“Motors, especially those with brushes, generate a lot of noise. To meet the requirements of electromagnetic compatibility standards, electrical appliances must deal with these noises. The means to solve electromagnetic compatibility are nothing more than capacitors, inductances (chokes), power filters and grounding.
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Motors, especially those with brushes, generate a lot of noise. To meet the requirements of electromagnetic compatibility standards, electrical appliances must deal with these noises. The means to solve electromagnetic compatibility are nothing more than capacitors, inductances (chokes), power filters and grounding.
Unfortunately, electromagnetic compatibility problems 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 faced with time constraints, but the project budget has been exhausted. The responsible engineer has been transferred to other projects and cannot solve related problems at any time.
The best time to solve these problems is in the product design stage, not the final stage of the product development cycle. Many tests can be performed before the product is loaded into the final enclosure.
capacitance
The capacitor bypasses the voltage spike by providing a low impedance path to the common end of the noise source. The spike voltage is mainly generated by the motor brushes. The capacitor can be connected between each lead of the motor and the ground, or between two leads. If the spike noise is common mode, the capacitance across the leads has little effect. But this kind of random noise generated by brushes is usually differential mode.
Despite this, connecting a capacitor between the brush and ground will have a great effect. Where to install the capacitor or how to connect it mainly depends on the type of noise faced. The voltage spike is caused by the disconnection of the brush and the commutator contact. The amplitude of the spike can be reduced by changing the brush material to a softer material or increasing the pressure of the brush on the commutator segment. But this will shorten the life cycle of the brushes and other problems.
To make the capacitor have a better filtering effect, the connection line between it and the common ground of the noise source should be as short as possible. The inductance of a wire in free space is about 1nH per inch. If the frequency of the noise generated by the brush is 50-100MHz, and the length of the wire connected to the capacitor is 4-6 inches, then even if the impedance of the capacitor is not considered, only the impedance of the wire inductance already has:
XL = 2πf L = 3.77
The total impedance also needs to add the impedance of the capacitor (0.1μF), XC = 1/2 π f C = 0.159 Ω.
It can be seen from the results that just looking at the impedance of the capacitor, this is a very good bypass filter. However, due to the influence of the lead inductance, it can no longer function as a filter at all. If the length of the wire is shortened to 1 inch, the impedance of the Inductor is only 0.628Ω, and the effect of the filter capacitor is increased by 20%.
When using the motor shell as the ground terminal, the paint on the shell 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 way. Even if the product shell is metal, it is a smart choice to install the filter directly on the noise source, rather than close to the noise source or a certain position of the shell. This eliminates the extra lead length, minimizes the impedance of noise returning to 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 and convenient way to solve the interference. The power line filter ensures that the power grid is protected from the internal noise pollution of the product. However, as with other filter components, the key point of using power line filters is to ensure that the wires connected to the common end of the noise source are as short as possible.
In the power line filter, there are inductors and capacitors that can filter differential mode and common mode noise. This 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 will cause the radiation interference generated inside the product to couple to the power input terminal, making the filter completely ineffective. Remember the following three points, your product has a greater chance of complying with electromagnetic compatibility standards:
1) Use power line filters;
2) Good system ground wire;
3) The attenuation frequency of the filter is as low as 150kHz;
It is difficult to select a suitable power line filter from the filter product samples. The industry standard stipulates that the characteristics of the filter should be measured under 50Ω input/output conditions. In reality, there is no environment that happens to be 50Ω. When conducting a conducted emission test, connect the line impedance stabilization network (LISN) at the input end of the power cord. This provides a standard test method for all testing organizations.
The main function of LISN is to provide a stable 50Ω impedance for the input of the filter within the test frequency range. The impedance at the output of the filter is determined by the household appliance product itself, which will never be exactly 50Ω. If it happens to be 50Ω, you can use the data on the filter samples to determine which filter performs better. In practice, it is usually necessary to determine the most suitable filter through experimentation.
As a principle, when the actual impedance condition is not clear, the data given in the filter sample can be reduced by 20dB to ensure the effect in the actual product. On two-wire appliances, the filter performance is not as good as on three-wire appliances. The filter has two filtering mechanisms, which are series and bypass. In a two-wire system, only the series filter (inductance) and the line-to-line filter capacitor work, and the line-to-ground capacitor does not work.
Inductance device
Another way to reduce noise is to place an inductance device directly on the brush. The function of the inductance is to prevent sudden changes in the current flowing into the brush when the brush passes through the commutator gap. The inductance of the inductor is about 10 ~ 25μH. The choke coil connected in series in the circuit can be combined with the bypass capacitor to the ground to form a low-pass filter, which can enhance the filtering effect of a single inductor or capacitor. This is very good for suppressing conducted noise.
The difference between a single capacitor and an LC filter is very large. The LC filter has a wider filter bandwidth than a single capacitor, so it has a greater effect on the broadband noise generated by the brush. The filtering technology introduced above can eliminate conducted interference, but the radiated interference generated by spikes also needs to be suppressed. This can be achieved by shielding.
Grounded
Grounding is also a very important method. If the filter is not connected to the ground, the designed performance will not be achieved. For bypass capacitors, if the ground impedance is too large, it will not provide a good bypass function. Excessive impedance here means that the impedance exceeds a few mΩ. The ground wire is the path to eliminate noise.
If a good-performance filter device is used, but a good grounding path is not provided, the filter effect of the device will be greatly reduced. In a non-metal chassis, there is no grounding plate that connects all devices, and filtering is very difficult at this time. To obtain a good ground, you can limit the noise-producing devices to a common ground board.
Designers should note that the state of the wire harness must be very close to the state of the wire harness in the final product. The treatment of the ground wire is very important. If there is no good ground wire, the filter and other electromagnetic compatibility devices cannot work effectively.
In addition, it is recommended to do a simple analysis of the actual circuit, including the motor windings, in order to understand how individual filter components affect noise. This does not require complex computer-aided analysis, as long as the simple relationship between impedance and filter components is understood.
Since the noise generated by the motor has great randomness in amplitude and frequency, it is necessary to make a large margin between the interference emission of the product and the specified limit value. Also, even if the same device is used, the differences between devices are much greater than imagined.
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