MAGNETIC HYSTERESIS & ITS IMPORTANCE. - ELECTRICAL ENCYCLOPEDIA

MAGNETIC HYSTERESIS & ITS IMPORTANCE.

Magnetic Hysteresis — Definition, B-H Curve, Loop Types & Importance

Magnetic hysteresis is a fundamental phenomenon in ferromagnetic materials where the flux density (B) lags behind the magnetizing force (H). Understanding hysteresis is critical for designing transformers, electric motors, and permanent magnets — it directly determines core losses and material selection in electrical machines.

What is Magnetic Hysteresis?

Magnetic hysteresis is the property of a ferromagnetic material due to which energy is dissipated when the material undergoes cyclic magnetization. The word "hysteresis" comes from the Greek word meaning "to lag" — referring to how flux density B always lags behind the applied magnetizing force H.

When an external magnetic field magnetizes a ferromagnetic material (like iron, cobalt, or nickel), the internal magnetic domains align with the field. However, when the field is removed, the domains do not return completely to their original random orientation. This irreversibility creates the characteristic hysteresis loop.

B-H Curve Explained Step by Step

To understand hysteresis, consider an unmagnetized iron bar placed inside a solenoid. By varying the current through the solenoid, we control the magnetizing force H and measure the resulting flux density B.


Circuit arrangement for plotting the hysteresis curve

Step 1 — Initial Magnetization (O to A): Starting from zero, as H increases, B increases along curve OA. The material reaches magnetic saturation at point A where all domains are aligned.

Step 2 — Reducing H to Zero (A to C): When H is decreased back to zero, B does not follow the same path. Instead, it follows curve AC. At point C (H = 0), the material retains some magnetism — this is the residual flux density (retentivity).

Step 3 — Reversing H (C to D): To completely demagnetize the bar, H must be applied in the reverse direction. The value of reverse H needed to bring B to zero is called coercivity (point D).

Step 4 — Negative Saturation (D to E): Continuing to increase reverse H drives the material to negative saturation at point E.

Step 5 — Completing the Loop (E to F to A): Bringing H back from negative saturation traces the lower half of the loop, completing the closed curve ACDEFGA.


Complete B-H hysteresis loop

Key Terms — Retentivity, Coercivity & Saturation

  • Retentivity (Br): The residual flux density remaining in the material when the magnetizing force is reduced to zero. High retentivity materials make good permanent magnets.
  • Coercivity (Hc): The reverse magnetizing force required to reduce the flux density to zero. High coercivity means the material resists demagnetization.
  • Saturation (Bsat): The maximum flux density achievable when all magnetic domains are fully aligned. Beyond this point, increasing H produces negligible increase in B.
  • Permeability (μ): The ratio B/H indicating how easily a material can be magnetized. Higher permeability means less H is needed for a given B.

Hysteresis Loss & Steinmetz Equation

The area enclosed by the hysteresis loop represents the energy lost per unit volume per cycle of magnetization. This energy dissipates as heat, raising the core temperature. For AC applications where the field reverses 50–60 times per second, minimizing this loss is critical.

Wh = η × Bmax1.6 × f × V  (watts)

Where:

  • η (eta) = Steinmetz hysteresis coefficient (depends on material)
  • Bmax = Maximum flux density (Tesla)
  • f = Frequency of magnetization (Hz)
  • V = Volume of the core (m³)

The exponent 1.6 (Steinmetz exponent) is empirical and varies between 1.5 to 2.5 depending on the material. Silicon steel used in transformers has η ≈ 500 J/m³, while cast iron has η ≈ 3000 J/m³.

Types of Hysteresis Loops


Three different types of hysteresis loops

Loop 1 — Wide Loop (Hard Magnetic Materials): High retentivity and high coercivity. Large loop area means high energy loss per cycle. Materials: ALNICO, neodymium, cobalt steel. Application: Permanent magnets.

Loop 2 — Medium Loop (Semi-Hard Materials): Moderate retentivity with fair coercivity and high permeability. Materials: Wrought iron, cast steel. Application: Electromagnet cores, relays.

Loop 3 — Narrow Loop (Soft Magnetic Materials): Low retentivity, low coercivity, high permeability, and minimal hysteresis loss. Materials: Silicon steel (CRGO), permalloy, ferrites. Application: Transformer cores, motor armatures, inductors.

Material Comparison Table

Material Retentivity Coercivity Loop Area Application
ALNICO / Neodymium High High Large Permanent magnets
Wrought Iron / Cast Steel Medium Medium Medium Electromagnets, relays
Silicon Steel (CRGO) Low Low Small Transformer cores
Permalloy (Ni-Fe) Very Low Very Low Very Small Magnetic shielding, sensors
Ferrite Low Medium Small High-frequency inductors

Importance of Hysteresis in Electrical Engineering

  • Transformer Design: Core material must have a narrow hysteresis loop to minimize heat generation. CRGO silicon steel reduces hysteresis loss by up to 75% compared to ordinary iron.
  • Electric Motor Efficiency: Armature cores undergo rapid magnetization reversals. Low-loss materials directly improve motor efficiency ratings (IE3/IE4 class).
  • Permanent Magnet Selection: Wide hysteresis loop materials retain magnetism under vibration and temperature changes — essential for speakers, sensors, and magnetic latches.
  • Magnetic Recording: Hard disk drives and magnetic tapes rely on high coercivity to retain data reliably over time.
  • Power System Economics: In India's grid operating at 50 Hz, hysteresis losses in distribution transformers account for 60–70% of no-load losses, directly impacting electricity costs.

How to Reduce Hysteresis Loss

  • Use soft magnetic materials with narrow B-H loops (silicon steel, amorphous metals)
  • Add silicon (3–4%) to iron — increases resistivity and narrows the loop
  • Use Cold Rolled Grain Oriented (CRGO) steel with grains aligned in the rolling direction
  • Anneal the core material to relieve mechanical stress that widens the loop
  • Use laminated cores to also reduce eddy current losses simultaneously
  • For high-frequency applications (>1 kHz), use ferrite cores instead of steel

Frequently Asked Questions

1. What is the difference between hysteresis loss and eddy current loss?

Hysteresis loss occurs due to molecular friction during domain realignment in each magnetization cycle — it depends on B1.6 and frequency. Eddy current loss occurs due to circulating currents induced in the core — it depends on B² and f². Laminating the core reduces eddy current loss but not hysteresis loss.

2. Why does B lag behind H in magnetic materials?

The lag occurs because magnetic domains resist realignment due to internal friction between domain walls. Energy is required to move domain boundaries (Bloch walls), and this energy is not fully recovered when the field reverses, creating the characteristic lag.

3. What is the unit of hysteresis loss?

Hysteresis loss is measured in Joules per cubic metre per cycle (J/m³/cycle). When multiplied by frequency, it gives power loss in Watts per cubic metre (W/m³). For practical transformer calculations, it is expressed in Watts per kilogram (W/kg) at rated flux density and frequency.

4. Why is silicon added to steel for transformer cores?

Silicon (3–4%) increases the electrical resistivity of steel (reducing eddy currents), narrows the hysteresis loop (reducing hysteresis loss), and increases permeability. However, silicon above 4.5% makes the steel brittle and difficult to roll into thin laminations.

5. Can hysteresis loss be completely eliminated?

No. Any ferromagnetic material will exhibit some hysteresis. However, it can be minimized using amorphous metals (metallic glasses) which have no crystal structure — their hysteresis loss is about 70–80% lower than conventional CRGO silicon steel. Superconducting materials at cryogenic temperatures show near-zero hysteresis.

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