SmartD: The Long Cable Run Dilemma – Where Should You Install the VFD?
May 1, 2025
How the Clean Power VFD Solves Long Cable Run Issues
Sophia, a dedicated electrical engineer in a large industrial complex, has been tasked with installing a drive system where the motor is located hundreds of meters away from the control room. Like many engineers before her, she considers the usual question:
Should I place the drive in the electrical room or near the motor itself?
Placing the Drive in the Electrical Room
- On the plus side, it keeps delicate electronics in a clean, temperature-controlled environment, making servicing easier and extending the drive’s lifespan.
- However, this setup demands long cable runs to the remote motor, which can mean voltage spikes, special filters, and potentially higher costs due to larger cable cross-sections and EMI concerns.
Moving the Drive Near the Motor
- This choice avoids long cables and the hassles they bring.
- Yet the shop floor environment—with dust, moisture, washdowns, and heat—poses serious challenges for sensitive power electronics. Achieving the necessary NEMA/IP enclosure ratings and safely servicing the drive becomes costly and complex.
Sophia sees that both traditional approaches have trade-offs—whether in added components, harsh environment protection, or overall reliability. Is there a middle path that combines the best of both worlds, allowing a drive to stay in a controlled room and eliminating the usual long-cable headaches?
In the past, she’s struggled with voltage spikes, high dV/dt, and motor insulation failures from standard VFDs. On the line side, her facility has also faced issues with harmonic distortion, sometimes incurring utility penalties and causing malfunctions in sensitive equipment.
But there’s a turning point. Sophia hears about SmartD Technologies’ Clean Power VFD, which claims to provide:
- A true sine wave output to the motor, eliminating the usual voltage overshoot and dv/dt stress.
- An Active Front End (AFE) that keeps a Total Harmonic Distortion of current better than the requirements of IEEE519 (THDi) below 3%, ensuring top-notch power quality. For more detailed technical insights, refer to this technical note.
Intrigued, she dives deeper. Could this be the game-changer that simplifies long cable runs and cleans up power quality in one go? This paper aims to detail how SmartD’s Clean Power VFD achieves these, and why it matters for engineers dealing with challenging VFD-to-motor distances.
Conventional Drawbacks
- High dV/dt and Reflections: Standard two-level VFDs often produce fast-switching PWM pulses (e.g.,600-800 V in a few tens of nanoseconds). Over long cables, these sharp transitions reflect and can create substantial voltage overshoot at the motor terminals, sometimes as high as 2 × the DC bus voltage.
- Motor Insulation Degradation: Repeated voltage spikes can degrade insulation, leading to premature motor failures and increased downtime.
- Line-Side Harmonics: Conventional diode-rectifier VFDs can draw distorted current from the grid, with harmonic content (THDi) ranging from 20–40%. This contributes to transformer overheating, poor power factor, and potential utility penalties.
SmartD’s Clean Power Advantage
SmartD Technologies challenges the status quo by delivering:
- True Sine Wave Output to the motor, effectively removing the problems of voltage overshoot and dv/dt stress.
- An embedded AFE (Active Front End) that synchronizes with the line voltage, yielding THDi < 3% on the input side.
Boosts the DC bus voltage, allowing for a higher output voltage than conventional 6-pulse drives. This helps compensate for voltage drop over long motor cables, ensuring proper voltage at the motor terminals. As a result, the drive supports longer cable runs without oversized cables or output filters.
Unlike conventional VFDs that rely on output filters to mitigate voltage spikes and reflections, the Clean Power VFD is designed to handle long motor cables without any external filtering. This eliminates the risk of skewed motor auto-tuning caused by filter-induced impedance, ensuring more accurate motor modeling and better control performance. Additionally, with the Clean Power VFD there’s no need for special motor insulation or expensive shielded cables, simplifying installation and reducing total system cost.
Reflection & dV/dt
Although SmartD’s Clean Power VFD all but eliminates these issues, it’s helpful to understand why conventional drives struggle.
1. Voltage Reflection: Transmission-Line Effects
When a standard VFD switches at high frequency, it generates steep voltage edges traveling down the cable toward the motor. A long cable can behave like a transmission line, and any mismatch between the cable’s characteristic impedance (Z0) and the motor’s input impedance (Zmotor) causes partial reflection.
The reflection coefficient is the ratio of the amplitude of the reflected wave to the amplitude of the incident wave, with each expressed as phasors, and the symbol of this coefficient is Γ (capital gamma). The reflection coefficient is a function of location, and the reflection coefficient at the load is dependent on the load impedance and the transmission line characteristic impedance. When load impedance is equal to the characteristic impedance of the wave, there is no reflection on the transmission line.
Estimation of the reflection coefficient at the motor connection point:
Γ=(Zmotor−Z0)/(Zmotor+Z0)
In a worst-case scenario (where Γ≈1), the reflected wave can nearly double the instantaneous voltage at the motor terminals:
Vmotor(t)≈Vincident(t)+Vreflected(t) with Vreflected(t)=Γ Vincident(t)
With a rapid rise time Δt of, say, 100 ns, the effective dV/dt for a 600 V step is:
dV/dt=600 V/100 ns=6 kV/µs, subjecting motor windings to severe stress.
2. Motor Insulation Stress
Repeated exposure to fast transitions weakens insulation over time. The frequency of switching events (often in the kHz range) means the motor sees thousands of these pulses per second. Insulation systems designed for 50/60 Hz sine waves can fail prematurely when faced with continuous, steep, high-frequency edges.
3. EMI & Capacitive Charging
High-frequency pulses can radiate electromagnetic interference (EMI) that disrupts nearby electronics. Also, the cable’s inherent capacitance means the drive must supply a charging current at each transition:
I=C.dV/dt
Longer cables have higher capacitance, increasing stress on both the VFD and motor.
SmartD Clean Power VFD Delivers a True Sine Wave Output
Sine Wave Reconstruction
Rather than outputting raw PWM pulses, SmartD’s Clean Power VFD reconstructs a true sinusoidal waveform at its output terminals:
Vmotor(t)≈Vnominalsin(ωt)
where Vnominal is the rated RMS line-to-line voltage. This means:
- No sharp edges to reflect and cause over-voltage.
- Minimal dV/dt, as the motor essentially sees a 50/60 Hz sine wave.
- Standard Motor Insulation is sufficient (Class F or H). You no longer need specialized “inverter-duty” motors for most applications.
2. Eliminating Reflection Issues
Because the waveform transitions are smooth and low-frequency (relative to PWM edges), the reflection phenomenon is negligible. At 50 Hz or 60 Hz, the cable can be considered a simple conductor rather than a high-frequency transmission line.
3. Reduced EMI
A sine wave has minimal high-frequency components, so there’s no significant EMI generation from rapid switching. Sensitive instrumentation and communication lines remain unaffected, even if they run near the motor cables.
Practical Implications for Long Cable Runs
1. Eliminating dV/dt Concerns
With a sine wave output, the famous reflection coefficient problem with fast edges is no longer relevant. The motor sees the same type of waveform it was originally designed for, drastically extending insulation life.
2. No External Filters Required
Gone are the days of adding dV/dt filters or sine wave filters after the drive. SmartD’s Clean Power technology integrates wave shaping and harmonic reduction internally. This reduces:
- System complexity.
- Installation footprint and wiring.
- Overall capital expenditure.
3. Versatility Across Industries
For example:
- Mining: Conveyors spanning hundreds of meters can operate reliably without voltage spikes damaging motors.
- Water Treatment: Remote pump stations benefit from reduced EMI and stable motor voltages.
- HVAC in High-Rises: Long vertical runs see minimal harmonic impact on building systems.
Implementation Guidelines
Cable Sizing
Even with a sine wave output, standard voltage-drop calculations apply over long distances. Ensure the conductor gauge is adequate: Vdrop=I×Rcable(per phase)
If the run is very long, you might still choose a larger gauge to keep the voltage above the motor’s minimum operating level.
Grounding & Bonding
Although the SmartD Clean Power VFD does a lot of the heavy lifting when it comes to reducing high-frequency noise at the source, grounding remains essential for overall electrical safety, equipment protection, and EMI control. Standards like IEC 60204-1 and NFPA 70 provide a detailed roadmap to achieve these goals in a compliant, reliable manner. Refer to standards like IEC 60204-1 or NFPA 70 (NEC). While high-frequency noise is significantly reduced, proper grounding ensures safety and further EMI containment.
Voltage & Current Verification
After commissioning, measure waveforms at the motor terminals with an oscilloscope or power analyzer. Expect a clean sine wave with minimal ripple.
On the input side, use a harmonic analyzer to verify THDi<3% under normal operating load.
Maintenance Simplification
Because the motor insulation is no longer subjected to harsh transients, planned maintenance can focus on mechanical wear rather than worrying about partial discharges or winding breakdowns
Conclusion: A Paradigm Shift
Sophia’s journey shows that SmartD’s Clean Power VFD does far more than marginally improve conventional drives. It fundamentally changes how engineers can approach long-distance motor connections:
- A True Sine Wave Output: By eliminating high dV/dt transitions, voltage spikes, and EMI, the SmartD Clean Power VFD removes the usual limits on cable length. Engineers can confidently place motors hundreds of meters away without fear of over-voltage or insulation breakdown.
- THDi<3% on the Input: The Active Front End (AFE) decreases line-side harmonics, protecting the rest of the plant from power-quality issues and utility penalties—even with substantial cable runs.
- No Need for External “Band-Aids”: Typical fixes like dV/dt or sine wave filters, oversizing cables, and specialized motor insulation become unnecessary. This saves on system cost, footprint, and complexity.
- Extended Motor Life, Simplified Maintenance: Because the motor sees a gentle, sinusoidal waveform, insulation wear is minimized, and unscheduled downtime almost disappears—even for motors located far from the drive.
Put simply, SmartD’s Clean Power VFD enables engineers to forget the old constraints of cable length and high-frequency noise. It tackles voltage stress and harmonics at their source, resulting in a robust, cost-effective, and straightforward solution. Whether you’re running a conveyor through a sprawling mine or installing pumps in remote water treatment facilities, you can place your drive wherever the application demands—without compromising reliability or power quality.
In short, SmartD Technologies has redefined the entire VFD landscape for long cable runs with the Clean Power VFD. By delivering a true sine wave at the motor and maintaining exceptional harmonic performance on the line side, they free engineers like Sophia from the hidden costs and headaches of older drive architectures. With the Clean Power VFD it’s a win-win: extended motor lifespan, stable facility power, and a cleaner, simpler approach to challenging distances—no matter how far.
