What Is Stall Torque?

Stall torque is the maximum torque a motor produces when:

• The motor shaft is not rotating
• The speed is 0 RPM
• Back EMF is zero

At this point, the motor draws its highest current. That is called stall current.

In simple terms:

Stall torque = Maximum twisting force at zero speed.

This is common in:

• DC motors
• BLDC motors
• Robotics actuators
• RC vehicles
• Electric bikes
• Industrial drives

Why Stall Torque Matters

Understanding stall torque helps you:

• Size a motor correctly
• Avoid overheating
• Prevent winding damage
• Choose the right ESC or driver
• Design gear systems properly

If a motor stalls under load for too long, it can burn out in seconds.

That is why a stall torque calculator is useful before you build anything.

Key Inputs in the Stall Torque Calculator

The calculator uses three main electrical inputs.

1. Nominal Voltage (V)

This is the operating voltage of your motor system.

Example:

• 12V
• 24V
• 48V

Higher voltage increases:

• Stall current
• Stall torque
• No-load speed

2. Terminal Resistance (R)

This is the phase-to-phase resistance of the motor windings, measured in ohms.

Typical values:

• Small motor: 0.5–2 ohms
• High power motor: 0.05–0.2 ohms

Lower resistance means:

• Higher stall current
• More heat
• Greater risk if stalled

3. Motor Constant (KV or Kt)

The calculator allows two modes:

Option A: KV Rating (RPM per Volt)

KV tells you how fast the motor spins per volt applied.

Example:

• 300 KV motor
• 1000 KV motor

Higher KV:

• More speed
• Less torque per amp

Option B: Torque Constant (Kt in N·m/A)

Kt tells you how much torque is produced per amp.

Example:

• 0.032 N·m/A
• 0.08 N·m/A

Higher Kt:

• More torque per amp
• Lower maximum RPM

Relationship Between KV and Kt

The calculator converts between KV and Kt automatically.

Formula used:

Kt (N·m/A) = 9.549296 / KV

This comes from motor physics and unit conversion between RPM and radians per second.

So if you enter KV, the calculator finds Kt.
If you enter Kt, it calculates KV.

Core Calculations Explained

Let’s walk through what the calculator computes.

1. Stall Current

At stall:

• Back EMF = 0
• Current is limited only by resistance

Formula:

Istall = V / R

Example:

If:

• Voltage = 24V
• Resistance = 0.2Ω

Then:

Istall = 24 / 0.2 = 120A

That is a very large current.

2. Stall Torque

Torque is:

Tstall = Kt × Istall

If:

• Kt = 0.03 N·m/A
• Istall = 120A

Then:

Tstall = 3.6 N·m

This is the maximum theoretical torque at zero speed.

3. No-Load Speed

The calculator computes theoretical no-load RPM:

RPM₀ = V × KV

If:

• 24V
• 300 KV

Then:

RPM₀ = 7200 RPM

This assumes no friction or losses.

4. Peak Mechanical Power

Maximum mechanical power does not happen at stall.

It happens at:

• Half of stall torque
• Half of no-load speed

The calculator uses:

Pmax = (Tstall / 2) × (ω_noLoad / 2)

Where angular speed is converted to radians per second.

This gives peak output power in watts.

5. Thermal Heat at Stall

At stall, all input power becomes heat.

Formula:

Pheat = V × Istall

This is dangerous.

If your motor stalls at:

• 24V
• 120A

Then:

Pheat = 2880 watts of heat

That can destroy a motor very quickly.

Unit Conversions Provided

The calculator also converts torque into:

• N·m (primary unit)
• oz-in
• kg-cm

This is helpful because:

• Robotics often uses kg-cm
• RC industry often uses oz-in
• Engineering uses N·m

Visual Power Comparison

The calculator includes a visual bar showing:

• Peak mechanical power (green)
• Heat loss at stall (red)

This helps you quickly see:

• How much useful power you can get
• How much energy becomes heat

In most motors, stall heat is much larger than usable mechanical output.

Engineering Insight: Why Stall Is Dangerous

When stall current is very high (for example over 150A):

• Copper windings overheat
• Insulation melts
• Neodymium magnets demagnetize
• ESC can fail

Even a few seconds at stall can cause permanent damage.

That is why current-limiting ESCs are critical.

Practical Example

Let’s say you have:

• 24V system
• 0.15Ω resistance
• 300 KV motor

Results would show:

• Very high stall current
• High stall torque
• Large heat generation
• Peak power at half no-load RPM

This tells you:

• Never let this motor remain stalled
• Use gearing
• Use current limiting

How to Use the Stall Torque Calculator Properly

1. Enter system voltage
2. Enter terminal resistance
3. Select KV or Kt mode
4. Input constant value
5. Click “Calculate Dynamics”

Then review:

• Stall torque
• Stall current
• Peak mechanical power
• Thermal warning message

If values look extreme, redesign before building.

Common Mistakes to Avoid

Ignoring Resistance Accuracy

Small error in resistance causes large current error.

Measure carefully with:

• Four-wire measurement
• Accurate ohmmeter

Assuming Stall Torque Is Continuous Torque

Stall torque is not safe operating torque.

Continuous torque is much lower.

Forgetting Cooling Limits

A motor on a bench behaves differently than:

• Inside a robot
• Inside a sealed housing
• In hot weather

Heat builds up fast.

When Should You Use a Stall Torque Calculator?

Use it when:

• Designing robotic arms
• Building electric scooters
• Choosing BLDC motors
• Selecting motor drivers
• Designing gear reductions
• Checking ESC limits

It saves time and prevents costly mistakes.