOUNT OF
MAGNETOSTRICTION AND HENCE, THE NOISE LEVEL.
• TRANSFORMER VOLTAGES ARE FIXED BY SYSTEM REQUIREMENTS, AND SO THE AMOUNT
OF MAGNETIZATION. THE AMOUNT OF FLUX AT THE NORMAL VOLTAGE IS INVARIABLY
FIXED, THUS SETTING THE NOISE AND VIBRATION LEVEL.
• FLUX DEPENDS ON THE RATIO OF THE TRANSFORMER VOLTAGES TO THE NUMBER OF
TURNS IN THE WINDING.
• THE DECISION ON WHAT THIS RATIO OF VOTAGE TO TURNS WILL BE, IS MAINLY DECIDE
BY ECONOMICAL FACTOR. (FLUX DENSITY IS INVERSELY PROPORTIONAL TO THE CORE
WEIGHT).

7. WHY NOT REDUCE THE TRANSFORMER NOISE
BY REDUCING AMOUNT OF FLUX?
• ADJUSTMENT TO A DESIGN TO REDUCE THE NOISE LEVEL CAN BE MADE AT COST
BUT DON’T EXPECT A LARGE REDUCTION IN THE NOISE LEVEL.
• REDUCING THE FLUX LEVEL CAN BE DONE, AT A COST AND FOR NOT MUCH
IMPROVEMENT IN NOISE LEVEL, SINCE MAGNETIZATION AND
MAGNETOSTRICTION RELATION IS NOT LINEAR.
• LOWERING THE FLUX DENSITY AND THIS MEANS INCREASED COST. THERE HAVE
TO BE TRADE-OFFS BETWEEN COST AND NOISE.

8. TRANSFORMER
NOISE VS LOAD
• LOAD NOISE IS CAUSED BY VIBRATIONS IN TANK WALLS,
MAGNETIC SHIELDS, AND TRANSFORMER WINDINGS
DUE TO THE ELECTROMAGNETIC FORCES RESULTING
FROM LEAKAGE FIELDS PRODUCED BY LOAD
CURRENTS.
• WHEN THE TRANSFORMER IS EXCITED, MECHANICAL
FORCES WILL EXIST BETWEEN INDIVIDUAL
CONDUCTORS IN A WINDING. THESE FORCES WILL
PRODUCE A VIBRATION AND NOISE, BUT ONLY AT
TWICE THE SUPPLY FREQUENCY. HOWEVER, THIS NOISE
IS SWAMPED BY THE FUNDAMENTAL AND HARMONICS
PRODUCED BY THE CORE.
• HARMONIC CONTENT IN THE LOAD CURRENT HAS A
LARGE IMPACT ON THE SOUND LEVEL. CURRENT
HARONICS ARE A MAJOR SOURCE OF INCREASE IN
SOUND LEVELS IN HVDC AND RECTIFIER
TRANSFORMERS. NON-LINEAR LOADS CAUSE
HARMONICS IN THE EXCITATION VOLTAGE RESULTING
IN AN INCREASE IN CORE SOUND LEVEL.
• AT A CONSTANT FLUX DENSITY, THE CONTRIBUTION OF
THE LOAD NOISE TO THE TOTAL TRANSFORMER NOISE
HAS REMAINED MODERATE.

9. METHODS TO
REDUCE
TRANSFORMER
NOISE
CORE AND COILS SHALL BE RESILIENTLY MOUNTED. THE STIFFNESS OF THE MOUNTS MUST BE
SUCH THAT THEY DO NOT WEAKEN TH INSTALLATION BY BEING TOO SOFT OR SPOIL THEIR
ATTENUATION PROPERTIES BY BEING TOO HARD.
BUSBAR AND OTHER CONNECTIONS TO THE CORE MUST NOT TRANSMIT VIBRATIONS. FLEXIBILITY
IS THE KEY.
TRANSFORMER CORE CONSTRUCTIONS HELP TO A DEGREE. i.e. GOOD JOINTS, FLAT STEEL,
CONSISTENT THICKNESS, GOOD CORE SUPPORTS, FEW BOLTS ETC.
NOISE REDUCTION BY SCREENS, ENCLOSURES etc.
USE OF ACOUSTIC DAMPING MATERIAL ON TRANSFORMER WALLS OR ROOM WALLS.
THE FAN NOISE IS A FUNCTION OF SPEED AND CIRCUMFENTIAL VELOCITY. IF THE SIZE OF FANS
REDUCED, NUMBER OF FANS WILL INCREASE. THEREFORE, IT IS BETTER TO INCREASE NUMBER OF
RADIATORS.
OPERATION OF FANS AND PUMPS AS PER TEMPERATURE SETTINGS., ONLY WHEN REQUIRED

10. WHAT CAUSES TRANSFORMER VIBRATION?
• THE NOISE FROM A TRANSFORMER IS CAUSED BY MECHANICAL MOVEMENT OF THE
INDIVIDUAL LAMINATION OF THE CORE UNDER MAGNETIZATION. THE PULSATION WILL
CAUSE NOT ONLY AIR DISTURBANCES, THUS PRODUCING NOISE, BUT ALSO PHYSICAL
VIBRATION OF THE CORE STRUCTURE AND EVERYTHING ATTACHED TO IT. THE VIBRATION
WILL HAVE SIMILAR FREQUENCIES TO THOSE MEASURED IN THE NOISE ANALYSIS.
• THE PRESENCE OF HARMONICS IN LOAD CURRENT AND VOLTAGE, CAN PRODUCE
VIBRATIONS, ESPECIALLY IN RECTIFIER TRANSFORMERS.
• NOISE IS USUALLY AIR BORNE; VIBRATION IS STRUCTURE BORNE.

11. METHODS TO
REDUCE
TRANSFORMER
VIBRATION
LOOSEN ANCHOR BOLTS BETWEEN THE TRANSFORMER AND ENCLOSURE
AND LET THE UNIT REST ON VIBRATION PADS ONLY (EXCEPT ON CERIFIED
SEISMIC UNITS).
ISOLATION OF TRANSFORMER FROM THE GROUND IS VITAL.
ALL CONNECTIONS – CABLES, BUSBARS ETC. TO ENCLOSED TRANSFORMER
SHOULD BE FLEXIBLE.
EXTERNAL COOLING TO THE TRANSFORMER ENCLOSURE SHALL BE
MOUNTED INDEPENDETLY VIA ANTI-VIBRATION MOUNTINGS.
BUSHINGS VIBRATE AND LOSSES (ACOUSTIC) ARE EXPERIENCED THROUGH
THEM. FLEXIBLE ACOUSTIC PROTETCION BETWEEN BUSHINGS AND
ENCLOSURE ARE NEEDED.
DAMPING MATERIALS ARE NEEDED IF STANDING WAVES OR
REVERBERATIONS ARE POSSIBLE.

12. METHODS OF
REDUCING THE
TRANSFORMER
VIBRATION
• ISOLATE THE CORE AND
COILS OF THE
TRANSFORMER FROM THE
GROUND (IN AIR COOLED
DRY TYPE TRANSFORMERS)
OR FROM ITS TANK BASE
(IN OIL FILLED UNITS), AND
ISOLATE ITS TANK BASE
FROM THE SUPPORTING
GROUND.

13. References
• Understanding transformer noise – Federal Pacific
• Ruchi Negi,Prateek Singh,Gaurav Kr. Shah, “Causes of noise Generation & its Mitigation in
Transformer” International
Journal of Advanced Research in Electrical, Electronics and Instrumentation Engineering,Vol. 2, Issue
5, May 2013
• IEC 60076-10
• IEC 60076-1