Spindle cables, reactors and filters

The cable between the drive and the spindle is not just three conductors. At switching frequencies it behaves like a transmission line, and the longer it is the more that matters.

What to use for the motor cable

Screened (shielded) cable, with the screen clamped 360° at the drive end. The screen is what keeps the switching noise inside the cable — see grounding and noise.
Four conductors where the spindle has an earth pin: three phases plus PE. Do not use the screen as the earth conductor.
Sized for the current, from the sizing calculator, with margin for the fact that a drive output is not a clean sine wave.
Rated for the voltage the bus can put on it, not just the nominal output.

Long runs: the reflected wave problem

The drive's output is a series of fast-edged pulses. When the cable is long enough for a pulse to travel to the motor and reflect before the edge has finished, the reflected and incident voltages add. At the motor terminals the peak can reach roughly twice the DC bus voltage, and in the worst case more.

That stresses the winding insulation, and it is why long motor cables are a known cause of spindle windings failing early.

Mitigation options for long motor runs
Option	What it does	Trade-off
Shorten the cable	Removes the problem	Not always possible
Output reactor (load reactor)	Slows the voltage edge; cheap and effective	Some voltage drop; a little heat
dv/dt filter	Limits the rate of change properly	Costs more than a reactor
Sine wave filter	Delivers a near-sinusoidal output	Most expensive, largest, some voltage drop

We are not publishing a cable-length threshold. Makers publish different limits — and the limit depends on the drive's switching characteristics, the cable type and the motor's insulation class, not on the number of metres alone. Find the stated maximum motor cable length in your drive's documentation and stay inside it. If you need longer than that, fit a reactor or filter rather than arguing with the number.

On the supply side

Line reactor — reduces harmonics drawn from the supply and protects the drive's rectifier from supply transients. Cheap, and worth having on anything but a tiny drive.
Input EMI filter — keeps switching noise from travelling backwards into your supply and out to other equipment. This is the part that fixes interference in the rest of the building.
Do not fit power-factor correction capacitors on the drive output. They resonate with the motor inductance.

Separation and routing

Keep the motor cable physically away from signal and USB cables. Where they must cross, cross at right angles.
Do not run signal and motor cable in the same conduit.
Keep the screen continuous through any junction — a break in the screen at a terminal box undoes the whole point of having one.

Things that must not be in the output circuit

Contactors or switches that operate while the drive is running. Use the drive's control inputs to start and stop. Breaking the output under current is a reliable way to destroy the output stage.
Capacitors for power-factor correction.
Any kind of earth-leakage device sized for a sine wave — see the note on single-phase input.

Cheapest effective order: shorten the cable → fit an output reactor → fit a dv/dt filter. And on the supply side, a line reactor before an EMI filter.

Parts

Filtering and suppression

Questions

Do I need a special cable between the drive and the spindle?

Yes. A symmetrical screened cable with low capacitance, intended for VFD output. Ordinary flexible cord radiates and ages badly under PWM.

What does an output reactor actually do?

It slows the voltage edges coming out of the drive, which reduces reflected-wave peaks at the motor and lowers the high-frequency current in the screen.

Can I put a filter in the output circuit?

A sine filter or a dV/dt filter is made for exactly that. A standard EMC input filter is not — it belongs on the supply side, and the voltage drop calculator will tell you what the run is costing you on that side.

Sources and method

Three kinds of statement appear on this page, and each is labelled where it occurs:

Derived — the reflected-wave and voltage-drop behaviour is described qualitatively, because the threshold at which it matters depends on your drive and cable.
Rule of thumb — workshop practice, not a standard. Given with the range it is meant to hold over, and it loses to your manual wherever the two disagree.
Maker-specific — parameter numbers, connector pin numbers, minimum brake resistance, maximum cable length, bearing speed limits. Not published here. They move between firmware revisions of the same model, and a wrong one is worse than none.

Standards and documents

IEC 61800-3 — adjustable speed drives: EMC requirements
IEC 61000-6-4 — EMC emission, industrial environments
IEC 60204-1 — electrical equipment of machines
The manual for your own drive and spindle — the only valid source for maker-specific values