How to spec out motor for my robotics project.

How to Choose a Motor for a Robotics Application (A Practical Guide)

Choosing a motor can feel overwhelming if you’re new to robotics. Datasheets are dense, specs look abstract, and it’s not obvious what actually matters. This guide walks through a simple, practical process anyone can follow to select a motor that actually works in the real world.

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Step 1: Understand What Your Robot Needs to Do
Before looking at motors, ask:

• How much force or torque is required?
• How fast does the system need to move?
• How long will it run (seconds, minutes, continuously)?

Motors are sized by what they do over time, not just peak numbers.

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Step 2: Convert Motion to Motor Speed
Your motor usually spins faster than the output moves. Gears, belts, or pulleys connect the two.

\[\omega_m = \frac{v}{R} \times G\]

Higher speed increases voltage demand and reduces available torque.

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Step 3: Convert Load to Motor Torque

\[\tau_m = \frac{\text{Load force} \times \text{radius}}{\text{efficiency} \times G}\]

Gears reduce torque demand but increase speed and losses.

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Step 4: Check How Much Current the Motor Draws

\[I = \frac{\tau_m}{K_t}\]

Current is what heats the motor. If current exceeds the motor’s rating, it will overheat—even if torque looks okay.

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Step 5: Estimate Heat Generation

\[P_{\text{loss}} = I^2 R\]

This is the primary source of temperature rise inside the motor.

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Step 6: Perform a First-Order Thermal Check

\[\Delta T = \frac{1}{m c_p} \int P_{\text{loss}}(t)\,dt\]

If this already exceeds safe limits, the motor is undersized.

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Step 7: Use a Dynamic Thermal Model (Optional)

\[G(s) = \frac{R_w}{\tau_w s + 1} + \frac{R_h}{\tau_h s + 1}\]

This models winding heating and housing cooling for cyclic operation.

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Step 8: Verify Voltage Availability

\[V = K_t \omega_m + I R\]

Ensure the power supply can provide the voltage required to reach the desired speed.

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Step 9: Check All Motor Limits
Always confirm:

• RMS current < rated current
• Torque < continuous torque
• Voltage < rated voltage
• Temperature < max allowable

Failing any one means the motor is unsuitable.

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Step 10: Check the Speed–Torque Curve

\[\omega = \omega_{\text{no-load}} - k\tau\]

Operating points should lie comfortably inside the continuous operation region.

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Step 11: Include a Safety Margin

• ~20–30% margin in current and thermal capacity
• Accounts for friction, disturbances, and uncertainty

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Key Takeaways

• Torque moves the load, but current and heat limit the motor
• Continuous operation matters more than peak specs
• Thermal modeling is essential for reliable design
• Gears help, but losses and heating remain

Rule of thumb: If the motor is thermally safe, it is usually mechanically safe too.