WHY DOES A TRANSFORMER HUM?

(Detailed Explanation with Engineering Insight & Relevant IS Standards)

A transformer humming sound is a normal operating characteristic, especially in power and distribution transformers used in substations and solar plants.

However, the physics behind it is interesting — and important for design, testing, and maintenance teams.

The primary cause is magnetostriction, supported by mechanical vibrations inside the transformer.

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1️⃣ Magnetostriction – The Primary Cause

🔹 What Happens Inside the Core?

A transformer core is made of laminated silicon steel, a ferromagnetic material.

When AC voltage is applied:

• Alternating magnetic flux is produced in the core.

• The magnetic domains inside the steel continuously realign.

• Due to magnetostriction, the core slightly changes its physical dimensions when magnetized.

🔄 Expansion–Contraction Cycle

• Flux increases → Core expands slightly

• Flux decreases or reverses → Core contracts

Since AC reverses every half cycle:

• At 50 Hz supply → vibration occurs at 100 Hz

• At 60 Hz supply → vibration occurs at 120 Hz

This double-frequency vibration produces the familiar humming sound.

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📘 Relevant IS Standards

🔹 IS 2026 (Part 1)

Specifies performance requirements and permissible noise levels for power transformers.

🔹 IS 1180

Covers distribution transformer design and construction, including noise considerations.

🔹 IS 10028

Provides guidance on installation and operational practices affecting vibration and noise.

These standards define acceptable sound pressure levels (dB) depending on transformer rating.

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2️⃣ Winding Vibrations

🔹 Electromagnetic Forces

When load current flows:

• Magnetic fields interact between conductors.

• Radial and axial forces develop inside the windings.

• Mechanical stress increases with load current.

If windings are not:

• Properly clamped

• Adequately braced

• Correctly impregnated

They may vibrate, increasing noise levels.

📌 Important Insight:

Core hum exists even at no-load (voltage dependent).
Winding noise increases with load current.

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3️⃣ Loose Core Laminations

Transformer cores are built from thin insulated laminations to:

• Reduce eddy current losses

• Improve efficiency

If laminations are not tightly clamped:

• Microscopic gaps form

• Individual sheets vibrate separately

• Noise level increases significantly

Proper clamping pressure during manufacturing is essential.

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4️⃣ Tank and Structural Resonance

Vibrations from the core and windings are transmitted to:

• Transformer tank

• Radiators

• Mounting frame

• Foundation

The transformer tank may act like a sound amplifier (sounding board).

If:

• Mounting bolts are loose

• Foundation is uneven

• Anti-vibration pads are missing

Noise may become excessive.

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5️⃣ Effect of Voltage and Load

🔹 Voltage Effect (Main Influence)

Transformer hum depends mainly on applied voltage, not load.

Magnetic flux ∝ Applied voltage

If voltage increases:

• Flux density increases

• Core approaches saturation

• Magnetostriction increases

• Noise level rises sharply

Overvoltage condition = noticeable louder hum.

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🔹 Load Effect

Heavy load causes:

• Higher winding current

• Greater electromagnetic forces

• Additional mechanical vibration

But load-related noise is usually secondary compared to core hum.

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6️⃣ When is Transformer Noise Abnormal?

Normal humming:
✔ Steady
✔ Low frequency (100 Hz in India)
✔ Uniform sound

Abnormal noise may indicate:

❌ Loose core bolts
❌ Loose clamping structure
❌ Overvoltage
❌ Shorted laminations
❌ Mechanical displacement after fault

IS standards specify acceptable noise limits in dB depending on kVA rating. Excess beyond limits requires inspection.

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7️⃣ Practical Site Insight (Solar Plant Context)

In solar pooling substations:

• Higher inverter harmonics may increase audible noise

• Poor earthing may amplify vibration

• Plinth-mounted transformers need vibration pads

• Always check bolt tightening during annual shutdown

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🔎 Engineering Summary

Transformer hum is primarily caused by:

1️⃣ Magnetostriction (Main Cause)

Core expands and contracts twice per cycle → 100 Hz vibration (in India)

2️⃣ Winding Electromagnetic Forces

Load-dependent vibration

3️⃣ Loose Laminations

Increase sound intensity

4️⃣ Tank Resonance

Amplifies vibration

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📌 Key Technical Conclusion

Transformer hum is:

• ✔ Normal and unavoidable

• ✔ Voltage-dependent

• ✔ Occurs at double supply frequency

• ✔ Controlled through proper design and clamping

However, excessive noise is not normal and must be investigated.

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