Why do we not use more than 100 m cables for VFD applications?

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 Cable length between VFD and Motor
The dU/dt at the output of the variable frequency drive combined with the motor cable length will result in very high voltage peaks at the motor terminals. This is a concern for the isolation in motors not designed to be driven by VFDs.
On the other hand the maximum motor cable length depends also on the switching frequency used due to the charging effect of the motor cable capacitance (this is a limitation on the variable frequency drive side, not on the motor isolation).
The dU/dt at motor terminals normally is very different from the dU/dt that you can calculate from IGBT and its driving characteristics (turn on time, gate resistor, etc) at variable frequency drive terminals. As the cable acts like a distributed LC impedance, the dU/dt calculation on VFD terminals will give you very high values that can be apparently dangerous, but in practice, will not happen at motor terminals.

For long cables, the combination of cable impedance, high frequency input impedance of motor and VFD switching frequency can lead to reflection of voltage pulses that gives origin to large voltage overshoots on motor terminals. The problem increases as increasing switching frequency because the time between voltage pulses will be smaller, so, a voltage pulse reaching the motor will add to the pulse being reflected. This "double pulsing" can results in extreme voltage overshoot and dU/dt that will result in motor insulation failures. For the variable frequency drives side the increasing switching frequency will be a problem (besides power losses) if you have a big capacitor filter at converter output, that can lead to high current pulses at inverter side.

The determination of the resulting dU/dt at motor terminals from the dU/dt at VFD drive terminals is very difficult if you try to use simulations. For this task you'll need the high frequency parameters of cables (that also depends on installation details) and motor, that will not be available from standard datasheets and are very difficult to obtain from measurements. In practice almost all VFD manufacturers make extensive measurements and establish some criteria in order to orient applications. The approach is to determine if it is necessary or not to have an output filter for a known application (cable length).

For instance, a common specification is:

For cable lengths up to 100 meters (and motor suitable for variable frequency drive applications) it is not necessary a filter; for lengths from 100 to 200 meters, a series reactance can be used; for greater lengths it is necessary an LC filter at VFD terminals. The limit lengths can be different from different manufacturers and voltage levels (LV/MV). Gozuk, for instance, can give complete orientation for application of its drives considering the needed cable length for the application.

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Every cable has, apart from its resistance, certain inductance and capacitance. These values are normally provided as specific values, i.e. inductance in nH/m (nano Henry per meter) and capacitance in pF/m (pico Farad per meter). The inductance and capacitance is roughly equally distributed along the cable length. As you know, inductance L and capacitance C in a circuit create a resonance.



From the equation above it is obvious that the longer the motor cable, the lower the resonance frequency. For short cables the resonance frequency is very high and not much excited from the inverter. There are two reasons: 1. inverter produces no harmonics at such frequencies or harmonics with very small magnitude (remember the amplitude law) and 2. high frequencies are generally much better damped (due to skin effect etc). However, as the motor cable length increases, the resonance frequency drops into a range where interaction of inverter and cable is likely to happen.
When the cable resonance frequency lies within the bandwidth of the inverter, the resonance can basically be excited continuously right after every switching instant.
We can observe characteristic "ringing". Such condition creates additional stress for the cable, additional inverter current to charge and discharge those capacitance and most importantly increased peak voltage at motor terminals. The inverter voltage is basically amplified in the resonance condition so that motor insulation system witnesses increased voltage peaks. Those peaks, when present over longer period of time, may lead to increased partial discharge activity and accelerated aging of insulation system.


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Let's briefly explore these different types of VFDs:

Voltage Source Inverter (VSI) drives are the most common type, suitable for various applications. They offer a good balance of performance and cost-effectiveness, making them ideal for general-purpose use in HVAC systems, pumps, and conveyors. VSI drives typically operate at switching frequencies of 2-16 kHz and can handle power ranges from 0.25 kW to 1 MW.

    Current Source Inverter (CSI) drives excel in high-power applications and situations requiring regenerative braking. They are particularly well-suited for large compressors, fans, and pumps in the megawatt range. CSI drives operate at lower switching frequencies (500 Hz to 1 kHz) but can handle power outputs up to 10 MW.
    Pulse Width Modulation (PWM) drives offer superior efficiency and harmonic reduction performance. They are versatile and can be used in various applications requiring precise speed control, such as machine tools, printing presses, and textile machinery. PWM drives typically operate at higher switching frequencies (4-16 kHz), and cover power ranges from 0.25 kW to 500 kW.
    Direct Torque Control (DTC) drives provide excellent dynamic performance and are ideal for applications with rapidly changing loads. They are commonly used in cranes, elevators, and electric vehicles. DTC drives can achieve full torque at zero speed and offer rapid torque response times of less than 2 ms.
    Multi-Level Inverter (MLI) drives are designed for high-power, medium-voltage applications. They offer very low harmonic distortion and are suitable for large industrial motors, wind turbines, and traction systems. MLI drives can operate at voltages up to 13.8 kV and power ratings up to 100 MW.

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Definition of Distance Between Frequency Inverter and Motor

The distance between the frequency inverter and the motor refers to the length of the cable that connects these two devices. This distance can significantly affect the performance and reliability of the motor control system.The distance between the frequency inverter and the motor can be classified into three categories:



Industrial Use Site Occasions

1. Short Distance

For short distances (≤ 20m), the frequency inverter and the motor can be directly connected without additional considerations. This straightforward setup minimizes potential issues caused by harmonics and EMI, ensuring stable operation.

2. Medium Distance

For medium distances (> 20m and ≤ 100m), the frequency inverter and the motor can still be directly connected. However, it is necessary to adjust the carrier frequency of the inverter to reduce harmonics and interference. Proper tuning of the carrier frequency helps maintain system stability and performance over these extended distances.

3. Long Distance

For long distances (> 100m), direct connection between the frequency inverter and the motor requires additional measures. Besides adjusting the inverter's carrier frequency, it is essential to install output AC reactors. These reactors help mitigate the effects of long cable runs by smoothing voltage spikes and reducing harmonic distortion, protecting the motor from potential damage.

Selection Situation in Highly Automated Factories

Highly automated factories, such as those found in the automotive, semiconductor, and electronics industries, rely heavily on precise motor control and rapid response times. In such environments, the cable length between the frequency inverter and the motor must be carefully managed to ensure optimal performance.

1. Short Distance

If the frequency inverter is installed within the central control room, the control console and the inverter can be directly connected using 0-5V or 0-10V voltage signals and some switching signals. However, the high-frequency switching signals of the inverter can cause electromagnetic interference with low-power control signals. Thus, it might be preferable not to place the inverter directly within the control room to avoid clutter and potential interference.

2. Medium Distance

For medium distances between the frequency inverter and the central control room, a 4-20mA current signal and some switching signals can be used for control connections. If the distance is even greater, RS485 serial communication can be employed for reliable connectivity.

3. Long Distance

When the distance between the frequency inverter and the central control room exceeds 100m, communication intermediate relays can extend the reach up to 1km. For distances beyond this range, fiber optic connectors are necessary, enabling connections up to 23km. These methods ensure robust and interference-free signal transmission over long distances.



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VFD Cable Selection: Understand the Changes to NFPA 79

Machine designers who want to take advantage of the precision and energy efficiency offered by variable frequency drives (VFDs) need to make sure the cable that connects the drive to the motor performs reliably. Choosing a poorly constructed VFD cable can hinder a machine's performance, or the cable may break down and compromise safety.

The National Fire Protection Association's NFPA 79 Electrical Standard for Industrial Machinery 2018 Edition includes an important update to the design requirements of VFD cable, which is believed to have been created to address problematic insulation materials. In long cable runs or wet environments, certain materials exhibit high capacitance characteristics which cause high charging currents, or corona discharges that, when moisture is present, can generate nitric acid that can cause them to melt.

NFPA 79 2018 Edition Article 4.4.2.8 Circuits Supplied From Power Conversion Equipment states:

Electrical conductors and equipment supplied by power conversion equipment as part of adjustable speed drive systems and servo drive systems shall be listed flexible motor supply cable marked RHH, RHW, RHW-2, XHH, XHHW, or XHHW-2.*

How to Select NFPA 79-Compliant VFD Cables

For any new installations and machinery going to U.S. states that have adopted NEC 79/NFPA 79 2018 Edition as their electrical standard to meet, the VFD cable must be marked RHH, RHW, RHW-2, XHH, XHHW or XHHW-W. SAB North America designs its VFD cables with cross-linked polyethylene (XLPE) insulation, which is rated for RHW-2 and XHHW-2, making it compliant to NFPA 79 2018.

XLPE insulation offers improved capacitance for longer installations and is extremely flexible and oil resistant. XLPE performs in VFD applications where increases in voltage can occur, such as spikes in harmonics, in-rush current and wave reflection. It is also particularly suited for wet environments because heat generated by nitric acid creation actually forms a thermally isolating charred layer on the XLPE surface that can prevent further degradation.

Updating VFD Specifications Assures Clarity

The latest edition of NFPA 79 can add difficulty to selecting the right cable for your VFD application, especially because not all U.S. states have adopted the standard. SAB North America recommends updating your VFD specifications to reflect the new standards to avoid any inspection issues. We can even provide cross references to ensure the correct cable is specified. Although staying current with the latest standard requires some time and attention, you'll have peace of mind knowing that the cable you select will not give rise to the issues associated with unsatisfactory insulation, and it will ensure the best protection for people and equipment.

*Source: You are not allowed to view links. Register or Login NFPA79 2018 archived revised information.

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