A variable frequency drive controls a motor by switching the DC bus on and off at high speed, producing a pulse-width-modulated output. That fast switching is what makes the drive efficient, but it also creates steep voltage rises that behave badly in a long motor cable. When the cable is too long, unshielded, or poorly grounded, the motor sees voltage spikes that puncture insulation, and the drive itself can trip on current noise. Cable length and shielding are not afterthoughts; they decide whether the motor survives the drive.
Why long cables cause spikes
The pulses from the drive travel down the cable at a finite speed. When a pulse reaches the motor, whose impedance differs from the cable, part of the energy reflects back toward the drive. If the cable is long enough that the pulse round-trip time is comparable to the rise time, the reflected wave adds to the incoming pulse and the voltage at the motor terminals can approach twice the DC bus voltage, sometimes more. Short cables see the reflection return before the pulse finishes rising, so it does not pile up. This is why every drive maker lists a maximum recommended motor cable length.
The numbers that matter
The critical length depends on the switching speed. Modern IGBT drives with very short rise times, tens of nanoseconds, can see reflection problems with cables as short as 10 to 15 meters, while older, slower drives tolerated longer runs. A 400 V drive on a 690 V DC bus can push peak motor terminal voltage toward 1200 to 1400 V, which stresses a motor rated for only 1000 V spikes. Check the motor’s insulation class and the drive’s cable-length table together rather than assuming any 400 V motor works with any 400 V drive.
Output reactors and dv/dt filters
When the motor must sit far from the drive, you can slow the edges or block the spikes. An output, or load, reactor adds inductance that softens the current rise and extends the effective critical length. A dv/dt filter reduces the steepness of the voltage pulses at the motor terminals. A sine wave filter converts the PWM output into something close to a smooth sine wave, letting you run very long cables and protect non-inverter-duty motors, at the cost of size, loss, and money. Match the filter to the cable length and switching frequency, and don’t run unfiltered long cables on a standard motor.
Shielded cable
The fast pulses radiate high-frequency noise that can disrupt sensors, encoders, and nearby equipment. A shielded VFD cable with a continuous copper foil and a braid, plus symmetrically arranged phase conductors and a low-impedance ground path, contains that noise. The shield must be bonded with 360-degree contact at the drive end, usually through an EMC gland, and terminated at the motor end as well. A pigtail ground wire left long and thin defeats the shield at high frequency, because inductance rises with length. Use the cable and glands the drive’s EMC instructions call for.
Grounding
Good grounding is what makes shielding work. Bond the drive, motor, and machine to a common equipotential ground with short, wide conductors, because high-frequency noise prefers low-inductance paths. The motor cable’s internal symmetric ground conductors, often three green-yellow wires arranged with the phases, carry common-mode current back to the drive. If the return path is poor, that current finds its way through signal shields and building steel, causing mysterious faults. Keep ground straps short and flat rather than relying on a single long round wire.
Cable construction
VFD-rated cables use cross-linked polyethylene, XLPE, insulation that withstands higher voltage and temperature than standard PVC, and they place the grounds symmetrically to cancel magnetic fields. Using ordinary building wire or flexible control cable between drive and motor saves money up front but invites insulation failure and noise problems. For runs in cable trays with instrumentation, the shielded, symmetric VFD cable is especially worth it. Specify the cable by its voltage rating, shield coverage, and insulation, not just its conductor size.
Separating cable runs
Even a shielded VFD cable should not lie alongside signal and communication wires. Segregate power and control cables in the tray, cross them at 90 degrees when they must meet, and keep a gap according to the electrical design. Encoder feedback and fieldbus cables are sensitive to the VFD’s switching noise. A noisy encoder that loses counts or a fieldbus that drops packets often traces to a VFD cable routed too close. Treat cable routing as part of the electrical design rather than filling the tray at random.
Bearing currents
The high-frequency common-mode voltage from the drive can induce current through the motor bearings, pitting the races and leading to failure. Symmetric grounding, shielded cable, and sometimes an insulated bearing or a shaft grounding ring provide a path or break the circuit. If a motor on a VFD develops a noisy bearing well before its rated life, common-mode bearing current is a likely cause. Proper cable and grounding reduce the common-mode current at the source.
Multiple motors on one drive
Running several motors from one drive adds the cable lengths and can worsen reflections and charging current. Each motor sees the switching, and the combined capacitance of long cables can cause overcurrent trips at enable. Size the drive for the total current, use output protection, and keep each branch as short as practical. When motors are spread across a large machine, individual small drives near each motor often beat one drive with long home-run cables.
Common mistakes
Ignoring the drive’s cable-length table, using unshielded or PVC building wire, terminating the shield with a long pigtail, sharing a poor ground, routing VFD cable beside signal wires, skipping filters on long runs, and overlooking bearing currents are recurring errors. Check length first, then shield, ground, and filter as needed.
Troubleshooting cable-related faults
A drive that trips on overcurrent the instant it enables, with the motor barely turning, may be seeing the charging current of long cables. The cable capacitance between phases and ground looks like a brief short to the drive, and longer cables hold more charge. Measure the total cable length, reduce the number of parallel runs, or enable any soft-start or carrier reduction setting the drive offers. If the trip follows a recent cable extension, the added length is the obvious suspect. Don’t keep increasing drive size to push through capacitive current; fix the cable.
Intermittent ground-fault trips that never reproduce on the bench often point to damaged insulation from repeated spikes. A motor that fails after months of service, with blackened winding slots or punctured phase paper, has been absorbing reflected voltage. Megger-test the motor and cable, inspect the insulation, and add a dv/dt filter rather than fitting another identical motor that will fail the same way. Thermal overload protects against sustained heat, but it does nothing against nanosecond spikes, so you need the right cable and filters for those.
Carrier frequency and cable length
The drive’s switching, or carrier, frequency ties into cable stress and heating. Raising the carrier makes the motor run quieter but increases switching losses, spike energy, and cable charging current, effectively shortening the allowable cable. Lowering the carrier extends cable reach and cools the drive, at the cost of audible motor noise. On long runs, dropping the carrier a step is often the cheapest first move before buying filters. Check whether the drive derates its current at high carrier frequency, and balance motor noise against cable and thermal limits.
A drive that trips on overcurrent at enable, with the motor barely turning, may see the charging current of long cables; cable capacitance looks like a brief short. Reduce parallel runs or enable soft-start. If trips follow a cable extension, the added length is the cause. Intermittent ground-fault trips often mean insulation damaged by repeated spikes. Megger-test motor and cable, then add a dv/dt filter rather than fitting another motor that fails the same way. Thermal overload does not protect against nanosecond spikes. On long runs, lowering the carrier frequency is the cheapest first move before buying filters, trading a little motor noise for longer cable reach.
A drive that trips at enable may see cable charging current; capacitance looks like a brief short. Reduce runs or enable soft-start. Intermittent ground faults often mean insulation damaged by spikes. Megger-test, then add a dv/dt filter rather than fitting another motor that fails the same way. Lowering the carrier is the cheapest first move.
A drive that trips at enable may see cable charging current. Reduce runs or enable soft-start. Lowering the carrier is the cheapest first move.
Bottom line
Fast IGBT switching reflects on long motor cables and can double the voltage at the terminals, so respect the drive’s length limit and the motor’s insulation. Use shielded symmetric VFD cable with XLPE insulation, bond the shield 360 degrees, and ground with short wide straps. Add reactors or dv/dt and sine filters for long runs, and segregate signal cables. Get the cable right and the motor runs quietly and lasts.
