When designing or maintaining a motor system driven by a variable frequency drive (VFD), one important factor is often overlooked—the cable length between the VFD and the motor. Although VFD drives bring clear benefits such as energy savings and speed control, the distance between these two devices directly affects performance, reliability, and motor lifespan. Understanding why cable length matters helps ensure stable operation and protects the system from unnecessary failures.
How VFD Output Signals Behave: PWM and High dv/dt
To begin with, it is important to understand the type of signal a VFD drive sends to the motor. A VFD does not output a pure sine wave. Instead, it delivers a PWM (pulse width modulation) signal, which is a series of fast voltage pulses. These pulses have very steep rising and falling edges, known as dv/dt.
As the PWM signal travels through the cable, its fast rise time becomes more sensitive to the electrical characteristics of the cable. This point leads directly to the next question—what happens when the cable is too long?
Why Long Cables Cause Voltage Reflections and Spikes
1. Impedance mismatch between motor and cable
The motor and the cable have different electrical impedances (impedance mismatch). When the VFD pulses travel down the cable and hit the motor, this mismatch causes part of the energy to reflect back toward the drive.
In short cables, this reflection settles quickly. However, as cable length increases, the travel time of the pulse becomes significant. This allows the reflected wave to overlap with the incoming wave, creating a phenomenon known as a standing wave.
2. Voltage spikes caused by reflected waves
When a reflected wave overlaps with the incoming PWM pulse, the voltage at the motor terminals can rise far above the expected level. This is called voltage reflection or voltage doubling.
In real industrial cases, peak voltage has been measured at over 1.4 kV, even in systems designed for only 400 V motors. This extreme value is caused purely by excessive cable length between the motor and the VFD drive.
3. High dv/dt causing additional stress
Besides the peak voltage, the speed of the voltage rise (dv/dt) also increases. High dv/dt stresses the motor insulation and reduces its long-term reliability.
Negative Effects of Excessive Cable Length on Motors
As cable length increases, the chance of voltage reflection also increases, leading to real and measurable damage.
1. Motor insulation damage
Voltage spikes and fast dv/dt can break down insulation materials, causing partial discharge or complete failure. Standard 400 V motors can typically withstand up to 1000 V peak, but reflected spikes may exceed this limit.
2. Electromagnetic interference (EMI)
Long cables behave like antennas. They radiate high-frequency noise, which can interfere with nearby equipment and reduce control accuracy of the motor system itself.
3. Bearing currents and premature wear
High dv/dt can lead to common-mode currents that travel through motor bearings. Over time, these currents cause electrical fluting and shorten bearing life.
4. Overheating and reduced motor lifespan
Fast switching pulses and reflected waves cause additional motor heating. This extra temperature rise accelerates aging and increases maintenance needs.
Key Factors That Influence Cable Performance Between VFD and Motor
Several key elements determine whether a VFD cable installation is safe and reliable:
1. Cable length
- ≤ 30 m: generally safe
- 30–100 m: possible risk, extra protection recommended
- 100 m: VFD manufacturer guidance is required
2. Cable type and insulation
Using high-quality, shielded VFD cables helps reduce noise, lower stress on insulation, and improve system reliability. Materials such as XLPE insulation offer better thermal and electrical performance.
3. Switching frequency
Higher switching frequencies mean more pulses per second. While this improves waveform quality, it also increases the rate of capacitive leakage current in long cables, leading to higher drive heating and more frequent stress events on the motor insulation.
4. Grounding and shielding
Proper grounding prevents unwanted noise and protects both the drive and the motor from additional electrical stress.
How to Mitigate Problems When Long Cable Runs Are Unavoidable
Even when the installation requires long cable distances, several proven solutions can control voltage reflection and protect the motor.
1. Using output reactors or line reactors
Reactors help smooth the VFD output by:
- reducing voltage spikes
- lowering dv/dt
- limiting electromagnetic interference
- improving motor lifespan
2. Installing dv/dt filters or sine wave filters
These filters provide even better protection:
- dv/dt filters reduce the rise time of the PWM pulses
- sine wave filters transform the output into a near-pure sine wave
Sine wave filters are especially useful for old motors or very long cable installations.
3. Adjusting VFD switching frequency
Lower switching frequencies reduce dv/dt and voltage spikes, making the system safer for long cable runs.
4. Selecting proper VFD-rated cables
Shielded and properly insulated cables reduce noise and help manage reflected waves more effectively.
5. Following manufacturer guidelines
Every VFD drive supplier offers recommended cable length limits. Following these guidelines ensures compliance and protects both the motor and the drive.
Best Practices for Reliable VFD–Motor Wiring
To maintain a stable system, the following practices are recommended:
- Keep cable length as short as possible
- Use proper VFD-rated shielded cables
- Ensure correct grounding and bonding
- Use reactors or filters when cable length exceeds recommended limits
- Keep VFD drive cooling systems clean and well-maintained
- Regularly check motor insulation resistance
- Follow installation instructions from the VFD drive supplier
Implementing these practices reduces the risk of motor insulation damage, EMI issues, and premature failure.
Conclusion: Managing Cable Length for Safe and Efficient VFD Operation
In summary, the cable length between a VFD and a motor plays a key role in determining system efficiency and reliability. Long cables increase the risk of voltage reflection, high dv/dt, insulation stress, and EMI issues. With proper cable selection, filtering, grounding, and system design, these problems can be effectively controlled. Paying attention to cable length ensures safe operation, longer motor life, and reduced maintenance costs in any VFD application.
If you are looking for reliable motor control solutions backed by strong engineering experience, GTAKE provides high-performance VFD drives designed for stable, efficient, and long-lasting operation. As a professional VFD drive supplier, GTAKE supports various industries with advanced control technologies and durable drive products tailored to real-world applications.
Feel free to contact GTAKE for expert guidance on VFD selection, installation, and system optimization.
