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Other countermeasures against the harmonic problem – EMC Testing

The above three kinds of solutions of the harmonic current problem is mainly applicable for the product powered by the high voltage rectifying way.
For the household electrical appliances powered only by the industrial frequency transformer, not by the power transform second circuit. In the common situation, the harmonic current is not big, and the wave current limits are much loose, even the harmonic current control measures are not taken, its qualified rate of the harmonic current test is very high. However, we still need to pay attention to the following aspects.
For those household electrical appliances powered by the high voltage rectifying way, the low harmonic current limits are rather relaxed; it is easy to be qualified. At this time, we should pay attention to those problems of the high harmonic current of more than 20 times.
For these higher harmonic over proof problem, usually, we increase some suitable higher harmonic filter inductance (high-frequency choke coil) in the power source circuit to solve the problem.
Since halfway rectifier way and phase cut way method adjustment (such as silicon controlled zero control) will produce very big harmonic current for the symmetric and asymmetric control of the power supplies. The harmonic standard usually doesn’t use the half rectifying way and symmetric and asymmetric control of the power supply.
When conducting the test, if the harmonic current exceeds the standards, we suggest that we should change the power supply half-wave rectification and the symmetric and asymmetric control method to other control method. For example, we change the half-wave rectification into the complete-wave rectification or bridge rectifier way. The way of using phase cut wave method to adjust the symmetric and asymmetric way to change into the symmetrical zero trigger control way. It can effectively solve the harmonic problem.

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