Documentation sections

DC Motor Speed Calculator

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

Slug dc-motor-speed. Calculate DC motor speed, torque, power, and efficiency from electrical parameters

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 dc-motor-speed
Name	Unit	Range	Default	Description
voltagenumber	V	at least 0	12	Supply Voltage
resistancenumber	Ω	at least 0.01	0.5	Armature resistance
backEmfConstnumber	V/RPM	at least 0.0001	0.01	Back-EMF constant
torqueConstnumber	N·m/A	at least 0.001	0.1	Torque constant
loadTorquenumber	N·m	at least 0	0.1	Load Torque
efficiencynumber	%	10 … 100	85	Mechanical Efficiency

Outputs

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

Outputs of dc-motor-speed
Field	Unit	Description
noLoadSpeed	RPM	No-Load Speed
loadSpeedheadline	RPM	Speed Under Load
current	A	Armature Current
outputPower	W	Output Power
inputPower	W	Input Power
torque	N·m	Torque
efficiencyPct	%	Overall Efficiency

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": "dc-motor-speed",
    "inputs": {
      "voltage": 12,
      "resistance": 0.5,
      "backEmfConst": 0.01,
      "torqueConst": 0.1,
      "loadTorque": 0.1,
      "efficiency": 85
    }
  }'

import rftools

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

result = client.calculate("dc-motor-speed", {
    "voltage": 12,
    "resistance": 0.5,
    "backEmfConst": 0.01,
    "torqueConst": 0.1,
    "loadTorque": 0.1,
    "efficiency": 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": "dc-motor-speed",
    "inputs": {
      "voltage": 12,
      "resistance": 0.5,
      "backEmfConst": 0.01,
      "torqueConst": 0.1,
      "loadTorque": 0.1,
      "efficiency": 85
    }
  }),
});
console.log(response.status, await response.json());

package main

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

func main() {
	body := []byte(`{
	  "slug": "dc-motor-speed",
	  "inputs": {
	    "voltage": 12,
	    "resistance": 0.5,
	    "backEmfConst": 0.01,
	    "torqueConst": 0.1,
	    "loadTorque": 0.1,
	    "efficiency": 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": "dc-motor-speed",
  "values": {
    "noLoadSpeed": 1200,
    "loadSpeed": 1150,
    "current": 1,
    "outputPower": 10.236356062946744,
    "inputPower": 12,
    "torque": 0.1,
    "efficiencyPct": 85.30296719122286
  },
  "warnings": [],
  "errors": [],
  "provenance": {
    "method": "calculator:dc-motor-speed",
    "version": "api@3f2c1a9b04de",
    "formulaRef": "Chapman, Electric Machinery Fundamentals",
    "assumptions": [],
    "validRange": {
      "status": "inside",
      "bounds": {
        "voltage": {
          "min": 0,
          "max": null,
          "unit": "V"
        },
        "resistance": {
          "min": 0.01,
          "max": null,
          "unit": "Ω"
        },
        "backEmfConst": {
          "min": 0.0001,
          "max": null,
          "unit": "V/RPM"
        },
        "torqueConst": {
          "min": 0.001,
          "max": null,
          "unit": "N·m/A"
        },
        "loadTorque": {
          "min": 0,
          "max": null,
          "unit": "N·m"
        },
        "efficiency": {
          "min": 10,
          "max": 100,
          "unit": "%"
        }
      },
      "outside": [],
      "model": null
    },
    "computedAt": "2026-09-24T14:05:11.003Z",
    "inputs": {
      "voltage": 12,
      "resistance": 0.5,
      "backEmfConst": 0.01,
      "torqueConst": 0.1,
      "loadTorque": 0.1,
      "efficiency": 85
    },
    "seed": null,
    "elapsedSeconds": 0.003
  }
}

{
  "detail": [
    {
      "type": "float_parsing",
      "loc": [
        "body",
        "inputs",
        "voltage"
      ],
      "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 pageDC Motor Speed Calculator, at these values