The principle and basic rectification of series excited motor EMC

What is EMC

Electromagnetic Compatibility (EMC) refers to the ability of a device or system to operate in its electromagnetic environment in compliance with requirements without causing intolerable electromagnetic interference to any device in its environment. Therefore, EMC includes two requirements: on the one hand, it refers to the requirement that the electromagnetic interference (Electromagnetic Disturbance) generated by the equipment to the environment during normal operation cannot exceed a certain limit; On the other hand, it refers to the device having a certain degree of immunity to electromagnetic interference present in the environment, that is, electromagnetic susceptibility.

Since the emergence of electronic system noise reduction technology in the mid-1970s, it has been mainly due to regulations on commercial digital products proposed by the Federal Communications Commission in the United States in 1990 and the European Union in 1992, which require companies to ensure that their products comply with strict magnetization and emission criteria. Products that comply with these regulations are called Electromagnetic Compatibility (EMC).

The History of EMC

In the past, research on electromagnetic compatibility was not rigorous outside of the military field, and most equipment manufacturers were not concerned about electromagnetic compatibility issues. But with the rapid increase in clock frequency of modern digital devices using lower signal voltages, electromagnetic compatibility issues have become increasingly important. Many countries have realized this prominent issue and issued decrees to relevant equipment manufacturers, requiring that only equipment that meets basic conditions can be sold.

Corresponding organizations in various countries have begun to develop standards and maintain government directives, among which well-known national organizations include FCC in the United States, CEN, CENELEC and ETSI in Europe, and BSI in the United Kingdom. There are also numerous international organizations dedicated to promoting international cooperation on standardization issues, including electromagnetic compatibility standards.

The most important international organization among them is the International Electrotechnical Commission (IEC), which has multiple full-time chapters on electromagnetic compatibility issues. The ACEC, the advisory committee for capacitor compatibility issues, coordinates these sub committees in IEC.

The hazards of EMC

1) In 1967, the RF energy emitted by the high-power radar on a US warship was coupled to the motor drive circuit of a missile rocket mounted on an aircraft, causing the motor to start, ignite the missile rocket, and trigger other missiles on the deck. Causing 134 people to lose, resulting in losses of up to 720 billion US dollars;

2) In 1980, radio frequency energy emitted from a land launch pad in Germany interfered with an avionics device, causing a military aircraft to explode;

3) In daily life, when we watch TV, if someone next to us uses household appliances such as hair dryers or razors, annoying snowflake stripes will appear on the screen. The rice cooker cannot cook cooked rice, and the air conditioner, which was turned off, starts up on its own after a while.

Introduction to EMC

Electromagnetic compatibility (EMC) is a discipline in which various electrical devices (including living organisms in a broad sense) can coexist within limited space, time, and spectrum resources without causing degradation. It includes two parts: electromagnetic interference and electromagnetic sensitivity. Electromagnetic interference testing measures the magnitude of the electromagnetic wave signal generated and emitted by the tested device under normal working conditions to reflect the strength of the interference on surrounding electronic devices. Electromagnetic sensitivity testing measures the ability of the tested equipment to resist electromagnetic interference. Electromagnetic interference mainly includes radiated emissions and conducted emissions.

Electromagnetic compatibility (EMC) is a comprehensive evaluation of the interference level (EMI) and anti-interference ability (EMS) of electronic products in electromagnetic fields. It is one of the most important indicators of product quality, and the measurement of EMC is composed of testing sites and testing instruments.

EMC includes two major components: EMI (interference) and EMS (sensitivity, anti-interference).
EMI
 testing components include:

l RE (radiation, emission)

l CE (Conducted Interference)

l Harmonic (harmonic)

l Flicker (flashing)

The EMS testing items include:

l ESD (electrostatic discharge)

l EFT (Transient Pulse Interference)

l DIP (Voltage Drop)

l CS (Conducted Anti Interference)

l RS (radiation anti-interference)

l Surge (surge, lightning strike)

l PMS (Power Frequency Magnetic Field Disturbance)

Introduction to Five Electromagnetic Interference

Electromagnetic interference (EMI), abbreviated as EMI, has two types: conducted interference and radiated interference. Conducted interference mainly refers to the interference signals generated by electronic devices that interfere with each other through conductive media or common power lines; Radiation interference refers to the transmission of interference signals generated by electronic devices to another electrical network or electronic device through spatial coupling. In order to prevent electromagnetic interference generated by some electronic products from affecting or damaging the normal operation of other electronic devices, governments of various countries or international organizations have successively proposed or formulated regulations or standards related to electromagnetic interference generated by electronic products. Products that comply with these regulations or standards can be called electromagnetic compatibility (EMC). Electromagnetic compatibility (EMC) standards are not constant, but constantly changing, which is also a common means adopted by governments or economic organizations around the world to protect their own interests.

Formation of six electromagnetic interferences

1) Interference source and interfered source

In any case, there are always two complementary aspects to electromagnetic compatibility issues: one is interference with the emission source, and the other is interference with sensitive and sensitive interference devices. If an interference source and the interfered device are both in the same device, it is called the internal EMC situation of the system, and the interference situation between different devices is called the inter system EMC situation. Most devices have antenna like components such as cables, PCB wiring, internal wiring, mechanical structures, etc. These components transfer energy through electric, magnetic, or electromagnetic fields coupled by circuits. In practical situations, the coupling between equipment and the interior of cargo equipment is severely affected by shielding and insulation materials, while the absorption of insulation materials is negligible compared to the cutting head. The coupling between cables can be capacitive or inductive and depends on the influence of orientation, length, and proximity.

2) Coupling of common impedance

The common impedance coupling line is caused by the interference source and the interfered equipment sharing the same circuit impedance. Public conductors are also caused by mutual inductance between two current loops or mutual capacitance coupling between two voltage nodes. The solution to the coupling of conductive common impedance is to separate the connecting lines so that the systems are independent and avoid forming a common impedance.

3) Launch

Emission from PCB: In most devices, the main source of current is flowing into the circuits on the PCB board, and this energy is radiated out through the antenna simulated by the PCB board.

Radiation from cable lines: Interference current is generated in common mode from ground noise formed at PCB and other locations of equipment, and flows along the shielding layer of conductors or shielded cables.

Conducted emission: Interference may also be coupled to the cable line in an inductive or capacitive manner from other cables.

The interference generated may appear in differential mode (between live and neutral lines or between signal lines) or common mode (between live/neutral/signal lines and ground), or in a mixed form of both. For the power supply port, it is necessary to measure the voltage between each phase/neutral line and the ground at the far end of the power cable. Differential mode emission is usually associated with low-frequency switching noise from the power supply. Common mode emission is caused by higher frequency switching elements, internal circuit sources, or internal coupling of cables. The distribution of capacitance in circuits is extensive. If there is no shielding object, it depends on the degree of proximity to other objects. Due to the high capacitance of the surrounding environment, partially shielded enclosures can actually exacerbate coupling.

Seven EMC rectification measures

Firstly, it is necessary to diagnose the product based on the actual situation, analyze the sources of interference and the ways and means of mutual interference. Based on the analysis results, targeted rectification will be carried out.

Generally speaking, the main rectification methods are as follows.

1. Weaken the interference source

On the basis of finding the interference source, the interference source can be weakened within the allowable range. The methods for weakening the source generally include the following:

1) Add a decoupling capacitor between the VCC and GND of the IC, with a capacity between 0.01 μ F and 0.1 μ F. When installing, pay attention to the capacitor leads to make them as short as possible.

2) Add an attenuator while ensuring sensitivity and signal-to-noise ratio. Crystal oscillators in VCD and DVD players have a serious impact on electromagnetic compatibility, and reducing their amplitude is one feasible method, but it is not the only solution.

3) Another indirect method is to keep the signal line away from the interference source.

Classification and organization of wires and cables

In electronic devices, line to line coupling is an important pathway and a significant cause of interference. Due to frequency factors, it can be roughly divided into high-frequency coupling and low-frequency coupling. Due to different coupling methods, the rectification methods are also different. The following will discuss them separately:

1) Low frequency coupling

Low frequency coupling refers to the situation where the length of the wire is equal to or less than 1/16 wavelength. Low frequency coupling can be divided into electric field and magnetic field coupling. The physical model of electric field coupling is capacitive coupling. Therefore, the main purpose of rectification is to reduce the distributed coupling capacitance or the coupling amount. The following methods can be used:

l Increasing the circuit spacing is the most effective method to reduce distributed capacitance;  

l Add a highly conductive shielding cover and ensure that the single point grounding of the shielding cover can effectively suppress low-frequency electric field interference;

l Adding a filter can reduce the coupling between the two circuits;

l Reduce input impedance, for example, the input impedance of CMOS circuits is very high and extremely sensitive to electric field interference. It is possible to connect a capacitor or a resistor with a lower resistance value in parallel at the input end within the allowable range.  

The physical model of magnetic field coupling is inductive coupling, which is mainly coupled through distributed mutual inductance between lines. Therefore, the main method of rectification is to destroy or reduce its coupling amount. Generally, the following methods can be adopted:

l When adding a filter, attention should be paid to the input and output impedance of the filter and its frequency impact;

l Reduce the loop area of sensitive circuits, that is, try to make the signal line cargo flow line and its return line as close or twisted together as possible;

l Increase the distance between two circuits to reduce mutual inductance between lines and decrease coupling;

l If possible, try to make the sensitive circuit orthogonal or nearly orthogonal to the source circuit plane to reduce the coupling between the two circuits;

l Using high magnetic conductivity materials to wrap sensitive wires can effectively solve the problem of magnetic field interference. It is worth noting that a closed magnetic circuit should be constructed, and efforts to reduce the magnetic resistance of the magnetic circuit will be more effective.

2) High frequency coupling

High frequency coupling refers to the increased coupling of wires with wavelengths greater than 1/4 due to standing waves of voltage and current in the circuit. The following methods can be used to solve this problem:

l Try to shorten the grounding wire as much as possible and use surface contact with the shell grounding as much as possible;

l Reorganize the input and output lines of the filter to prevent coupling between them and ensure that the filtering effect of the filter remains unchanged

l The shielding layer of the shielded cable adopts multi-point grounding

l Connect the suspended pins of the connector to ground potential to prevent antenna effects

3. Improve the grounding system

The ideal ground wire is a physical entity with zero impedance and zero potential. It is not only a reference point for signals, but also does not produce a voltage drop when current flows through it. In specific electrical and electronic devices, this ideal ground wire does not exist. When the current flows through the ground wire, a voltage drop will inevitably occur. Based on this, the mechanism of interference in the ground wire can be attributed to the following two points: first, reducing low impedance and power supply line impedance. Secondly, choose the correct grounding method and barrier grounding loop. According to the grounding method, there are floating grounding, single point grounding, multi-point grounding, and mixed grounding. If the interference of sensitive lines mainly comes from external space or system casing, floating ground can be used to solve it. However, floating ground equipment is prone to static electricity accumulation, and when the charge reaches a certain level, static discharge will occur. Therefore, floating ground is not suitable for general electronic devices. Single point grounding is suitable for low-frequency circuits. To prevent potential differences between points on the signal ground caused by power frequency currents and other stray currents, the signal ground is isolated from the power supply and safety ground, and a single point connection is made at the grounding point of the power supply line. Single point grounding is mainly suitable for frequencies below 3MHz. Multi point grounding is the only practical grounding method for high-frequency signals, which exhibits transmission line characteristics during radio frequency. To ensure the effectiveness of multi-point grounding, when the length of the grounding wire exceeds 1/8 wavelength of the highest frequency, an equipotential grounding plane is required for multi-point grounding. Multi point grounding is suitable for frequencies above 300KHz. Hybrid grounding is suitable for electronic circuits with both high and low frequencies.

4 shielding

Shielding is one of the important measures to improve the electromagnetic compatibility of resistance systems and electronic devices, which can effectively suppress various electromagnetic interferences propagated through space. Shielding can be divided into magnetic field shielding and electromagnetic shielding according to their mechanisms. The following points should be noted for electric field shielding:

l Choose materials with high conductivity and good wiring;

l Choose the correct grounding point and a reasonable shape, preferably with the shielding body directly grounded.

Magnetic field shielding usually refers to the shielding of DC or very low frequency magnetic fields, and its shielding effectiveness is far inferior to electric field shielding and electromagnetic shielding. Magnetic shielding is often the focus of engineering, and when it comes to magnetic shielding:

l To choose ferromagnetic materials

l The magnetic shielding body should be kept away from magnetic components to prevent magnetic short circuits

l Double layer shielding or even triple layer shielding can be used

l Attention should be paid to the direction of the openings on the shielding body, and the longer side of the seam should be parallel to the direction of the magnetic field flow as much as possible to minimize the increase in magnetic circuit length.

Generally speaking, magnetic shielding does not require grounding, but to prevent electric field induction, grounding is still better. When the electromagnetic field passes through a metal or a barrier that attenuates the electromagnetic field, it will experience a certain degree of attenuation, which produces a shielding effect on the electromagnetic field. In the actual rectification process, the shielding and shielding body shape, size, grounding method, etc. should be selected according to specific needs.

5. Change the wiring structure of the circuit board

Some frequency points are determined by the distribution parameters of the wiring on the circuit board, and the aforementioned methods are not very useful. Such rectification involves adding small inductors, capacitors, and magnetic beads in the wiring to change the circuit parameter structure and move it to frequency points with higher limit requirements. To fundamentally address the impact of such interference, rewiring is necessary.

Basic rectification of EMC for eight AC small household appliance motors

1. The EMC testing standard for AC small household appliance motors can refer to EN 55014

2. AC motor EMC testing mainly tests RP (radiation) and CE (conducted interference)

1) CE is low frequency, with a test frequency between 0.15~30MHz, divided into neutral and live wires

Differential mode interference is the main interference with frequencies between 0.15~0.5MHz. X2 capacitor can be connected in parallel on the power line and improved by changing the capacity of X2 capacitor. For motors, X2 capacitor should be placed inside the body as much as possible. The coexistence of frequency difference and common mode interference between 0.5~30MHz can be improved by increasing the Y2 capacitor and adjusting its capacity. The role of capacitors in circuits is to block direct current, connect alternating current, and prevent low frequencies.

2) RP is high frequency, with a testing frequency between 30~3000MHz (as shown in the figure below)

Common mode interference is predominant between frequencies of 30~3000MHz, which can be improved by connecting inductors in series in the coil. The main function of inductors is to block AC, connect DC, and prevent high frequencies.

3. There are also several ways to make adjustments:

1) Turning the commutator to reduce the number of cutting lines, roundness, and inter piece runout during commutator turning;

2) Choose carbon brushes with anti EMC function (low density, high resistivity), which can make the end face of the carbon brush serrated;

3) Adjust the motor winding parameters and reduce the motor performance appropriately.

Summary: When carrying out EMI rectification on motors, do not expect one component to solve all problems. Because the radiation problem of the motor covers the entire frequency band, it is not possible to achieve such a large filtering frequency band without that component. We need to analyze the testing and make targeted improvements. In addition, some subtle adjustments can be made to the material and structure of the motor itself, such as reducing the copper content of the carbon brush and commutator of the motor, and so on. Because the entire radiation is fundamentally caused by the motor itself, solving the problem from the root is the most effective. External auxiliary circuits also exist to reduce the radiation of the motor. Therefore, when developing motors, it is necessary to confirm some parameter combinations of the motor and how to maximize the reduction of EMI problems, rather than making improvements after production!