Wire a VFD to a CNC spindle, and size it correctly

A free reference for anyone running a CNC spindle off a variable frequency drive: what connects to what, how big the drive has to be, and the five checks that stop you from letting the smoke out on the first power-up.

Start here

Size the drive

Spindle power and current in; drive rating and supply current out, with the arithmetic shown.

VFD sizing calculator →

Speed and torque

What a 2.2 kW spindle actually delivers at 6,000 rpm against 24,000, and where constant power starts.

Speed / torque calculator →

Wire it up

Power, earth, brake resistor, run/stop and the 0–10 V reference in one schematic.

Wiring diagram →

Identify the connector

No pinout tables. A meter, four measurements, and the order to take them in.

Connector pinouts →

Check the cable run

Voltage drop and copper losses from current, length and cross-section. Resistivity only, no tables.

Cable drop calculator →

Convert the units

kW to hp, N·m to in·lb, rpm to Hz with the pole count. Constants shown on the page.

Unit converter →

Start with the calculators

VFD sizing calculator — put in the nameplate, get the drive current class, the supply current and the cable size back
Speed / torque calculator — what your spindle actually delivers at 6,000 rpm versus at 24,000 rpm
Wiring diagram — power, control and brake resistor, one page
Connector pinouts — how to find out which pin is which when the seller does not tell you

Five checks before you apply power

These are the failures that cost a spindle or a drive within the first minute. None of them need special equipment beyond a multimeter.

Pre-power checklist — do these with the VFD disconnected from the spindle
#	Check	What “wrong” looks like
1	Read the spindle nameplate: rated voltage, rated current, rated frequency, poles	380 V spindle on a 220 V drive output — common and fatal to expectations, if not to hardware
2	Resistance U–V, V–W, W–U balanced and low	One pair open = a break; one pair much lower = shorted turns
3	Every phase reads open to the spindle body (ideally megohms)	Anything but open = do not power up
4	Earth (PE) bonded to the VFD chassis and the spindle frame	Frame floating — the drive enclosure can sit at a hazardous voltage
5	Cooling running before the first cut: water flowing, or fan unobstructed	A water-cooled spindle with an air lock or a dry pump dies quietly in minutes

Measuring resistance is not optional. On a typical 2.2 kW spindle the winding resistance is well under one ohm per phase, which most cheap meters struggle to read accurately. You are looking for balance between the three pairs more than an absolute number. Do it with the spindle unplugged from the drive: a drive connected across the windings will read back as a fault, and insulation testers will damage drive electronics that are still connected.

Browse by topic

Wiring

Wiring basics

What a VFD actually does to your supply, why U/V/W order sets direction, and the terminal names that are not consistent between brands.

Wiring diagram

Power, control, brake resistor and speed reference in one schematic, plus the five wiring mistakes that break things.

Connector pinouts

There is no universal pinout for the aviation connector on a spindle. Here is how to find out yours in ten minutes.

Parameter setup

The five parameters that matter and why we will not publish a parameter number table.

Single-phase input

Running a three-phase spindle from a domestic socket: what gets derated, and the 220 V output ceiling you cannot get around.

Grounding & noise

Why USB drops out and limit switches false-trigger when the spindle starts, and the order to fix it in.

Sizing & performance

VFD sizing

Size on current, not on kilowatts. Why, plus what to do when the nameplate is missing.

Speed & torque

The constant-torque region, the constant-power region, and why 2.2 kW at 24,000 rpm is a much smaller motor than it sounds.

Sizing calculator

Nameplate in, drive class and supply current out.

Speed / torque calculator

Torque at every speed across the range, with the field-weakening limit made explicit.

Hardware

Spindle types

Air versus water cooled, bearing classes, duty ratings, and what the power figure is measured at.

Cooling

Loops, pumps, coolant, flow switches and the failure modes that quietly shorten spindle life.

Braking resistors

When you need one, how to pick the ohms and the watts, and the minimum value you must not go under.

Cables & filters

Screened motor cable, long-run reflected waves, line reactors and EMI filters.

Reference

Fault codes

Overcurrent, overvoltage, overload, earth fault — grouped by what the drive is actually telling you.

How to choose

One buying sequence, in the order that prevents the expensive mistakes.

Data sources and method

This site separates three kinds of statement, and the distinction is the point:

Derived — worked out from physics and shown on the page. Synchronous speed, torque from power and speed, the √3 input-current relationship, DC bus voltage, brake resistor ohms. You can check the arithmetic yourself.
Rule of thumb — widely used practice with no standard behind it: cable separation distances, carrier frequency ranges, derating margins. Labelled as such, and deliberately given as ranges rather than single numbers.
Maker-specific — parameter numbers, pin numbers, terminal labels, minimum brake resistance, allowable cable length. We do not publish these. They move between firmware revisions of the same model, and a wrong one is worse than none. Every page that needs one tells you where to find it in your own documentation instead.

Where a value genuinely has no single answer — pinouts, long-cable thresholds, single-phase derating — the page says so rather than picking a number for you.

Standards and documents

IEC 61800 series — adjustable speed electrical power drive systems (ratings, EMC, safety)
IEC 60204-1 — electrical equipment of machines: general requirements
IEC 60034-1 — rotating electrical machines: rating and performance
The manual for your own drive and spindle — the only valid source for maker-specific values