BLDC motor sizing estimate calculator

Mechanical power and approximate DC bus current at a supplied efficiency.

motors-bldc-sizing.napkin / editable

SCENARIO COMPARISON

Compare the numbers.

This source describes an analytical model. Its quantities and checks are the view.

Scenario	shaftPowerW	busCurrentA
	314.159	15.4
	251.327	12.32

All calculated quantities are listed with the inputs. Switch to Source to inspect the equations.

SCENARIOS / CALCULATED VALUESSAME SOURCE

Change an input to see the consequences.

bus current budget15.40 A✓ passes

BASELINE INPUTS

torque1 N*mspeed3000 rpmvoltage24 Vefficiency0.85

CALCULATED

shaftPower314.16 W

busCurrent15.4 A

Edits change the baseline source.

THE SOURCE

Edits below update the same model.

# BLDC motor sizing estimate
# Scope: Mechanical power and approximate DC bus current at a supplied efficiency.
# Status: illustrative worked example; engineering review pending.
# Inputs are example values. Checks are editable example requirements, not standards.

torque = 1 N*m
speed = 3000 rpm
voltage = 24 V
efficiency = 0.85

shaftPower := torque*speed [W]
busCurrent := shaftPower/(voltage*efficiency) [A]

check bus current budget: busCurrent <= 20 A

scenario alternative: torque = 0.8 N*m

Calculated from your source · not saved

Baseline · Baseline

bus current budget15.40 A ✓ passes

alternative · alternative

bus current budget12.32 A ✓ passes

Illustrative first-order example. Checks are editable requirements, not design standards. The broader topic may require additional equations, data, and validation.

Formula and units

This example uses the following equations. Input names and units match the editable source above.

shaft power: shaftPower = torque*speed [W]
bus current: busCurrent = shaftPower/(voltage*efficiency) [A]

Original worked example

These are the baseline values in the downloadable source. Edits above let you test your own values.

Given

torque: 1 N*m
speed: 3000 rpm
voltage: 24 V
efficiency: 0.85

Calculated

shaft power: 314.159 W
bus current: 15.4 A

Assumptions and limits

Mechanical power and approximate DC bus current at a supplied efficiency.

The requirement checks use the limits written in this example. Passing a check does not establish that the model covers every load, material behavior, or operating condition in a real design.

Download this .napkin example ↓

Take it further with your agent

Napkin keeps the inputs, equations, checks and assumptions together in one study. Your agent sets it up; you change the numbers.

Try with Claude ↗

codex 'Use Napkin (https://napkin.engineering/mcp, or the local Napkin plugin) to build a study: BLDC motor sizing estimate. Inputs: load torque-speed envelope; duty; bus voltage. Outputs: current demand; power; candidate motor constraints. Model scope: Motor and load parameters with explicit operating limits. Read napkin_capabilities and the current snapshot. Ask me for the values that matter, label assumptions with units, author .napkin source, validate it and apply it with napkin_source using the current revision, then open the study. Add checks for the limits I supply. Keep unsupported analyses explicit.'

Claude needs the Napkin connector; Codex needs the Napkin plugin. Set up your agent ↗

The broader study

The topic brief below describes the broader scope. The worked example above states the particular calculation it implements.

Inputs: load torque-speed envelope, duty, bus voltage
Outputs: current demand, power, candidate motor constraints
Modes: calculate, compare, sweep
Drawing: Torque-speed and duty plots

EXECUTABLE WORKED EXAMPLE · FIRST-ORDER MODEL · CHECK THE SCOPE & ASSUMPTIONS