Engineering Principle

Active magnetics is a method to synthesize inductive behavior using a controlled power-electronic system, instead of relying solely on magnetic energy storage.

In a conventional inductor, the impedance is defined by:

v(t)=L⋅di(t)dtv(t)=L⋅dtdi(t)​

This requires physical energy storage in a magnetic field proportional to L⋅i2L⋅i2, driving the need for large cores and copper windings.

In contrast, an active inductor separates the electrical function (impedance shaping) from physical energy storage by introducing a controlled auxiliary circuit. The system dynamically injects a compensating voltage and current such that the terminal behavior matches that of a much larger passive inductor over the frequency range of interest. 

System Architecture

An active magnetics implementation typically consists of four tightly coupled subsystems:

1. Reduced Physical Inductor Core

A small inductance (often 20–30% of the equivalent passive value) handles the high-frequency ripple and provides a physical current path. 

2. Power Electronic Conversion Stage

A bidirectional converter (typically H-bridge or full-bridge topology) generates the required compensating voltage across the terminals.

The converter injects a controlled current component that is phase-shifted relative to the ripple content, effectively canceling undesired harmonics in the main current path.

3. Energy Buffer (DC-side Capacitor)

A capacitor provides temporary energy storage to enable bidirectional power flow within the auxiliary circuit. It supports the emulation of inductive energy exchange without requiring large magnetic storage. 

4. Control and Measurement System

The control system is the defining element. It continuously measures:

• terminal voltage

• inductor current

• harmonic content

Based on this, it computes a reference current that enforces the desired impedance.

Typical control structure includes:

• inner current loop (fast, hysteresis or PWM-based)

• outer voltage loop (stabilization and loss compensation) 

Impedance Synthesis Mechanism

The effective impedance seen from the terminals is determined by the interaction between the physical inductor and the controlled current source:

Zactive(s)=L1s1+Gr(s)Zactive​(s)=1+Gr​(s)L1​s​

Where:

• L1L1​ = physical inductance

• Gr(s)Gr​(s) = control transfer function

By shaping Gr(s)Gr​(s), the system can extend the apparent inductance beyond the physical limitation of the core. 

Minimal Energy Processing Principle

A key innovation in this architecture is that the auxiliary circuit processes only the ripple components:

• it handles a fraction of the total current

• it sees only low-frequency harmonic voltage components

This results in a minimum apparent power requirement, significantly improving efficiency and reducing semiconductor stress compared to earlier active inductor concepts. 

Dynamic and Adaptive Behavior

Unlike passive inductors with fixed impedance, active magnetics enables real-time impedance control.

The equivalent inductance LeqLeq​ becomes a controlled variable:

• increased under high harmonic distortion

• reduced under light load

This is achieved through adaptive control strategies that monitor:

• THD

• DC-link ripple

• system operating conditions

The system adjusts inductance dynamically to maintain performance targets across varying conditions. 

Multi-Objective Control (Advanced Implementation)

Advanced implementations introduce multi-loop control strategies, where the system simultaneously optimizes:

• grid-side current distortion (THD)

• DC-link ripple

• system stability under imbalance

A supervisory control layer selects the dominant objective depending on operating conditions, enabling robust performance even under:

• grid voltage imbalance

• load transients

• harmonic disturbances

Integration into Power Electronic Systems

Active magnetics is not a standalone laboratory concept—it is designed to integrate directly into standard topologies:

• DC-link filtering in motor drives

• AC grid-side filtering

• rectifier/inverter systems

From a system perspective, it behaves as a two-terminal plug-and-play component with identical electrical interfacing as a passive inductor.

Engineering Implications

Decoupling Function from Physics

The key paradigm shift is that inductance is no longer constrained by magnetic material limits. Instead, it becomes a software-defined parameter.

Performance Scalability

• Higher power density

• Reduced material dependence (copper, ferrite)

• Improved system integration flexibility

System-Level Impact

• Reduced footprint and weight

• Improved power quality

• Reduced stress on capacitors and downstream components

Summary

Active magnetics replaces conventional inductors by synthesizing their electrical behavior using a controlled power-electronic architecture.

It combines:

• reduced physical inductance

• power conversion

• real-time control

to achieve:

• equivalent impedance behavior

• significantly lower energy storage

• adaptive system performance

This transforms the inductor from a passive constraint into an active, controllable system element, enabling a new level of integration and optimization in modern power electronics.