How to size a VFD for a spindle

Size on current, not on kilowatts. That single rule resolves most of the confusion, and it is the reason two people can look at the same spindle and correctly order drives with different power ratings on the label.

Why current and not power

A drive is limited by what its output transistors can carry. Its power rating is a convenience label, set by assuming a typical motor: a certain voltage, a certain power factor, a certain efficiency, at a certain frequency. Your spindle is not that motor.

A high-speed spindle is the clearest case. It runs at 400 Hz, far above the 50/60 Hz the drive's power label assumes, and the current it draws per kilowatt is not the current a standard four-pole motor draws. Match the label and you can be an amp or two short without anything on the box telling you so.

The rule: the drive's rated output current must be at least the spindle's rated current, and comfortably above it. If you have both numbers, the power figures are decoration.

Reading the nameplate

Everything you need is printed on the spindle. If it is not, see the fallback below.

What to take from the nameplate, and what to do with it
Nameplate says	Use it for
Rated voltage	The drive's output voltage class must match. This is a hard constraint
Rated current	The number you actually size against
Rated frequency	Base frequency — the top of the constant-torque region
Rated speed / poles	How output frequency maps to rpm; also tells you the maximum frequency you need
Rated power	Sanity check, and the label on the drive you will be sold
Duty / insulation class	Whether the rating is continuous or intermittent

When the nameplate is missing or unreadable

Common with unbranded spindles. Work in this order:

Measure the winding resistance across each pair. It tells you the winding is intact, not its rating — but it is the first thing to check anyway.
Count the leads and look for a link arrangement in the terminal box. Six leads suggests a dual-voltage winding and therefore a lower per-phase voltage.
Use the physical size as a bound. Power scales roughly with the rotor volume, so a spindle of a given body diameter and length has a plausible power band. This is an estimate, not a measurement — label it as one and leave margin.
Order a drive one class above your best guess, set rated current conservatively, and watch the drive's current readout on the first real cut. The drive will tell you what the spindle draws, and it is the cheapest instrument you have.

Do not guess the voltage, and do not guess upward. Applying 380 V to a winding rated 220 V is not a conservative error; it over-fluxes the motor. If you cannot establish the voltage, establish it before buying anything.

How much margin

Two things argue for headroom and one argues against it:

For: drives derate. Above base frequency many makers publish a reduced continuous current, ambient temperature above the rating point reduces it further, and so does altitude. If your spindle spends its life at 400 Hz, you are living in the derated region permanently.
For: cutting loads are spiky. A drive sized exactly at rated current will sit near its limit on every heavy pass.
Against: oversizing costs money, and a drive far larger than the motor protects it less precisely — the overload curve is set for the drive, not for your spindle.

A modest margin is the usual answer, and the calculator defaults to one you can change. What matters more than the exact figure is that you check the drive's derating documentation for your actual conditions, because that is where a nominally correct choice turns out to be short.

Also check, before you buy

Maximum output frequency. Not every drive will go to 400 Hz. Check the specification, not the marketing.
Input class. Single-phase input is a different rating from three-phase on the same frame — see single-phase input.
Enclosure and cooling. A drive in a sealed cabinet with no airflow will derate until it trips on temperature, whatever its rating says.
Braking. If you need a short stop, you need a brake resistor, and the drive needs the braking transistor and terminals to drive it. See braking resistors.

Then run the numbers. The VFD sizing calculator takes the nameplate, applies the margin, and returns the drive current class plus the supply current and cable size.

Parts

Drives

Questions

The nameplate is faded. How do I size the drive?

Measure the winding resistance to identify the phases and the voltage class, then work from the frame size and rated speed. When in doubt, size on the largest plausible current.

Why does my spindle draw more current than the arithmetic suggests?

Because power factor and efficiency are both below one. Use the nameplate current, not the number you get by dividing watts by volts.

Do I need to oversize for a single-phase supply?

Yes. The input stage carries more current per leg on single phase and makers derate the drive accordingly. Check the single-phase rating, not the headline one.

Sources and method

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

Derived — the drive rating falls out of spindle current and the duty you actually run, and the page shows which of the two governs.
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 61800-2 — adjustable speed drives: rating specifications
IEC 60034-1 — rotating electrical machines: rating and performance
NEMA MG 1 — motors and generators (service factor and overload)
The manual for your own drive and spindle — the only valid source for maker-specific values