In this video prof. Sam Ben-Yaakov provides deep examination of inner transformer process to understand how transformer works in details.
Transformer is a basic device that is used in many electrical, electronic and power electronic systems. It is used in multiple applications for power transfer and sensing.
It would seem however, that since we take it for granted, some might have lost the understanding how exactly does the transformer transfer voltage, power and in particular current.
This presentation video dives into inner working of the transformer using simulation tools.
Introduction
Transformers are foundational components in electrical and power electronics systems, yet their internal mechanisms—particularly the interplay between voltage, current, flux, and magnetomotive force (MMF)—are often misunderstood. This presentation explores the inner workings of transformers using reluctance-based magnetic circuit modeling and LTspice simulations, offering a deeper understanding of how energy transfer and magnetization occur.
Key Points
- Reluctance modeling reveals the magnetic circuit behavior of transformers.
- Voltage and current transfer mechanisms are orthogonal and governed by distinct laws.
- MMF differences drive flux and explain magnetization current requirements.
- Simulation validates theoretical predictions and highlights transient dynamics.
Reluctance Modeling of Magnetic Circuits
The transformer core is modeled as a magnetic circuit using the concept of reluctance, analogous to electrical resistance. The MMF is treated as a voltage source, reluctance as resistance, and magnetic flux as current. This analogy enables simulation using standard circuit tools like LTspice.
Where:
- l = magnetic path length
- A = cross-sectional area
- μ = permeability of the core material
Flux and MMF Relationship
This duality is resolved by modeling the magnetic circuit explicitly, showing that voltage induces flux, which in turn generates MMF and current. The simulation confirms that flux lags voltage by 90°, indicating purely reactive power in unloaded conditions.
Simulation of Magnetization
Using LTspice, a one-turn inductor is excited by a sinusoidal voltage source. The resulting flux, MMF, and current are analyzed. The simulation shows:
| Parameter | Value | Units |
|---|---|---|
| Peak Current | 0.5 | mA |
| Peak-to-Peak Flux | 320 | nWb |
| Inductance | 31 | mH |
These values match theoretical predictions, validating the reluctance model.
Power Transfer in Loaded Transformer
Adding a secondary winding with a resistive load introduces real power transfer. The simulation reveals:
- Voltage induces flux (reactive component).
- Flux induces MMF in secondary (via Lenz’s Law).
- MMF generates current in the load (real power component).
The MMFs of primary and secondary oppose each other, with a small difference driving the magnetization flux. This explains why input current exceeds output current slightly—due to magnetization requirements.
MMF Balance and Flux Drive
The difference in MMF between primary and secondary is responsible for sustaining flux:
This difference is small but essential. It corresponds to the magnetization current and is independent of load, governed by core reluctance.
Impact of DC Bias
Introducing DC to the secondary causes flux offset, increasing the risk of core saturation. For example:
| Condition | Flux |
|---|---|
| Without DC | 330 nWb |
| With 100 mA DC | 2.8 µWb |
This highlights the importance of avoiding DC bias in transformer applications.
Conclusion
This paper demonstrates that transformer behavior is governed by two orthogonal processes: voltage-induced flux and current-induced MMF. Reluctance modeling and simulation reveal the subtleties of magnetization, power transfer, and MMF balance. Understanding these principles is essential for accurate transformer design and analysis, especially in high-frequency or precision applications.