Wave Reflection and VFD Cable Length Limits

Short answer

VFD voltage pulses travel along the cable as a wave. If the motor impedance does not match the cable impedance, part of the pulse reflects at the terminals and adds to the next one, and the motor voltage can approach twice the DC bus voltage. The effect grows with cable length, edge speed (dv/dt) and system voltage, and damages motor insulation over time. It is limited with short cables, inverter-duty motors, output filters and, in some cases, the carrier frequency.

The phenomenon

A drive output is not a sine wave but a sequence of voltage pulses with very fast edges, as explained in the guide on how a variable frequency drive works. For those edges, the cable to the motor behaves as a transmission line: it has a characteristic impedance, and the pulse travels along it at a finite speed, a fraction of the speed of light.

Seen from the fast edges, the motor has a much higher impedance than the cable. When the wave reaches the motor terminals, the difference between the two impedances makes part of the pulse reflect and return toward the drive. If the cable is long enough, the next pulse or the edge on another phase arrives while the reflection is still present, and the voltages add. In the worst case, the voltage at the motor terminals approaches twice the DC bus voltage. On a 460 V system the bus peaks around 650 V, and published values reach figures of about 3.1 times the rated line voltage.

What sets the magnitude

Factor	Effect on overvoltage
Cable length	Grows until the critical length is reached; then levels off near the maximum
Edge rise time (dv/dt)	Faster edges increase the effect and shorten the critical length
DC bus voltage	Higher voltage means a higher absolute peak
Inverter technology	Modern IGBTs have fast edges; silicon carbide devices are faster still and cause the effect on very short cables
Cable and motor impedance	The difference determines how much of the wave reflects
Several motors on the same cable	Changes the impedances and the reflection points
Carrier frequency	More pulses per second means more chances for reflections to coincide

A figure published by a test instrument manufacturer, for 400 V motors, says the insulation suffers no significant damage as long as the peak stays below 1,000 V and dv/dt below 500 V/µs. A severe-case measurement recorded a peak of 1.41 kV at the motor terminals. These thresholds are a general reference: each motor has its own, and inverter-duty motors tolerate higher values.

What it damages and how it shows up

The overvoltage does not kill the motor at once; it wears it out. Voltage distributes unevenly along the winding: with fast edges, a large share falls across the first turns of each coil. Repeated pulses, thousands per second, promote partial discharges in the turn-to-turn insulation, degrading it until a turn-to-turn or ground short circuit appears. The process speeds up with temperature, humidity and dirt.

Typical signs:

Insulation failures in motors with few years in service, concentrated in the first turns.
Motors that fail repeatedly on long cables and not on short ones.
A high-pitched audible noise in the motor, related to the carrier.
Drive ground fault trips with no obvious wiring cause.

There are also side effects in the installation: common-mode currents through bearings and frame, and electromagnetic disturbances, covered in the guide on electromagnetic compatibility and shielding.

How the risk is limited

Shorten the cable. Installing the drive close to the motor is the most effective measure, sometimes possible with panels near the machine.
Use an inverter-duty motor. With reinforced insulation and, in some types, insulation systems qualified for drive pulses. The guide on motors for drive use explains the criterion.
Reduce dv/dt or smooth the output with an output reactor or a dv/dt or sine wave filter. They are covered in the guide on dv/dt and sine wave filters.
Adjust the carrier frequency, within what the drive allows without derating, to reduce the number of pulses.
Choose the right cable. Symmetrical shielded cable of known capacitance; cable length and capacitance affect reflection as much as leakage current.
Avoid branches. A cable with several motors in cascade is designed with the manufacturer, paying attention to the impedances.

A public manufacturer reference states that an inductor filter to limit dv/dt allows cables of 30 to 100 m, depending on switching frequency. It is an order of magnitude, not a design value: the filter manufacturer’s table, for the specific drive and voltage, rules.

Illustrative example

Illustrative data, not from a real installation. A 460 V system with a DC bus of about 650 V peak feeds a motor through a long cable.

Theoretical maximum voltage from full reflection: close to 2 · 650 V = 1,300 V at the motor terminals.
If the motor insulation allows, on its datasheet, peaks of 1,000 V at that dv/dt, the unprotected design is above the limit.
Options: shorten the cable, switch to a motor with reinforced insulation and a higher allowable peak, or install a dv/dt filter that brings the peak below the motor limit.

Reflection rarely reaches the theoretical maximum, because the cable attenuates the wave. But the calculation shows the order of magnitude and why the data is requested from the motor manufacturer.

How to assess an existing installation

When reflection is suspected (repeated motor insulation failures, long cable, replaced drive), the diagnosis follows an order:

Gather the data of the chain: system voltage, actual cable length, cable type, drive technology, carrier frequency and motor insulation data.
Review the failure history: if failures are turn-to-turn in the first coils, edge overvoltage is a probable cause. If they are to ground at any point of the winding, other causes are possible, such as moisture or contamination.
Compare with the drive and motor manufacturers’ limits for that length and voltage.
Measure at the motor terminals the voltage peak and dv/dt with an oscilloscope of suitable bandwidth and differential probes rated for that voltage and speed. This is a measurement for qualified personnel, with the equipment in service under the site’s electrical safety rules.
Test the motor insulation with the tests in the guide on electrical tests on low-voltage motors, to learn how much margin remains.
Choose the corrective measure by cost and effect: shorten the cable, output filter, reduce the carrier within what is allowed, or replace the motor with one with reinforced insulation.

One point to keep in mind is that a fix on one side can move the problem to another. Reducing the carrier cuts the number of pulses, but raises audible noise and current ripple. A sine wave filter reduces stress on the motor, but takes some voltage and requires respecting the minimum carrier. For that reason the decision is made with the drive and filter manufacturers’ tables, not with a general length rule.

Common mistakes

Applying a length rule without considering voltage. What is safe at 230 V may not be at 460 V or more.
Ignoring that changing the drive changes the edge. A new drive with faster devices can worsen an installation that worked.
Measuring voltage at the drive and not at the motor terminals. The overvoltage appears at the end of the cable.
Using cable without the characteristics the drive or filter manufacturer asks for.
Installing several motors in parallel without reviewing the total length and impedances.
Accepting the motor nameplate as proof. Being a three-phase induction motor does not mean it is ready for drive pulses.

Checklist

Actual cable length between drive and motor, including raceways.
System voltage and DC bus voltage.
Drive technology and edge data (datasheet).
Motor insulation: class, allowable peak voltage and whether it is inverter-duty.
Maximum length allowed by the drive and filter manufacturer.
Output filter or reactor selected if needed.
Carrier frequency set and its effect on drive capacity.
Measurement at the motor terminals during commissioning, if the case warrants it.

How we help

An installation with a long cable is solved in the design, not after burning the first motor. In Engineering design and studies we verify the combination of drive, cable, filter and motor, and in testing and commissioning, in Colombia or elsewhere on request, we check the condition of the insulation. Write to us through the contact page and we will send you a quote.

References

Fluke, “Why does cable length between VFDs and motors matter?” (insulation thresholds and dv/dt filters).
Wikipedia, “Variable-frequency drive” (reflections and cable length).
IEC/TS 60034-25 and IEC 60034-18-41, motors in drive systems and insulation qualification.
Application notes from drive manufacturers on reflected wave.

Frequently asked questions

At what cable length should I start to worry?

There is no single number. The critical length depends on the drive edge speed, the system voltage and the cable. Public references mention tens of meters on 460 V or higher systems with modern IGBTs, and less with faster switching devices. The drive and motor manufacturers publish the limit for their combination.

Does a longer cable always produce more overvoltage?

Yes, up to a critical length, because the time the wave takes to travel there and back grows with the cable. When that time exceeds the pulse rise time, the reflection reaches its maximum and approaches twice the pulse voltage. Beyond the critical length, a longer cable does not raise that maximum, although other effects appear, such as voltage drop and capacitive currents.

Can a standard motor be used with a drive?

It depends on the voltage and the length. On short low-voltage systems, many standard motors work well. With higher voltages, long cables or high carrier frequencies, motors with reinforced insulation for drive use are specified. The decision is made with the motor manufacturer's data.

How do I know if reflection is present in an existing installation?

Measure the voltage at the motor terminals with an oscilloscope of suitable bandwidth and rated probes, and compare the peak and dv/dt with what the insulation allows. The field warning sign is insulation failure in the first turns of the winding, or a history of burned-out motors on long cables.