How to Choose the Right Brushed DC Motor

Selecting the right brushed DC motor involves much more than matching a voltage or choosing the smallest available motor.

A properly selected motor should provide sufficient torque, operate efficiently within its rated range, achieve the desired service life, and meet the environmental requirements of the application.

Choosing an undersized motor can lead to overheating, excessive brush wear, and premature failure. Conversely, selecting an oversized motor increases cost, weight, and power consumption without providing meaningful benefits.

This chapter presents a practical, engineering-oriented approach to selecting a brushed DC motor.

(Illustration: An engineer reviewing motor specifications alongside application requirements, with a motor datasheet and performance curves visible.)

Step 1 — Define the Application Requirements

Before comparing motor models, clearly define the application’s operating conditions.

Typical questions include:

What function will the motor perform?
What type of load will it drive?
Is the load constant or variable?
Will the motor operate continuously or intermittently?
How much installation space is available?
What environmental conditions will the motor experience?

Understanding these requirements helps eliminate unsuitable motor options early in the selection process.

(Illustration: Flowchart showing application analysis leading to motor selection.)

Step 2 — Determine the Required Torque

Torque is usually the most important parameter when selecting a brushed DC motor.

Consider the following:

Starting torque
Continuous operating torque
Peak torque
Safety margin

Applications with high starting loads—such as gear mechanisms, pumps, or actuators—often require significantly higher torque during startup than during normal operation.

A good engineering practice is to select a motor with sufficient reserve capacity rather than operating continuously near its maximum rated torque.

(Illustration: Example of a motor driving a gearbox with arrows indicating load torque and output torque.)

Step 3 — Select the Required Speed

Determine the desired operating speed of the driven equipment.

Typical examples include:

Application	Typical Speed Requirement
Cooling Fan	High speed
Conveyor	Moderate speed
Door Lock	Low speed
Medical Pump	Stable controlled speed
Power Tool	High speed with variable load

If the required output speed is significantly lower than the motor’s natural speed, adding a gearbox may be a better solution than operating the motor at very low voltage.

(Illustration: Comparison of direct-drive and geared motor solutions for different output speeds.)

Step 4 — Match the Supply Voltage

Choose a motor whose rated voltage matches the available power supply.

Common DC supply voltages include:

3 V
6 V
12 V
24 V
36 V
48 V

Using the correct voltage simplifies system design and helps ensure predictable performance.

Avoid relying on overvoltage or undervoltage operation to achieve different speeds, as this can reduce efficiency and shorten motor life.

(Illustration: Various DC power sources connected to motors with matching rated voltages.)

Step 5 — Evaluate the Duty Cycle

How long will the motor operate during each cycle?

Different applications impose very different duty requirements.

Continuous Operation

Examples include:

Ventilation fans
Laboratory equipment
Conveyor systems

Continuous-duty motors require effective thermal management.

Intermittent Operation

Examples include:

Power windows
Electric door locks
Seat adjustment systems

These motors typically operate only for a few seconds before stopping, allowing time for cooling.

The expected duty cycle has a major influence on motor size, cooling requirements, and expected brush life.

(Illustration: Timeline comparing continuous-duty and intermittent-duty operating cycles.)

Step 6 — Consider Brush Life

Unlike brushless motors, brushed motors contain components that wear during normal operation.

Brush life depends on factors such as:

Speed
Current
Load
Operating hours
Ambient temperature
Number of start-stop cycles

Applications that run continuously for thousands of hours may require scheduled brush replacement.

For applications where maintenance is difficult or impossible, a brushless motor may be a better long-term choice.

(Illustration: Carbon brush wear progression over time, from new to replacement condition.)

Step 7 — Evaluate the Operating Environment

Environmental conditions can significantly affect motor reliability.

Consider factors such as:

Temperature

High ambient temperatures reduce cooling efficiency and may shorten brush and bearing life.

Dust

Dust can accumulate around the brushes and commutator, increasing wear and reducing electrical contact quality.

Moisture

Humid or wet environments may require motors with improved sealing or corrosion-resistant materials.

Vibration

Applications involving continuous vibration should use motors with robust bearings and secure mounting methods.

(Illustration: Brushed DC motors operating in dusty, humid, high-temperature, and vibration-prone environments.)

Step 8 — Determine Whether a Gearbox Is Needed

Many applications require high torque at relatively low output speeds.

Instead of selecting an unusually large motor, engineers often combine a brushed DC motor with a gearbox.

Common gearbox options include:

Planetary gearboxes
Spur gearboxes
Worm gearboxes

Advantages include:

Higher output torque
Lower output speed
Improved positioning capability
Better load matching

However, gearboxes also introduce:

Additional cost
Increased size
Mechanical losses
Extra maintenance requirements

(Illustration: Comparison between a standard brushed DC motor and a brushed DC gear motor.)

Step 9 — Review the Motor Datasheet

Once several candidate motors have been identified, compare their datasheets carefully.

Pay particular attention to:

Rated voltage
Rated speed
Rated torque
Rated current
Output power
Efficiency
Motor dimensions
Shaft specifications
Brush life
Bearing type
Operating temperature

Avoid selecting a motor based solely on one specification, such as maximum speed or peak power.

A balanced comparison provides a much better indication of real-world performance.

(Illustration: Example datasheet with key specifications highlighted for comparison.)

Common Selection Mistakes

The following mistakes are frequently encountered during motor selection.

Choosing a Motor Based Only on Voltage

Matching the supply voltage is important, but voltage alone does not determine whether a motor can meet the required torque, speed, or duty cycle.

Ignoring Starting Torque

Some applications require much higher torque during startup than during steady operation.

If the motor cannot overcome the initial load, it may stall and draw excessive current.

Overlooking Brush Life

Brush wear is a normal characteristic of brushed motors.

Applications requiring continuous, maintenance-free operation should consider brush life during the design stage.

Ignoring Thermal Performance

Even if a motor can produce the required torque, prolonged operation beyond its thermal limits may result in overheating and premature failure.

Selecting the Smallest Possible Motor

A motor with little or no performance margin may operate continuously near its limits, reducing efficiency, shortening service life, and increasing maintenance requirements.

A reasonable safety margin generally leads to better long-term reliability.

(Illustration: Infographic summarizing common motor selection mistakes and their consequences.)

Engineering Note

Many engineers focus primarily on speed and torque when selecting a motor.

In practice, the most reliable motor selection considers the complete operating system, including:

Electrical supply
Mechanical load
Duty cycle
Cooling conditions
Available installation space
Expected maintenance interval
Product life expectancy

A motor that appears ideal on paper may perform poorly if these practical factors are ignored.

Motor Selection Checklist

Before finalizing your motor selection, verify the following:

✓ Rated voltage matches the available power supply.

✓ Rated torque exceeds the application’s continuous torque requirement.

✓ Starting torque is sufficient to overcome the initial load.

✓ Rated speed meets the application’s operating requirements.

✓ Duty cycle is compatible with the expected operating pattern.

✓ Brush life satisfies maintenance expectations.

✓ Environmental conditions are within the motor’s operating limits.

✓ Motor dimensions and shaft configuration fit the mechanical design.

✓ Bearing type is appropriate for the expected load.

✓ A suitable safety margin has been included.

(Illustration: A printable motor selection checklist beside a brushed DC motor.)

Key Takeaways

Successful motor selection begins with understanding the application’s mechanical and electrical requirements.
Torque, speed, voltage, and duty cycle are the primary performance parameters to evaluate.
Brush life and operating environment are especially important for brushed DC motors.
Gearboxes provide an effective solution when high torque and low speed are required.
Selecting a motor with an appropriate performance margin improves reliability, efficiency, and service life.