Have You Noticed That the Distance Between the VFD and the Motor Is Limited?

As a kind of advanced AC motor transmission control equipment, frequency converter has been widely used in various industries of mechanical equipment or engineering fields. During the design process, the power supply and distribution designers of industrial and mining enterprises will often choose frequency converters if the motors involved need to be started with reduced voltage and speed regulation. When designing the inverter control circuit, designers generally check whether the capacity of the inverter matches the capacity of the motor it controls, and also pay attention to the selection of some main accessories of the inverter.

If you are an experienced engineer in engineering design, after completing the above work, you know whether the distance between the inverter and the motor meets the requirements, so you can skip the content of this article. If you are a novice in engineering design and have not been exposed to and applied inverters for a long time, it is recommended to take the time to read the content of this article, because the distance between the inverter and the motor is limited, and it may cause design quality accidents.

1. Why is the distance between the inverter and the motor ignored?

Why do novice engineers ignore the distance between the frequency converter and the motor? There may be several reasons:

1. The frequency converter is usually installed in the frequency converter cabinet located in the control room or power distribution room. In most cases, the frequency converter cabinet is relatively close to the frequency converter j it controls, so the distance between the frequency converter and the motor will not become a problem and will not cause any adverse effects. Therefore, in the design and selection process, the distance between the frequency converter and the motor is not considered.

2. In the selection samples provided by inverter manufacturers, the content mainly introduces the performance parameters, characteristics, size, capacity, accessories, etc. of the inverter. Generally, there is no introduction about the distance between the inverter and the motor, but the introduction about the distance regulations between the inverter and the motor often appears in the inverter installation and use manual. Because the designer mainly selects the inverter and determines the capacity of the inverter, usually due to tight construction schedules and heavy tasks, he rarely has time to collect, read, and view some details about the use, installation, and commissioning of the inverter in the inverter installation and use manual.

3. There are many books on the market that introduce frequency converter technology, most of which mainly introduce the technology and knowledge of the principle, performance, and application of frequency converters, and do not involve the distance between frequency converters and motors.

2. Why is the distance between the inverter and the motor limited?

In the actual use process, various or various manufacturers have restrictions on the distance between the inverter and the motor, that is, the length of the cable that is allowed to be connected to the output end of the inverter. The main reasons are as follows:

1. The output voltage waveform of the inverter contains a large number of harmonics. The voltage waveform output by the inverter is similar to a sine wave, but not a real sine wave. Therefore, the output voltage waveform of the inverter contains a large number of harmonic components.

2. A large number of harmonics contained in the output voltage waveform of the frequency converter will generate radio interference and cause misoperation of other electronic equipment.

3. Between the motor connection cable and the ground, there will be a parasitic capacitance to the ground during operation. If the parasitic capacitance is too large, it will easily cause the fast switching transistor in the inverter to generate an excessive peak current at the moment of switching off, which may damage the power inverter module of the inverter.

4. The combination of the fast switching transistor in the frequency converter and the motor connection cable may generate a higher voltage, and its peak voltage may reach about twice the normal value. This high peak voltage will cause the motor winding insulation to continue to age, resulting in motor failure.

The longer the distance between the frequency converter and the motor, the longer the connecting cable will be, the more serious the above-mentioned adverse effects will be, and the distance between the frequency converter and the motor will be limited to a certain extent.

3. Relevant regulations on the distance between the frequency converter and the motor

Since different manufacturers or different models of inverters of the same manufacturer have different internal processing methods (such as the setting of dead time when the upper and lower tubes are alternately turned on, etc.), different manufacturers have different regulations on the maximum allowable laying length of cables for different types of inverters they produce, that is, the maximum distance between the inverter and the motor.

The maximum allowable distance between the inverter and the motor depends not only on the selected inverter manufacturer and model, but also on the characteristics of the controlled motor (insulation material), the type of connecting cable between the inverter and the motor (shielded/unshielded), the cable laying method (cable channel, underground installation…) and the use plan.

The distance between the inverter and the motor can be divided into three situations: long distance (above 100m), middle distance (20~100m) and short distance (within 20m).

4.The method of extending the allowable length of the connecting cable between the inverter and the motor

If a frequency converter has been selected and the allowable length of the connecting cable does not meet the engineering requirements, the following methods can be adopted to extend the allowable length of the connecting cable between the frequency converter and the motor according to the actual project situation and the technical data of the frequency converter as a remedial measure.

• 1. Choose a motor with good insulation performance.
• 2. Select the frequency converter integrated with voltage reflection superposition software suppression function.
• 3. Use non-screen knocking cables.
• 4. Switching frequency of small inverter
• 5. Measures should be taken to suppress the influence of the coupling capacitance to the ground, and the capacity of the frequency converter should be selected by amplifying 1~2 gears or installing an output reactor at the output end of the frequency converter.

The specific measures to extend the allowable length of the connecting cable between the inverter and the motor are subject to the information of the inverter manufacturer. The best way is to check the allowable length of the connecting cable of the inverter in advance and select a suitable inverter.