Motor cable length with variable frequency drives — voltage reflection and choosing between a dv/dt and a sine-wave filter

The guide on symmetrical construction of a VFD motor cable covers why the earth conductor(s) of a variable-frequency-drive motor cable must be arranged symmetrically around the phases, to limit common-mode bearing currents. This article covers a different physical phenomenon involving the same cable: the voltage reflection that occurs as the cable between drive and motor gets longer, which can occur independently of the cable's symmetrical construction.

The phenomenon: the cable as a transmission line

A variable frequency drive switches its output transistors (typically IGBTs) with very steep switching edges — a voltage rise time on the order of a few hundred nanoseconds to a few microseconds. When the cable between drive and motor is long enough relative to this rise time, the cable behaves electrically as a transmission line: the cable's characteristic impedance differs from that of the motor winding, causing the voltage pulse to be partially reflected at the motor end. This reflection can add to the incoming pulse, resulting in a voltage peak at the motor terminals that can reach roughly twice the drive's DC-bus voltage — a stress that a standard motor's winding insulation is not designed for.

NEMA MG1 Part 30 versus Part 31

NEMA MG1 Part 30 (for general-purpose motors, not specifically designed for drive supply) assumes an insulation system able to withstand repeated voltage peaks of up to 1000 V with a rise time of at least 2 microseconds. NEMA MG1 Part 31 ("inverter-duty" motors, specifically designed for drive supply with a more robust insulation system) allows for considerably higher stress: peak voltages up to 1600 V, at a rise time as low as 0.1 microsecond. A standard Part 30 motor supplied over a long cable by a modern IGBT drive can therefore easily exceed the voltage peaks it was designed for.

Note: the exact cable length above which reflection problems occur is not a fixed number — it depends on the rise time of the drive used, the cable characteristics and the installation's rated voltage. Consult the drive manufacturer's cable-length tables for a specific design rather than applying a universal rule of thumb.

Two types of filters, two application ranges

To limit voltage reflection at cable lengths exceeding the limit specified by the drive manufacturer, two main categories of output filters exist:

Dv/dt filter (also called an output reactor or load reactor in its simplest form): slows the switching-edge rise time enough to keep the peak voltage within acceptable limits at moderate cable lengths. This is typically the most cost-effective solution as long as the cable length does not far exceed the manufacturer's stated limit.
Sine-wave filter: converts the drive's rectangular PWM output signal into a near-pure sine voltage, removing the reflection problem at its root rather than merely limiting it. A sine-wave filter therefore has no fixed maximum cable length, but is considerably more expensive than a dv/dt filter — in practice mainly used for very long cable runs (for example hundreds of metres, such as a remotely located pump motor).

Practical relevance

When designing a variable-frequency-drive motor circuit with a cable length approaching or exceeding the manufacturer's limit, first establish whether the motor is a Part 30 or Part 31 design (or its European equivalent), and then whether a dv/dt filter suffices or a sine-wave filter is needed — independently of whether the cable itself is already built symmetrically against bearing currents (see the guide on symmetrical motor cable construction). Both measures may be needed at the same time, since they solve two different problems.

Common mistakes

Confusing voltage reflection due to cable length with common-mode bearing currents — these are two separate physical phenomena, each with its own solution (filter versus symmetrical cable construction and shield earthing).
Connecting a standard Part 30 motor over a long cable without considering a filter, assuming the motor is "resistant enough" to a variable frequency drive regardless.
Applying a dv/dt filter at a cable length far beyond the manufacturer's limit — for very long runs a sine-wave filter is needed; a dv/dt filter alone will not suffice.
Assuming a fixed universal cable-length limit (for example "always 50 metres") instead of consulting the drive manufacturer's specific table, which depends on voltage, switching frequency and cable type.