Documentation sections

BLDC Efficiency Analyzer

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

Slug bldc-efficiency. Analyze BLDC motor efficiency at any operating point. Breaks down copper, iron, and mechanical losses. Finds the optimal current and RPM for peak efficiency.

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-efficiency
Name	Unit	Range	Default	Description
kvRatingnumber	RPM/V	1 … 10000	920	Kv Rating
phaseResistancenumber	mΩ	at least 0.1	45	Winding resistance measured between two motor leads, as datasheets quote it; the supply current flows through it under block commutation
noLoadCurrentnumber	A	at least 0.001	1.5	Current drawn at no mechanical load (iron + friction losses)
supplyVoltagenumber	V	at least 1	22.2	Supply Voltage
operatingCurrentnumber	A	at least 0.01	10	Actual load current to analyze
targetEfficiencynumber	%	0 … 100	85	The efficiency the motor must reach at the operating current. The status fails below 50 %.

Retired, still accepted and ignored with a warning: polePairs (Pole Pairs, 2026-10-12).

Outputs

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

Outputs of bldc-efficiency
Field	Unit	Description
copperLoss	W	I²·R_tt: the supply current through the terminal resistance (block commutation)
ironLoss	W	Iron Loss
mechanicalLoss	W	Mechanical Loss
totalLoss	W	Total Loss
outputPower	W	Output Power
efficiencyheadline	%	Efficiency
optimalCurrent	A	Current that maximizes efficiency: √(I₀ × I_stall)
optimalRpm	RPM	Optimal RPM
copperPct	%	Copper Loss Share
ironPct	%	Iron Loss Share
mechPct	%	Mechanical Loss Share

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-efficiency",
    "inputs": {
      "kvRating": 920,
      "phaseResistance": 45,
      "noLoadCurrent": 1.5,
      "supplyVoltage": 22.2,
      "operatingCurrent": 10,
      "targetEfficiency": 85
    }
  }'

import rftools

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

result = client.calculate("bldc-efficiency", {
    "kvRating": 920,
    "phaseResistance": 45,
    "noLoadCurrent": 1.5,
    "supplyVoltage": 22.2,
    "operatingCurrent": 10,
    "targetEfficiency": 85,
})
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-efficiency",
    "inputs": {
      "kvRating": 920,
      "phaseResistance": 45,
      "noLoadCurrent": 1.5,
      "supplyVoltage": 22.2,
      "operatingCurrent": 10,
      "targetEfficiency": 85
    }
  }),
});
console.log(response.status, await response.json());

package main

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

func main() {
	body := []byte(`{
	  "slug": "bldc-efficiency",
	  "inputs": {
	    "kvRating": 920,
	    "phaseResistance": 45,
	    "noLoadCurrent": 1.5,
	    "supplyVoltage": 22.2,
	    "operatingCurrent": 10,
	    "targetEfficiency": 85
	  }
	}`)
	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-efficiency",
  "values": {
    "copperLoss": 4.5,
    "ironLoss": 32.19446127288026,
    "mechanicalLoss": 0.10124999999999999,
    "totalLoss": 36.79571127288026,
    "outputPower": 185.20428872711975,
    "efficiency": 83.42535528248636,
    "optimalCurrent": 27.202941017470888,
    "optimalRpm": 19297.798241876706,
    "copperPct": 12.229686135505306,
    "ironPct": 87.49514592644583,
    "mechPct": 0.27516793804886935
  },
  "warnings": [],
  "errors": [],
  "provenance": {
    "method": "calculator:bldc-efficiency",
    "version": "api@3f2c1a9b04de",
    "formulaRef": "Drela, First-Order DC Electric Motor Model (MIT, 2007), eqs. 3, 7 and 9; maxon, Formulae Handbook (J. Braun, maxon academy, 2012), p. 37; Hanselman, D. — Brushless Permanent Magnet Motor Design",
    "assumptions": [
      {
        "code": "block-commutation-terminal-resistance",
        "text": "The motor is driven by block (six-step) commutation, so the supply current flows through the terminal resistance measured between two leads and the copper loss is I²·R (Drela 2007; maxon Formulae Handbook 2012, p. 37). A sinusoidal drive with I the phase rms current would give 1.5·I²·R for the same resistance."
      },
      {
        "code": "no-load-loss-split",
        "text": "The no-load input power V·I₀ less the no-load copper loss I₀²·R is iron loss, scaled with speed to the power 1.5; the mechanical loss is taken equal to the no-load copper loss."
      }
    ],
    "validRange": {
      "status": "inside",
      "bounds": {
        "kvRating": {
          "min": 1,
          "max": 10000,
          "unit": "RPM/V"
        },
        "phaseResistance": {
          "min": 0.1,
          "max": null,
          "unit": "mΩ"
        },
        "noLoadCurrent": {
          "min": 0.001,
          "max": null,
          "unit": "A"
        },
        "supplyVoltage": {
          "min": 1,
          "max": null,
          "unit": "V"
        },
        "operatingCurrent": {
          "min": 0.01,
          "max": null,
          "unit": "A"
        },
        "targetEfficiency": {
          "min": 0,
          "max": 100,
          "unit": "%"
        }
      },
      "outside": [],
      "model": null
    },
    "computedAt": "2026-09-24T14:05:11.003Z",
    "inputs": {
      "kvRating": 920,
      "phaseResistance": 45,
      "noLoadCurrent": 1.5,
      "supplyVoltage": 22.2,
      "operatingCurrent": 10,
      "targetEfficiency": 85
    },
    "seed": null,
    "elapsedSeconds": 0.003
  }
}

{
  "detail": [
    {
      "type": "float_parsing",
      "loc": [
        "body",
        "inputs",
        "kvRating"
      ],
      "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 Efficiency Analyzer, at these values