Documentation sections

BLDC Winding Calculator

POSThttps://rftools.io/api/py/v1/calculate

Slug bldc-winding. Calculate BLDC motor winding parameters: turns per coil, wire gauge, fill factor, winding factor, and phase resistance. Visual winding scheme diagram for delta and wye configurations.

Inputs

Send every one of these in inputs, in the units shown. Most are required: a call without one is refused (400) and costs nothing; one the calculator can do without takes the default shown. An input outside its range is still computed and flagged in the provenance.

Inputs of bldc-winding
Name	Unit	Range	Default	Description
targetKvnumber	RPM/V	1 … 10000	935	Desired motor velocity constant — determines turns per coil
poleCountnumber	poles	2 … 60	14	Number of permanent magnet poles (always even)
slotCountnumber	slots	3 … 72	12	Number of stator slots (should be multiple of 3)
statorInnerDianumber	mm	5 … 300	22	Inner bore diameter of the stator
statorStackLengthnumber	mm	1 … 200	13	Axial depth of the stator lamination stack
maxCurrentnumber	A	0.1 … 500	5	Maximum continuous phase current for wire gauge selection
supplyVoltagenumber	V	1 … 400	11.1	Supply Voltage
windingTypenumber	none	0 … 1	0	Winding TypeOne of: 0 Wye (Y) · 1 Delta (Δ)

Outputs

Returned in values, by key. A value that is unbounded or undefined is null.

Outputs of bldc-winding
Field	Unit	Description
turnsPerCoilheadline	turns	Number of wire turns per stator coil
wireAwg	none	American Wire Gauge for target current density of 6.5 A/mm²
wireDiameter	mm	Wire Diameter
fillFactor	%	Bare copper of both coil sides in a slot over the slot area, 2·N·A_wire/A_slot — above 75% is very difficult to wind
phaseResistance	mΩ	Phase Resistance
coilSpan	slots	Coil Span
windingFactor	none	Fundamental winding factor = pitch factor × distribution factor
actualKv	RPM/V	Actual Kv with integer turns (may differ from target)
deltaEquivKv	RPM/V	Equivalent Kv if winding connection type were changed (Kv_delta = Kv_wye × √3)
noLoadSpeed	RPM	Achieved Kv × supply voltage: the unloaded speed on this supply, where the back-EMF equals it

Calling it

Calculate: one call, the request and response in full, and its errors.
Batch: this calculator with others, or a sweep, in one request.
Solve for a target: the value of one input that brings an output to a target.

curl -X POST https://rftools.io/api/py/v1/calculate \
  -H "X-API-Key: $RFTOOLS_API_KEY" \
  -H "Content-Type: application/json" \
  -d '{
    "slug": "bldc-winding",
    "inputs": {
      "targetKv": 935,
      "poleCount": 14,
      "slotCount": 12,
      "statorInnerDia": 22,
      "statorStackLength": 13,
      "maxCurrent": 5,
      "supplyVoltage": 11.1,
      "windingType": 0
    }
  }'

import rftools

client = rftools.Client()  # reads RFTOOLS_API_KEY, or the key `rftools login` saved

result = client.calculate("bldc-winding", {
    "targetKv": 935,
    "poleCount": 14,
    "slotCount": 12,
    "statorInnerDia": 22,
    "statorStackLength": 13,
    "maxCurrent": 5,
    "supplyVoltage": 11.1,
    "windingType": 0,
})
print(result.values)
print(result.warnings, result.provenance.formula_ref)

const response = await fetch("https://rftools.io/api/py/v1/calculate", {
  method: "POST",
  headers: {
    "X-API-Key": process.env.RFTOOLS_API_KEY,
    "Content-Type": "application/json",
  },
  body: JSON.stringify({
    "slug": "bldc-winding",
    "inputs": {
      "targetKv": 935,
      "poleCount": 14,
      "slotCount": 12,
      "statorInnerDia": 22,
      "statorStackLength": 13,
      "maxCurrent": 5,
      "supplyVoltage": 11.1,
      "windingType": 0
    }
  }),
});
console.log(response.status, await response.json());

package main

import (
	"bytes"
	"fmt"
	"io"
	"net/http"
	"os"
)

func main() {
	body := []byte(`{
	  "slug": "bldc-winding",
	  "inputs": {
	    "targetKv": 935,
	    "poleCount": 14,
	    "slotCount": 12,
	    "statorInnerDia": 22,
	    "statorStackLength": 13,
	    "maxCurrent": 5,
	    "supplyVoltage": 11.1,
	    "windingType": 0
	  }
	}`)
	req, err := http.NewRequest("POST", "https://rftools.io/api/py/v1/calculate", bytes.NewReader(body))
	if err != nil {
		panic(err)
	}
	req.Header.Set("X-API-Key", os.Getenv("RFTOOLS_API_KEY"))
	req.Header.Set("Content-Type", "application/json")
	res, err := http.DefaultClient.Do(req)
	if err != nil {
		panic(err)
	}
	defer res.Body.Close()
	out, _ := io.ReadAll(res.Body)
	fmt.Println(res.Status, string(out))
}

application/json

{
  "slug": "bldc-winding",
  "values": {
    "turnsPerCoil": 4,
    "wireAwg": 18,
    "wireDiameter": 1.024,
    "fillFactor": 47.23709144930302,
    "phaseResistance": 13.876901160598317,
    "coilSpan": 1,
    "windingFactor": 0.9330127018922194,
    "actualKv": 1027.607626225481,
    "deltaEquivKv": 1779.8686188677814,
    "noLoadSpeed": 11406.444651102838
  },
  "warnings": [],
  "errors": [],
  "provenance": {
    "method": "calculator:bldc-winding",
    "version": "api@3f2c1a9b04de",
    "formulaRef": "Hanselman, D. — Brushless Permanent Magnet Motor Design, 2nd ed.; Bianchi, N. & Dai Pré, M., IEE Proc. Electric Power Applications, 2006 (winding factor); Pyrhönen, Jokinen & Hrabovcová, Design of Rotating Electrical Machines, Wiley 2008, section 2.9 (slot fill)",
    "assumptions": [
      {
        "code": "slot-area-inrunner-bore",
        "text": "The slot area is estimated from the stator bore as for an inrunner, 0.55·πd/S wide by 0.2·d deep. An outrunner's slot is smaller, so its real fill factor is higher than shown."
      }
    ],
    "validRange": {
      "status": "inside",
      "bounds": {
        "targetKv": {
          "min": 1,
          "max": 10000,
          "unit": "RPM/V"
        },
        "poleCount": {
          "min": 2,
          "max": 60,
          "unit": "poles"
        },
        "slotCount": {
          "min": 3,
          "max": 72,
          "unit": "slots"
        },
        "statorInnerDia": {
          "min": 5,
          "max": 300,
          "unit": "mm"
        },
        "statorStackLength": {
          "min": 1,
          "max": 200,
          "unit": "mm"
        },
        "maxCurrent": {
          "min": 0.1,
          "max": 500,
          "unit": "A"
        },
        "supplyVoltage": {
          "min": 1,
          "max": 400,
          "unit": "V"
        },
        "windingType": {
          "min": 0,
          "max": 1,
          "unit": ""
        }
      },
      "outside": [],
      "model": null
    },
    "computedAt": "2026-09-24T14:05:11.003Z",
    "inputs": {
      "targetKv": 935,
      "poleCount": 14,
      "slotCount": 12,
      "statorInnerDia": 22,
      "statorStackLength": 13,
      "maxCurrent": 5,
      "supplyVoltage": 11.1,
      "windingType": 0
    },
    "seed": null,
    "elapsedSeconds": 0.003
  }
}

{
  "detail": [
    {
      "type": "float_parsing",
      "loc": [
        "body",
        "inputs",
        "targetKv"
      ],
      "msg": "Input should be a valid number, unable to parse string as a number",
      "input": "abc"
    }
  ]
}

An input sent as text. Nothing was counted.

{
  "detail": "More than 120 requests in 60 seconds from this address.",
  "errorKind": "rate_limited",
  "limit": 120,
  "windowSeconds": 60,
  "upgradeUrl": "https://rftools.io/pricing/"
}

Too many requests in a minute, with Retry-After.

Same inputs on the calculator pageBLDC Winding Calculator, at these values