Magnetic hysteresis is an important concept in electrical engineering, especially in the study of magnetic materials used in machines like transformers, motors, and generators. Let’s understand this phenomenon in simple terms with proper technical depth.
🔁 What is Magnetic Hysteresis?
Magnetic hysteresis can be defined as:
The lagging of magnetic flux density (B) behind the magnetising force (H) when a magnetic material is subjected to a cycle of magnetisation.
Alternatively, it refers to the property of a magnetic material where energy is lost as heat when the material undergoes magnetisation and demagnetisation repeatedly.
🧲 Basic Experiment to Understand Hysteresis
Consider an unmagnetised iron bar AB placed inside a solenoid. When current flows through the solenoid, it creates a magnetic field, and the magnetic field strength is given by:H = NI/l
Where:
- N = Number of turns
- I = Current through coil
- l = Mean length of magnetic path
As we increase the current, the magnetising force H increases, and so does the flux density B. If we plot B vs. H, we get a curve known as the magnetisation curve, starting from point O to A .
At a point, the material gets magnetically saturated (point M), and the corresponding flux density is Bmax.
Reversing the Magnetising Force
Now, if we reduce H to zero, B does not return to zero. It follows a different path (AC) and retains some magnetism even when H = 0. This retained magnetic flux density is called:
Remanence or Retentivity (Br)- To fully demagnetise the iron bar, we apply H in the reverse direction, and B becomes zero at point D. The value of H at this point is called:
- If we continue increasing H in the reverse direction, the material again gets saturated in the negative direction (point L). Reversing H again produces the curve EFGA, forming a closed loop called the Hysteresis Loop
🔁 What is a Hysteresis Loop?
The closed curve obtained during one full cycle of magnetisation (i.e. from O → A → C → D → E → F → G → A) is called the hysteresis loop.
It shows how B varies with H during a complete magnetising cycle. The shape and size of the hysteresis loop depend on the magnetic properties of the material.
🔥 Area of the Hysteresis Loop = Energy Loss
The area enclosed by the hysteresis loop represents the energy lost per cycle of magnetisation. This energy is dissipated as heat due to molecular friction inside the magnetic material and is known as hysteresis loss.
🧠 Explanation Using Molecular Theory
According to Weber’s Molecular Theory:
- Magnetising a material aligns its molecular magnets in one direction.
- Energy is used to align them.
- If the material has no retentivity, this energy can be fully recovered.
- However, in real magnetic materials, some energy is always lost due to retentivity and coercivity.
🔍 Mathematical Expression for Hysteresis Loss
Let:
l = Mean length of iron barA = Cross-sectional area
N = Number of turns
B = Flux density
H = Magnetising force
Then,
- Change in flux: Φ = B × A
- Induced EMF: e = N × dΦ/dt = N × A × dB/dt
- Magnetising current: I = Hl/N
So, Power loss:
P = e × I = NA × dB/dt × Hl/N = Al × H × dB/dt
Energy loss in time dt:
dW = Al × H × dB
For one complete cycle:
W = ∮ H dB × Al (integrated over the loop)
Thus,
✅ Hysteresis Loss per cycle = Al × (Area of the Hysteresis Loop)
- And if Al = Volume of the material, then
✅ Hysteresis Loss per m³ per cycle = Area of the B-H Loop
📌 Conclusion
- Magnetic hysteresis represents the energy loss in magnetic materials during magnetisation cycles.
- It is visualised through the hysteresis loop, where the area = energy loss.
- Materials with narrow loops (like silicon steel) have low hysteresis loss and are preferred for transformer cores and electrical machines.
📚 FAQs on Magnetic Hysteresis
Q1. What causes hysteresis in magnetic materials?
Hysteresis is caused by the internal friction between magnetic domains that resist changes in magnetisation direction.
Q2. What is the unit of hysteresis loss?
Joule per cubic meter per cycle (J/m³/cycle)
Q3. Which material has minimum hysteresis loss?
Silicon steel has a narrow hysteresis loop and hence minimum loss.
Q4. Why is hysteresis loss undesirable?
Because it causes energy loss in the form of heat, reducing the efficiency of machines.
Q5. What is coercivity?
The value of H required to reduce B to zero after it has reached saturation. It measures the resistance to demagnetisation.