VFDSPINDLE VFD & CNC spindle wiring, sizing and setup

VFD sizing calculator

Put the spindle nameplate in, get the drive current class, the supply current and a cable size out. Every number is derived on the page, and every maker-specific value is deliberately left for your own manual.

Size a drive from a spindle nameplate

Leave rated current blank if the nameplate does not give it — it will be estimated from power, voltage, power factor and efficiency.

What it does and does not do

Does: turns nameplate power into a current, applies your margin, checks your supply against your spindle voltage, and estimates what the supply circuit has to carry.

Does not: know your drive's derating curve, your ambient temperature, your altitude, or your local wiring rules. A calculator that pretended to would be giving you a number you could not defend.

The two numbers that decide everything

  1. Rated current. The drive's continuous output current rating must exceed it. Read why current beats kilowatts.
  2. Rated voltage. Must match the drive's output class. No amount of margin fixes a mismatch.
About the single-phase figure. The √3 comes from equating input and output power at the same voltage. Real drives also have losses, and a single-phase input often has a lower published rating than the three-phase rating on the same frame. Treat the calculator's supply current as the least you should design for.

Worked example: 2.2 kW, 220 V, single-phase 230 V supply

Take a common water-cooled router spindle: 2.2 kW, 220 V three-phase, no rated current on the plate, running to 400 Hz.

  1. Estimate the current. With cosφ = 0.85 and η = 0.85: I = 2200 / (√3 × 220 × 0.85 × 0.85) ≈ 8.0 A. That is the figure a nameplate would confirm or correct — and if the plate says something materially different, the plate wins.
  2. Apply the margin. At 20 %, the drive needs to offer at least about 9.6 A continuous output. Note what that does to the shopping: plenty of drives labelled “2.2 kW” are rated below 9.6 A on the output, because their label assumes a standard motor at 50 Hz.
  3. Size the supply. On a single-phase 230 V input the supply current is roughly √3 × 8.0 ≈ 13.9 A, before drive losses. Design the circuit for more than that, not for 2.2 kW divided by 230 V.
  4. Sanity-check the voltage. A single-phase 230 V input gives about 230 V out, which matches a 220 V winding. If this spindle had been the 380 V version, step four would have failed and the fix would have been a different spindle, a relinkable winding, or a transformer — not a bigger drive.
  5. Then check the derating. The spindle lives at 400 Hz, permanently above base frequency. Whatever the drive's nameplate says, look for its continuous current at that output frequency.

After you have the number

Parts

Drives

Questions

Should I size the drive on kW or on current?

On current. Drives are rated by output current, and the current your spindle draws depends on its voltage and efficiency, not on the power figure alone.

How much margin should I leave?

Enough that the drive is not sitting at its continuous rating at full load, and more again if you are feeding it from a single-phase supply. The calculator shows both cases.

Can an oversized drive hurt?

It costs more and it will not protect a small spindle as tightly, but electrically it is safe. An undersized drive trips under load — usually with a code that looks like something else.

Sources and method

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

Standards and documents

Further reading