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Reduce transformer magnetic leakage and electromagnetic interference
2026-07-07
Reduce transformer leakage flux and electromagnetic interference, ensuring the stable and noise-free power supply for industrial control equipment
PLC controllers, communication modules, sensor acquisition units, servo drives, industrial touch screens and other industrial control equipment have extremely high requirements for the purity of the power supply. Grid surges, harmonic distortion, transformer leakage magnetic radiation, and distributed capacitance of windings all bring electromagnetic noise, often causing a series of typical faults: analog signal drift, data acquisition jumps, frequent disconnections of 485 buses, unexpected resets of microcontrollers, and incorrect triggering of relays.
Many engineers focus their rectification efforts on adding filtering circuits at the rear end, installing common-mode inductors, and optimizing the grounding system, but they overlook that the power frequency transformer itself is the largest source of interference. EI and ordinary winding structures will generate a large amount of leakage flux, radiating alternating magnetic fields; the distributed capacitance between the primary and secondary windings will directly allow grid common-mode noise to enter the secondary side, causing distortion in the power supply waveform. Even with multiple-stage filtering at the rear end, the power supply noise cannot be completely eradicated.
To achieve long-term stable operation of the industrial control system as a whole, it is necessary to start from the power source, suppress transformer leakage magnetic radiation, cut off electric field coupling interference, and regulate the output voltage to a smooth and clean sine wave. This article starts from the generation mechanism of leakage magnetic and EMI interference, and focuses on five dimensions: core selection, winding structure, multi-layer shielding, process optimization, and overall layout. It systematically provides a complete set of solutions for reducing leakage magnetic and suppressing noise waves, making the industrial control power supply stable, smooth, and clean for a long time, and completely solving various soft faults caused by electromagnetic interference.
- Two major mechanisms for magnetic leakage and electromagnetic noise in industrial control power supplies
1. Magnetic flux leakage causes spatial magnetic field interference
During energy conversion in transformers, not all magnetic flux is confined within the iron core; some of it spreads outward, forming magnetic leakage. The alternating 50Hz magnetic field cuts through the copper foil of the PCB and signal cables, inducing alternating electromotive forces in the weak electrical circuits. In EI-type transformers, there are multiple air gaps in the magnetic path, and magnetic leakage radiation is the most severe. When the transformer is close to sensor signal lines and analog quantity acquisition circuits, the weak induced voltage will directly superimpose on the useful signal, causing the measured values to fluctuate up and down. Especially in power supply schemes with multiple branches and multi-output, the asymmetric winding arrangement will further aggravate magnetic imbalance, and the stray magnetic field continuously disturbs the weak electrical system of the entire machine.
During energy conversion in transformers, not all magnetic flux is confined within the iron core; some of it spreads outward, forming magnetic leakage. The alternating 50Hz magnetic field cuts through the copper foil of the PCB and signal cables, inducing alternating electromotive forces in the weak electrical circuits. In EI-type transformers, there are multiple air gaps in the magnetic path, and magnetic leakage radiation is the most severe. When the transformer is close to sensor signal lines and analog quantity acquisition circuits, the weak induced voltage will directly superimpose on the useful signal, causing the measured values to fluctuate up and down. Especially in power supply schemes with multiple branches and multi-output, the asymmetric winding arrangement will further aggravate magnetic imbalance, and the stray magnetic field continuously disturbs the weak electrical system of the entire machine.
At the same time, excessive leakage inductance will cause poor transient response of the load. When servo, electromagnetic valves, etc., impact loads start or stop, the leakage inductance will generate voltage spikes and waveform oscillations, causing the power supply to have sharp peaks of noise, resulting in abnormal operation of the main control chip. The larger the leakage inductance, the more severe the voltage dynamic drop and spikes, and the worse the power supply stability.
2. The primary-secondary distribution capacitance causes conductive common-mode interference
The primary and secondary windings are close to each other, resulting in parasitic capacitance of several hundred picofarads or even nanofarads. High-frequency harmonics from the power grid, lightning surges, and high-frequency noise generated by frequency converters will couple through the capacitance and directly pass through the transformer and enter the secondary low-voltage power supply circuit. This type of conductive interference is a common-mode noise, and ordinary π-type filters are difficult to completely filter out. Eventually, it manifests as numerous spikes in the output voltage waveform and uneven baseline. Once the noise enters the analog acquisition circuit, high-precision temperature control, pressure detection, and flow monitoring data will continuously drift, significantly reducing the stability of the equipment.
The leakage magnetic field causes spatial radiation interference, and the distribution capacitance causes conductive interference. The combination of the two ultimately results in dirty and messy industrial control power supply waveforms and mutual interference between strong and weak circuits. Only dealing with the end circuit and not optimizing the shielding at the transformer source will inevitably lead to repeated recurrence of electromagnetic interference problems.
- First optimization: Select low leakage magnetic cores to reduce stray magnetic fields at the source
The magnetic circuit structure of the core directly determines the basic level of leakage magnetic field and is the first threshold for electromagnetic management.
1. Priority ranking of core selection
Ring-shaped core > C-shaped plug-in core > High-laminated EI core. Ordinary open EI transformers have many air gaps and discontinuous magnetic resistance, and the magnetic flux is very likely to overflow outward, making them suitable only for power supply for strong electrical relays. For any equipment with built-in high-precision acquisition circuits and industrial communication modules, the use of ordinary EI cores should be minimized as much as possible. The ring-shaped transformer has a seamless closed magnetic circuit, and the magnetic flux is almost completely enclosed within the core, reducing the leakage magnetic radiation by more than 80% compared to ordinary EI, and the magnetic field radiation to the outside is extremely weak, not interfering with nearby weak electrical lines. For industrial control systems with compact chassis and strict EMC requirements, the ring structure is the preferred solution. If cost constraints force the use of EI type, high-conductivity oriented silicon steel sheets must be selected, and the interleaved pair insertion process must be adopted to reduce the gap between the core connections and fully compress the laminations to reduce magnetic flux leakage. Loose laminations will cause the leakage magnetic field to increase exponentially, and the electromagnetic interference will also worsen.
2. Reasonably control the magnetic flux density to avoid magnetic saturation-induced amplification interference
When the input voltage is high and remains close to full load for a long time, the core is prone to enter a magnetic saturation state. Once it is saturated, a large amount of magnetic flux rushes out of the core boundary, causing the leakage magnetic flux to surge instantly, while the excitation current becomes distorted, generating a large number of harmonic noise waves, and the power supply waveform becomes severely distorted. Most industrial control equipment requires 24-hour uninterrupted operation, and the transformer power must reserve more than 30% of the margin. Reducing the working magnetic flux density and always keeping the core working within the linear range can not only reduce the no-load harmonic current but also avoid magnetic saturation caused by load fluctuations, effectively suppressing voltage spikes and magnetic field leakage at the source.
- The second core point: Optimize the winding structure and simultaneously reduce leakage inductance and parasitic capacitance
The core of the iron core can only determine the lower limit, while the winding arrangement is the key to controlling leakage inductance and reducing distributed capacitance, and is also the core process for suppressing supply noise.
1. Adopting staggered symmetrical winding, significantly reducing leakage inductance
The traditional winding method involves winding the entire primary winding first, then the secondary winding. The coupling between the primary and secondary windings is uneven, resulting in high leakage inductance. The optimized solution adopts a "primary - secondary - primary" staggered structure: Divide the primary winding evenly into two halves, wind them separately on the inner and outer layers, and place the secondary winding in the middle. The mutual turns of the primary and secondary windings are balanced, and the magnetic fields cancel each other out, resulting in a direct reduction of leakage inductance by more than half. After reducing leakage inductance, the voltage spikes generated during load changes are effectively suppressed, the output voltage waveform becomes smoother, and there are no frequent pulse noise. For multi-output industrial control transformers, the secondary windings of each group are evenly distributed, ensuring that the coupling coefficient of each group of windings is consistent, and the output voltages of each path are synchronized and stable, avoiding voltage distortion in other circuits due to fluctuations in a single path load.
2. Reduce the spacing between the primary and secondary windings to decrease the capacitance between the windings
The insulation layer of the primary and secondary windings should not be too thick. Under the premise of meeting the creepage distance requirements for safety standards, try to make the windings closely adhere to each other to enhance magnetic coupling. At the same time, strictly control the total thickness of the coils. The more compact the windings are, the smaller the parasitic capacitance between the inner and outer layers will be. The intensity of high-frequency noise from the power grid coupling into the secondary side through the capacitance will be weakened accordingly. Multiple secondary windings are independently wound separately, and common leads are not used. Common leads will introduce common impedance. A fluctuation in one current will cause a fluctuation in the entire power supply voltage, and mutual interference between each path of power supply will occur, with the noise adding up. Each group of output windings and independent lead terminals are isolated from each other, significantly improving the stability of power supply.
3. The wires are evenly and tightly arranged to avoid local distortion of the magnetic field.
The wires are neatly and densely wound, eliminating any unevenness in the density of the local coils. Inhomogeneous coil arrangement can cause magnetic field imbalance, resulting in directional magnetic leakage and continuous radiation interference with nearby signal lines. The fully automatic and uniform wire arrangement ensures uniform magnetic field distribution around the circumference or the framework, without any concentrated magnetic flux leakage points, minimizing the interference from space radiation.
- The third key point: Multi-layer shielding structure, to cut off both conductive and radiative interference
Simply optimizing the windings can only reduce leakage inductance. To completely block the common-mode noise caused by electric field coupling, a multi-layer shielding structure must be added. This is an indispensable design for industrial control transformers to meet EMC testing requirements.
1. Add a copper foil electrostatic shielding layer between the primary and secondary windings
Wrap an oxygen-free copper foil around the primary and secondary windings to form a Faraday shielding layer, and strictly adhere to the single-point connection to the chassis ground. The copper foil shielding layer can block the distributed capacitance between the primary and secondary windings, cutting off the coupling channel of high-frequency common-mode interference. Harmonics from the frequency converter in the power grid, switch spikes, and lightning surges will be intercepted by the shielding layer and cannot penetrate the windings and enter the low-voltage industrial control circuit, ensuring a clean and flat voltage waveform on the secondary side. Key construction points: Reserve an opening at the copper foil interface, it cannot be closed to form a short circuit loop, to prevent eddy currents from causing abnormal heating; The shielding layer is only grounded at a single point, and Strictly prohibited multiple-point grounding to form a ground loop, otherwise it will introduce new AC interference.
1. Add a copper foil electrostatic shielding layer between the primary and secondary windings
Wrap an oxygen-free copper foil around the primary and secondary windings to form a Faraday shielding layer, and strictly adhere to the single-point connection to the chassis ground. The copper foil shielding layer can block the distributed capacitance between the primary and secondary windings, cutting off the coupling channel of high-frequency common-mode interference. Harmonics from the frequency converter in the power grid, switch spikes, and lightning surges will be intercepted by the shielding layer and cannot penetrate the windings and enter the low-voltage industrial control circuit, ensuring a clean and flat voltage waveform on the secondary side. Key construction points: Reserve an opening at the copper foil interface, it cannot be closed to form a short circuit loop, to prevent eddy currents from causing abnormal heating; The shielding layer is only grounded at a single point, and Strictly prohibited multiple-point grounding to form a ground loop, otherwise it will introduce new AC interference.
2. Add an iron-magnetic shielding shell on the outside to lock the residual leakage magnetic field.
For high-precision measurement and control equipment, even with optimized windings, there will still be a small amount of leakage magnetic field. By adding a low-carbon steel or Palladium alloy shielding cover outside the transformer, the residual alternating magnetic flux is confined within the shielding shell, completely blocking the magnetic field from radiating outward, and avoiding spatial magnetic induction interference with the analog signal lines. The dual-layer structure of magnetic shielding and static shielding simultaneously solves both radiation interference and conduction interference. The electromagnetic emission from the transformer itself can be reduced to an extremely low level, making the entire machine easier to pass the industrial EMC electromagnetic compatibility test.
For high-precision measurement and control equipment, even with optimized windings, there will still be a small amount of leakage magnetic field. By adding a low-carbon steel or Palladium alloy shielding cover outside the transformer, the residual alternating magnetic flux is confined within the shielding shell, completely blocking the magnetic field from radiating outward, and avoiding spatial magnetic induction interference with the analog signal lines. The dual-layer structure of magnetic shielding and static shielding simultaneously solves both radiation interference and conduction interference. The electromagnetic emission from the transformer itself can be reduced to an extremely low level, making the entire machine easier to pass the industrial EMC electromagnetic compatibility test.
3. Vacuum impregnation curing, stabilizing the electrical parameters of the windings
The coils undergo vacuum pressure impregnation treatment, and all gaps are filled with insulating paint, resulting in the windings being solidified into a rigid whole. On one hand, it avoids the electromagnetic force pulling the wires after power-on, preventing the deformation of the windings and random changes in leakage inductance; on the other hand, it can stabilize the parasitic capacitance between the windings, avoiding intermittent noise caused by long-term vibration. Loose windings not only produce high noise but also have their electrical parameters constantly drifting, causing the power supply noise to fluctuate repeatedly. After impregnation curing, the parameters of the transformer remain stable for a long time, and the power supply quality remains consistent.
The coils undergo vacuum pressure impregnation treatment, and all gaps are filled with insulating paint, resulting in the windings being solidified into a rigid whole. On one hand, it avoids the electromagnetic force pulling the wires after power-on, preventing the deformation of the windings and random changes in leakage inductance; on the other hand, it can stabilize the parasitic capacitance between the windings, avoiding intermittent noise caused by long-term vibration. Loose windings not only produce high noise but also have their electrical parameters constantly drifting, causing the power supply noise to fluctuate repeatedly. After impregnation curing, the parameters of the transformer remain stable for a long time, and the power supply quality remains consistent.
- Fourth Guarantee: External Circuitry and Overall Layout to Prevent Secondary Coupling of Interference
After the transformer has achieved leakage magnetic reduction and shielding, if the wiring of the chassis is not reasonable, residual interference will still be picked up and introduced into the weak current circuit. The initial rectification results will be greatly compromised.
1. Division of Strong and Weak Current Zones: Place the power frequency transformer in the strong current area of the chassis, away from the PLC analog quantity module, signal acquisition cables, and 485 communication bus. The alternating magnetic field decays rapidly with distance. By increasing the physical distance, the magnetic induction electromotive force can be significantly reduced. The radial magnetic field of the transformer is the strongest. Adjust the placement angle to avoid signal lines that run parallel.
2. Separate the power lines and signal lines for wiring. Do not parallel-bundle the transformer inlet and outlet lines with audio and analog signal cables to prevent the alternating magnetic field from cutting through the wires and generating induced noise. Try to keep the power lines as short as possible to reduce the area of the loop formed by the wires and minimize electromagnetic coupling.
3. Improve the grounding system and separate the chassis ground from the signal ground. The electrostatic shielding layer and magnetic shielding cover should be uniformly connected to the chassis protection ground; the weak signal ground wire should be single-point grounded, and should not be extensively short-circuited to the chassis ground to avoid ground loops that cause power frequency noise. Disordered ground connections are an important cause of voltage baseline drift in industrial control equipment.
4. Add pre-filter at the input end: Install a power filter, a voltage-sensitive resistor, and a common-mode inductor at the front end of the transformer to pre-absorb the surges and high-frequency harmonics on the power grid side, reducing waveform distortion on the high-voltage side. This reduces the source of coupling interference at the very beginning, ensuring that the alternating current input to the transformer is already sufficiently smooth.
- Common Misunderstandings in Rectification
Misconception 1: Only installing post-filtering without addressing transformer leakage flux. The spatial magnetic field is a form of radiation interference. Capacitor and inductor filtering can only mitigate conductive noise but cannot eliminate the signal drift caused by magnetic field induction. It merely treats the symptoms but not the root cause.
Misconception 2: Only performing electrostatic shielding without installing magnetic shielding. Copper foil can only block electric fields but cannot block alternating magnetic fields. The spatial interference caused by leakage flux still exists.
Misconception 3: Winding is loose and the number of turns is asymmetric, relying solely on external shielding. The imbalance of winding magnetic field will continuously emit stray magnetic flux. No matter how thick the shielding is, it is difficult to completely eliminate it.
Misconception 4: The transformer operates under full load for a long time. As the iron core approaches saturation, a large number of harmonics will be generated, the voltage waveform will be full of spikes, and all power supplies will interfere with each other, resulting in continuous deterioration of stability.
Misconception 2: Only performing electrostatic shielding without installing magnetic shielding. Copper foil can only block electric fields but cannot block alternating magnetic fields. The spatial interference caused by leakage flux still exists.
Misconception 3: Winding is loose and the number of turns is asymmetric, relying solely on external shielding. The imbalance of winding magnetic field will continuously emit stray magnetic flux. No matter how thick the shielding is, it is difficult to completely eliminate it.
Misconception 4: The transformer operates under full load for a long time. As the iron core approaches saturation, a large number of harmonics will be generated, the voltage waveform will be full of spikes, and all power supplies will interfere with each other, resulting in continuous deterioration of stability.
The noise and electromagnetic interference of industrial control power supplies can be mainly classified into two types: the leakage magnetic radiation caused by asymmetric windings, and the common-mode conducted noise brought by the parasitic capacitance between the primary and secondary coils. To maintain a long-term stable and clean power supply, the principle of source control must be followed.
Firstly, choose a closed magnetic circuit ring or high-quality laminated EI iron core, and reserve sufficient power margin to prevent the generation of harmonics due to magnetic saturation; secondly, adopt a sandwich symmetrical winding process to significantly reduce leakage inductance and suppress the transient voltage spikes of the load; then, through copper foil electrostatic shielding to cut off the electric field coupling, and combine with an outer ferromagnetic cover to lock the leakage magnetic field, achieving dual suppression of conducted interference and radiation interference; finally, cooperate with vacuum impregnation to stabilize the winding structure, ensuring that the electrical parameters remain unchanged for a long time; lastly, optimize the layout of the strong and weak electrical components in the chassis and the grounding method, to block the secondary coupling of interference. Control the electromagnetic emission of the transformer itself well, and the output voltage waveform will naturally be smooth and flat, without harmonic waves at power frequency, instantaneous spikes, or baseline drift. Only when the source of the power supply is low in leakage magnetic field and low in interference, can PLC, sensors, and industrial communication modules work stably, completely solving stubborn soft faults such as signal jumps, communication disconnections, and chip misreset, and ensuring the continuous and stable operation of the entire industrial control system.
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