How to Choose a Robot Motor Based on Technical Requirements

Before looking at motor datasheets, you must define what the motor needs to do. In engineering, this is known as the motion profile, which consists of acceleration, constant speed, and deceleration phases [1].

To determine the requirements, evaluate these three factors:

• Torque $(\tau)$: The rotational force required to move the load.

• Speed $(\omega)$: How fast the motor must spin to meet the robot’s operational goals, usually measured in Revolutions Per Minute (RPM).

• Inertia $(J)$: The resistance of the load to changes in rotation.

For mobile robots, torque calculation must account for friction, mass, and wheel radius. For robotic arms, the “worst-case scenario” (the arm fully extended horizontally) determines the peak torque needed to counteract gravity [2]. As you refine these physical requirements, you might also consider how your choice of end-effector impacts the load, much like the considerations found in our guide on how to choose the right robotic gripper for your application.

Different motor types excel in different niches. Selecting the wrong technology can result in a control system that is either too complex or lacks the necessary accuracy.

Brushed DC Motors

These are the simplest and most cost-effective options. According to technical guides from Mouser, brushed motors are best for simple applications where precision is not a priority, such as toys or basic conveyors. However, they suffer from mechanical wear and electrical noise (EMI) [3].

Brushless DC (BLDC) Motors

BLDCs are the industry standard for high-performance robotics. They are lighter and more efficient than brushed motors but require external Electronic Speed Controllers (ESCs) [4].

• Best for: Drones, mobile robot drive wheels, and high-duty cycle industrial robots.

• Key Metric: Look for the kV rating (RPM per volt). High kV motors spin fast with low torque; low kV motors provide more “pulling power” but slower speeds [4].

Stepper Motors

Steppers move in discrete increments (typically 1.8° per step). They offer excellent “holding torque,” meaning they can stay in one position without drifting [1].

• Best for: 3D printers, CNC machines, and applications requiring precise, open-loop positioning.

• Drawback: They are less energy-efficient and can lose steps if overloaded.

Servo Motors

A servo is not a specific motor type but a system that includes a motor, a position sensor (encoder), and a controller. This closed-loop system allows the robot to know exactly where its joints are at all times [2]. Advanced automation often relies on these, a topic we explore further in our article on advanced robot modeling and control systems techniques.

To avoid “guessing,” use the following fundamental formulas to size your motor correctly.

For Mobile Robots (Wheels)

Total Force $(F)$ is the sum of acceleration force and friction.

1. Force $(F) = \text{Mass} \times \text{Acceleration}$
2. Torque $(\tau) = F \times \text{Wheel Radius}$
3. Safety Factor: Engineers typically multiply the final torque by 1.5 to 2.0 to account for real-world friction and slopes [1].

For Robotic Arms (Joints)

The torque is at its maximum when the arm is parallel to the ground.

• Torque $(\tau) = \text{Load Weight} \times \text{Distance from Pivot}$ If an arm link is 0.5 meters long and the payload is 2kg (approx. 20 Newtons), the motor at the joint needs at least 10 Nm of torque plus a safety margin.

Most DC motors spin at very high speeds (3,000–10,000+ RPM) with very low torque. To make them useful for robotics, a gearhead is attached to reduce the speed and increase the torque [2].

• Gear Ratio: A 100:1 gear ratio reduces speed by 100x but increases torque by roughly 100x (minus efficiency losses).

• Trade-off: High gear ratios introduce backlash (mechanical play), which can reduce the precision of the robot’s movement.

Discussions on robotics subreddits emphasize that hobbyists often overlook thermal management. Even if a motor meets a torque requirement, running it at its limit for long periods will cause it to overheat and fail. Community consensus suggests choosing a motor that can handle your “continuous load” at 50-60% of its rated capacity to ensure longevity [5].

• Define Pathing First: Calculate your peak torque (acceleration/slopes) and continuous torque (constant speed).
• Apply a Safety Factor: Always size your motor 50% to 100% larger than your theoretical calculations $(1.5x – 2x$ torque factor).
• Select for Precision: Use Steppers or Servos for positioning; use BLDC for continuous motion.
• Check Efficiency: Brushless motors offer the best power-to-weight ratio for mobile, battery-operated robots.
• Account for Modern Integration: Consider “Smart Motors” that integrate the controller and encoder into one package to simplify wiring.

Action Plan

1. Identify the total mass (including batteries and payload).
2. Determine the required top speed and how fast you need to reach it (acceleration).
3. Calculate the torque required based on wheel radius or arm length.
4. Apply a 1.5x safety factor.
5. Choose a motor/gearbox combination that provides that torque at the desired RPM.