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what is EMC?

Jul 12, 2019

Definition

Electromagnetic compatibility means that electrical equipment operates under environment of electromagnetic interference, but it does not interfere electromagnetic environment and realizes the function stably.

Introduction of EMC standard

According to requirements of national standard GB/T12668.3, frequency convertor shall conform to requirements of two aspects: electromagnetic interference and anti-electromagnetic interference.

Our current products execute latest international standards: IEC/EN61800-3: 2004 (Adjustable speed electrical power drive systems part 3: EMC requirements and specific test methods), which is equal to national standard GB/T12668.3.

IEC/EN61800-3 mainly checks frequency convertor from two aspects: electromagnetic interference and anti-electromagnetic interference. Electromagnetic interference mainly tests radiated interference, conducted interference and harmonic interference of frequency convertor (requirements on the frequency convertor for civil use). Anti-electromagnetic interference mainly tests conduction immunity, radiated immunity, surge immunity, fast changed pulse group, ESD immunity and immunity of low-frequency terminal of power (specific test items include: 1. immunity test for input voltage sag, interruption and change; 2. immunity test for commutation notch; 3. immunity test for harmonic input; 4. change test for input frequency; 5. unbalance test for input voltage; 6. fluctuation test for input voltage). The test is conducted as per strict requirements of above IEC/EN61800-3, and please install our company products as per instructions of 7.3, which own good electromagnetic compatibility under general industry environment.

EMC guidance

1. Influence of harmonic: higher harmonic of power will damage frequency convertor, so it’s suggested to install AC input reactor in the places with weak quality of power grid.

2. Electromagnetic interference and installing precautions: there are two kinds of electromagnetic interference. One is interference of surrounding electromagnetic noise for frequency convertor, and another is interference produced by frequency convertor for peripheral equipment.

Installing precautions:

1) Earthing wire of frequency convertor and other electrical products should be well grounded;

2) Do not layout power input and output line or weak current signal line (e.g. control circuit) of frequency convertor in parallel, layout them vertically if possible;

3) It’s suggested to use shield cable or steel tube shield power line for output power line of frequency convertor, and keep reliable earthing of shield layer. For the lead of equipment with interference, it’s suggested to use dual twisted pair shield control line and keep reliable earthing of

shield layer;

4) For the motor cable exceeding 100m, output filter or electric reactor should be installed.

3. Handling method of interference produced by peripheral electromagnetic equipment for frequency convertor: in general, the cause of frequency convertor produces electromagnetic influence is that many relays, contactors or electromagnetic brakes are installed nearby frequency convertor. If any malfunction of frequency convertor due to interference, it’s suggested to adopt below methods:

1) The devices producing interference are installed with surge suppressor;

2) Install filter in input terminal of frequency convertor as per 7.3.6 for operation;


3) Control signal line and lead of detection circuit adopt shield cable, and keep reliable earthing.

4. Handling method of interference produced by peripheral equipment for frequency convertor: there are two kinds of noise, namely radiated interference of frequency convertor and conducted interference of frequency convertor. These two interference lead to electromagnetic or electrostatic induction of peripheral electrical equipment, and then cause malfunction of equipment. Aiming at different interference, below solutions can be referred:

1) The signal of instruments, receivers and sensor for measurement is generally weak. If they are

near to frequency convertor or in the same control cabinet, the frequency convertor is easily interfered and malfunction produces. It’s suggested to adopt below solutions: keep away from interference source as far as possible; do not layout signal line and power line in parallel, or bundle up them in parallel; signal line and power line adopt shield line, keep reliable earthing; install  ferrite

core (range of blanketing frequency is 30  1000MHz) on output side of frequency convertor and

wind 2~3 turns in the same direction. For serious situation, EMC output filter can be installed;

2) If interfered equipments share the same power with frequency convertor, conducted interference will produce. If interference can’t be eliminated through above method, EMC filter shall be installed between frequency convertor and power (refer to 7.3.6 for model selection operation);

3) Independent earthing of peripheral equipment can eliminate the interference produced by leak current of ground lead of frequency convertor.

5. Leak current and handling: there are two kinds of forms of leak current when using frequency convertor: leak current to ground, and leak current between lines.

1) Factors influencing leak current to ground and solutions:

There is distributed capacitance between wire and ground. The larger the distributed capacitance, the larger the leak current will be, so reduce the distance between frequency convertor and motor to decrease distributed capacitance. The larger the carrier frequency, the larger the leak current will be, so decrease carrier frequency to reduce leak current. However, decreasing carrier frequency will lead to increasing motor noise. Please note that installing reactor is an effective way to solve leak current.

Leak current increases with enlargement of loop current, so the larger the motor power, the larger  the corresponding leak current will be.

2) Factors influencing leak current between lines and solutions:

There is distributed capacitance between output wiring of frequency convertor. If the current passing circuit contains higher harmonic, resonance may be caused to produce leak current. If using thermal relay at this time, malfunction may occur.

The solution is to reduce carrier frequency or install output reactor. When using frequency convertor, it’s not suggested to install thermal relay between frequency convertor and motor, but use electric over- current protection function of frequency convertor.

6. Precautions about installing EMC input filter on power input terminal:

1)  Caution: please strictly abide by rated value when using filter. As filter is I-class electric appliance, metal shell of filter should contact well with metal of installing cabinet, and good electric conduction continuity is required, otherwise there is risk of electric shock and EMC effect will be seriously influenced;

2) According to EMC test, filter and PE terminal of frequency convertor should be connected on the same ground, otherwise EMC effect will be seriously influenced;

3) The filter should be installed nearby power input terminal of frequency convertor as far as possible.