VFD’s require proper cabling. Here is why.

A short overview of cause and effect:

1. Pulse Width Modulation (PWM) Switching causing electrical distortions

VFDs use Pulse Width Modulation to generate variable AC voltage from a DC bus. This involves rapid switching between “on” and “off” states at high frequencies (4–20 kHz). These fast rise times (up to 3 kV/µs) create steep voltage impulses that stress cable insulation and motor windings. 

2. Cable Capacitance and Charging Currents

Cables between the VFD device and the motor act like capacitors. Longer cables and certain insulation materials increase capacitance, causing high charging currents especially in the combination of long cable runs and small VFD’s. These currents do not power the motor but heat the cable, reducing efficiency and lifespan. 

3. Reflected Wave Phenomenon

When motor impedance is higher than cable impedance, voltage pulses reflect toward the drive, creating standing waves. This can double the nominal voltage at the motor end (up to 2,150 V in a 460 V system), leading to insulation breakdown and premature cable failure but can also contribute to bearing fluting.  

4. Corona Discharge

High voltage spikes can ionize air around conductors, causing corona discharge. This produces ozone and UV light, degrading insulation and accelerating aging. This is especially true if moisture is present. CIV (Corona Inception Voltage) is lower for PVC insulation than XLPE, making material choice critical.

5. Electromagnetic Interference (EMI) and Common Mode Noise

High-frequency switching generates EMI, which can disrupt nearby electronics and control systems. Common mode currents return through ground paths unpredictably if cables lack proper shielding or aren’t terminated correctly, causing equipment malfunctions and can contribute to motor bearing fluting.

6. Mechanical Stress

Proper cable routing, especially in tight confined areas, is often difficult. Tight bends and improper routing add mechanical stress to insulation, increasing failure risk.

Summary and best practice:

• VFD applications require careful selection of proper installation materials, and the installer should consult the manual on the following points for best operation:

• Use of filters, line reactors

• Cable type and construction (Shielded VFD cable with XLPE insulation recommended)

• Maximum cable length and cable size

• EMI mitigation such as shield termination, grounding rings, ferrite rings etc.

• Environmental factors such as temperature, moisture, sunlight, chemicals etc.