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Effects of Too Long Distance between VFD and Motor

As the output voltage of VFD belongs to the high voltage pulse train, its frequency equals to the carrier frequency and the peak equals to DC circuit voltage (513V). When the connection line between the variable frequency drive and motor is too long, the effect of wire distributed inductance and capacitance between wires can not be neglected. When the distance between motor and VFD is long, the distributed capacitance between wires and leakage inductance of motor may be close to resonance point, thus leading to high input voltage of motor and damaging the motor, or vibration during operation.

In application site, there are three cases for the installation distance between the VFD and motor: long distance, medium distance and short distance. The distance above 100m belongs to the long distance, the distance between 20~100m belongs to the medium distance, and the distance within 20m belongs to the short distance.

The output voltage waveform of VFD contains a large amount of harmonic components in its operation. As has been said before, these harmonics will have a great negative effect on the function of the VFD system. Appropriate design of VFD installation position and connection distance between Variable frequency drive and motor can reduce the influence of harmonics. Long-distance connection will generate surge voltage at both ends of motor winding, the superimposed surge voltage will increase the winding current of the motor and the motor temperature, thus damaging the winding insulation.
Therefore, the VFD is wished to install near the controlled motor. But actually, there’s always a certain distance between the VFD and motor, which can be connected directly. If the distance between VFD and motor is within the range of 20~100m, it’s necessary to adjust the carrier frequency of VFD to reduce the harmonics and interference. If the distance between the VFD and motor is more than 100m, not only the carrier frequency should be reduced properly, but also the ssvs or AC reactor for output shall be added. Different types of VFD have different performances on this respect.  

In the distributed control system (DCS), the electromagnetic radiation of high frequency switching signal of VFD will cause some interference to the electronic control signal. Therefore, large-scale VFD is generally placed in the central control room. However, most medium and small capacity VFDs are installed on the production site, and RS485 serial communication is generally adopted for connection. If the distance is longer, communication repeater can be used and the distance could be 1km. If an optical fiber connector is used, the distance could be as long as 23km. With communication cable connection, it’s rather convenient to form multilevel drive control system and implements the master/slave and synchronous control requirements.

At present, field bus control technology is more popular, and typical field bus includes Profibus, LonWonks and FF. Its greatest characteristic is to replace analog signal with digital signal, to simulate the field signal cable to be replaced by high capacity field bus network, thus greatly improving the data transmission speed and realizing complete decentralization of control. Such a decentralization can shorten the distance between variable frequency drive and motor, so as to make the system layout more reasonable.

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