Documentation sections

VFD Motor Speed & Torque Calculator

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

Slug vfd-motor-speed. Calculate AC induction motor speed under VFD (Variable Frequency Drive) control. Enter pole count, line and drive frequencies to get synchronous speed, actual RPM with slip, torque derating above base speed, and V/Hz characteristics.

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 vfd-motor-speed
Name	Unit	Range	Default	Description
polesnumber	none	2 … 48	4	2-pole = 3000/3600 RPM, 4-pole = 1500/1800 RPM
lineFrequencynumber	Hz	25 … 400	60	50 Hz (EU/Asia) or 60 Hz (US/Americas)
driveFrequencynumber	Hz	0.5 … 400	45	Drive Output Frequency
ratedSlipnumber	%	0 … 15	3	Typical: 2–5% for standard motors

Outputs

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

Outputs of vfd-motor-speed
Field	Unit	Description
actualSpeedDriveheadline	RPM	Actual Motor Speed
syncSpeedDrive	RPM	Synchronous Speed
actualSpeedLine	RPM	Speed at Line Frequency
torqueDerating	%	Available Torque
powerDerating	%	Available Power
speedRatio	none	Speed Ratio
frequencyRatio	none	Frequency Ratio

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": "vfd-motor-speed",
    "inputs": {
      "poles": 4,
      "lineFrequency": 60,
      "driveFrequency": 45,
      "ratedSlip": 3
    }
  }'

import rftools

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

result = client.calculate("vfd-motor-speed", {"poles": 4, "lineFrequency": 60, "driveFrequency": 45, "ratedSlip": 3})
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": "vfd-motor-speed",
    "inputs": {
      "poles": 4,
      "lineFrequency": 60,
      "driveFrequency": 45,
      "ratedSlip": 3
    }
  }),
});
console.log(response.status, await response.json());

package main

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

func main() {
	body := []byte(`{
	  "slug": "vfd-motor-speed",
	  "inputs": {
	    "poles": 4,
	    "lineFrequency": 60,
	    "driveFrequency": 45,
	    "ratedSlip": 3
	  }
	}`)
	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": "vfd-motor-speed",
  "values": {
    "syncSpeedLine": 1800,
    "syncSpeedDrive": 1350,
    "actualSpeedLine": 1746,
    "actualSpeedDrive": 1309.5,
    "torqueDerating": 100,
    "powerDerating": 75,
    "voltageAtDrive": 0.75,
    "speedRatio": 0.75,
    "frequencyRatio": 0.75
  },
  "warnings": [],
  "errors": [],
  "provenance": {
    "method": "calculator:vfd-motor-speed",
    "version": "api@3f2c1a9b04de",
    "formulaRef": null,
    "assumptions": [],
    "validRange": {
      "status": "inside",
      "bounds": {
        "poles": {
          "min": 2,
          "max": 48,
          "unit": ""
        },
        "lineFrequency": {
          "min": 25,
          "max": 400,
          "unit": "Hz"
        },
        "driveFrequency": {
          "min": 0.5,
          "max": 400,
          "unit": "Hz"
        },
        "ratedSlip": {
          "min": 0,
          "max": 15,
          "unit": "%"
        }
      },
      "outside": [],
      "model": null
    },
    "computedAt": "2026-09-24T14:05:11.003Z",
    "inputs": {
      "poles": 4,
      "lineFrequency": 60,
      "driveFrequency": 45,
      "ratedSlip": 3
    },
    "seed": null,
    "elapsedSeconds": 0.003
  }
}

{
  "detail": [
    {
      "type": "float_parsing",
      "loc": [
        "body",
        "inputs",
        "poles"
      ],
      "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 pageVFD Motor Speed & Torque Calculator, at these values