How to reduce the noise of the motor to meet electromagnetic compatibility standards?
Motors, especially those with electric brushes, generate a significant amount of noise. Electrical appliances must meet the requirements of electromagnetic compatibility standards by processing these noises. The means to solve electromagnetic compatibility are nothing but capacitors, inductors (chokes), power filters, and grounding.

Unfortunately, electromagnetic compatibility issues are usually discovered when the product has been thoroughly designed and assembled. It is very difficult to consider electromagnetic compatibility at this time. Manufacturers are not only facing time constraints but also project budgets have been exhausted, and responsible engineers have been transferred to other projects, unable to solve related problems at any time.

The best time to address these issues is during the product design phase, rather than the final stage of the product development cycle. Many experiments can be conducted before the product is loaded into the final casing.

capacitance

Capacitors bypass voltage spikes by providing a low impedance path to the common terminal of the noise source. The peak voltage is mainly generated by the motor brushes. Capacitors can be connected between each lead of the motor and ground, or between two leads. If the spike noise is common mode, the capacitance across the leads has almost no effect. However, the random noise generated by the electric brush is usually differential mode.

However, connecting a capacitor between the brush and the ground can have a significant effect. The location or connection of capacitors mainly depends on the type of noise they are facing. Voltage spikes are generated by the disconnection of electric brushes and commutator contacts. The amplitude of the peak can be reduced by replacing the brush material with a softer one or increasing the pressure on the commutator of the brush. But this will shorten the lifespan of the electric brush and other issues.

To achieve good filtering effect, the connection between the capacitor and the common ground of the noise source should be as short as possible. The inductance of a wire in free space is approximately 1nH per inch. If the noise frequency generated by the electric brush is 50-100MHz and the length of the wire connected to the capacitor is 4-6 inches, then even without considering the impedance of the capacitor, the impedance of the wire inductance alone would already be:

XL = 2πf L = 3.77

The total impedance also needs to add the impedance of the capacitor (0.1 μ F), XC=1/2 π f C=0.159 Ω.

From the results, it can be seen that looking at the impedance of the capacitor alone, this is a very good bypass type filter. However, due to the influence of lead inductance, it no longer serves as a filter. If the length of the wire is shortened to 1 inch, the impedance of the inductor is only 0.628 Ω, and the effect of the filtering capacitor is improved by 20%.

When using a motor casing as a grounding terminal, the paint on the casing must be removed so that the wire can make good contact with the ground. Relying on the 4 or 5 threads of the connecting screw to connect is not a good method. Even if the product casing is made of metal, installing the filter component directly on the noise source rather than near the noise source or casing is a smart choice. This eliminates the additional lead length, minimizes the impedance of the noise source, and has the best filtering effect.

power line filter

In many products, power line filters are necessary. When the power line filter is installed correctly, it is a simple method to solve interference. The power line filter ensures that the power grid is protected from internal noise pollution caused by the product. However, like other filtering devices, the key to using a power line filter is to ensure that the wires connected to the common end of the noise source are as short as possible.

There are inductors and capacitors in the power line filter that can filter out differential and common mode noise. This type of filter is a simple and economical way to filter out power line interference. The power line filter should be installed at the entrance of the power line. In some products, the filter is installed in the middle of the product, which can cause the radiation interference generated inside the product to couple to the power input terminal, causing the filter to completely fail. Remember the following three points, your product is more likely to comply with electromagnetic compatibility standards:

1) Use power line filters;

2) Good system grounding;

3) The attenuation frequency of the filter is as low as 150kHz;

It is difficult to select a suitable power line filter from the sample of filter products. The industrial standard specifies measuring the characteristics of filters under 50 Ω input/output conditions. In reality, there is no environment that is exactly 50 Ω. When conducting conduction emission tests, a Line Impedance Stabilization Network (LISN) should be connected to the input end of the power line. This provides a standard testing method for all testing agencies.

The main function of LISN is to provide a stable 50 Ω impedance for the input of the filter within the testing frequency range. The impedance at the output of the filter is determined by the household appliance itself, and it will never be exactly 50 Ω. If it happens to be 50 Ω, you can use the data on the filter samples to determine which filter has better performance. In practice, it is usually necessary to determine the most suitable filter through experimentation.

As a principle, when the actual impedance conditions are unclear, the data given in the filter sample can be reduced by 20dB to ensure its effectiveness in actual products. On two-wire appliances, the performance of filters is not as good as on three wire appliances. There are two filtering mechanisms for filters, which are series connection and bypass. In a two-wire system, only series filtering (inductance) and line to line filtering capacitors work, and line to ground capacitors do not work.