Understanding Brushed DC Motor Specifications

Selecting the right brushed DC motor requires more than simply choosing a voltage or motor size.

Manufacturers provide a wide range of technical specifications that describe a motor’s electrical, mechanical, and thermal performance. Understanding these parameters is essential for comparing motors, interpreting datasheets, and selecting the best motor for a specific application.

Some specifications, such as voltage, speed, torque, and power, are common to almost all electric motors. Others—such as brush life and commutator wear—are particularly important for brushed DC motors because of their mechanical commutation system.

This chapter explains the most important brushed DC motor specifications and how they influence real-world performance.

(Illustration: A brushed DC motor datasheet with key specifications such as voltage, speed, torque, current, power, efficiency, and brush life highlighted.)

Rated Voltage

Rated voltage is the DC voltage at which the motor is designed to operate.

Common rated voltages include:

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

Operating the motor close to its rated voltage generally provides the best balance between performance, efficiency, and service life.

What Happens if the Voltage Is Too Low?

When supplied with a voltage below its rated value, the motor typically:

  • Rotates more slowly
  • Produces less output power
  • Delivers less torque (under voltage-limited conditions)
  • May fail to start under heavy loads

What Happens if the Voltage Is Too High?

Excessive voltage may result in:

  • Overspeed
  • Higher current
  • Increased heat generation
  • Faster brush and commutator wear
  • Reduced motor life

(Illustration: Graph showing the effect of supply voltage on motor speed and operating temperature.)

Rated Speed

Motor speed is usually expressed in revolutions per minute (RPM).

Manufacturers commonly specify:

  • No-load speed
  • Rated speed
  • Maximum permissible speed

No-Load Speed

This is the speed when the motor operates without any external mechanical load.

It is typically the highest speed the motor can achieve at its rated voltage.

Rated Speed

Rated speed is measured when the motor operates under its specified rated load.

This value better represents normal operating conditions than no-load speed.

(Illustration: Speed comparison between no-load and rated operating conditions.)

Torque

Torque is the motor’s rotational force.

It determines how much load the motor can drive.

Common torque values include:

Starting Torque

Also called locked-rotor torque, this is the torque produced when the motor starts from rest.

Brushed DC motors generally provide excellent starting torque.

Rated Torque

Rated torque is the continuous torque the motor can deliver safely under normal operating conditions.

This is the most important torque value when selecting a motor.

Stall Torque

Stall torque is the maximum torque produced when the shaft is prevented from rotating.

Although stall torque is useful for understanding motor capability, operating a motor in a stalled condition should generally be avoided because it results in extremely high current and rapid heating.

(Illustration: Speed–torque curve highlighting no-load speed, rated torque, and stall torque.)

Engineering Note:
Manufacturers may use different terminology, such as starting torque, stall torque, or locked-rotor torque. Always check the datasheet definitions before comparing motors.

Current

Current represents the electrical current drawn by the motor.

Several current values are commonly specified.

No-Load Current

The current consumed when the motor operates without an external load.

This current mainly overcomes:

  • Bearing friction
  • Brush friction
  • Windage losses

Rated Current

The current drawn during normal operation under rated load.

Stall Current

The current drawn when the motor is prevented from rotating.

This is the highest current the motor can draw and is often several times greater than the rated current.

Continuous stall operation can quickly damage the motor due to excessive heat.

(Illustration: Graph showing current increasing as mechanical load increases.)

Output Power

Motor output power describes the rate at which useful mechanical work is performed.

Mechanical output power depends on both:

  • Torque
  • Rotational speed

A motor producing high torque at very low speed may deliver less power than a motor producing moderate torque at high speed.

For this reason, torque and speed should always be considered together rather than independently.

(Illustration: Diagram showing the relationship between torque, speed, and mechanical output power.)

Efficiency

Efficiency describes how effectively a motor converts electrical energy into mechanical output.

Some electrical energy is inevitably lost due to:

  • Copper losses in the windings
  • Brush contact losses
  • Bearing friction
  • Iron losses
  • Air resistance

Because brushed motors include mechanical commutation, they generally have lower efficiency than comparable brushless motors.

Typical efficiency depends on motor size and design but often falls within the range of 60% to 85%.

(Illustration: Energy flow diagram comparing electrical input, useful mechanical output, and energy losses.)

Duty Cycle

Duty cycle describes how long a motor operates relative to its rest period.

Common duty cycles include:

Continuous Duty

The motor operates continuously without exceeding its allowable temperature.

Intermittent Duty

The motor operates for short periods followed by cooling intervals.

Many brushed DC motors are designed for intermittent operation.

Examples include:

  • Power windows
  • Door locks
  • Automotive actuators

Understanding the duty cycle is essential because overheating is often determined by operating time rather than instantaneous load.

(Illustration: Timeline comparing continuous duty and intermittent duty operation.)

Brush Life

Brush life is a specification unique to brushed motors.

It represents the expected operating life of the carbon brushes before replacement becomes necessary.

Brush life depends on factors such as:

  • Operating current
  • Speed
  • Load
  • Temperature
  • Brush material
  • Commutator condition

Applications involving frequent starts, stops, or overloads typically shorten brush life.

(Illustration: Carbon brushes at different stages of wear, from new to replacement condition.)

Commutator Wear

The commutator is also a wear component.

Over time, repeated contact with the brushes can result in:

  • Surface wear
  • Grooving
  • Pitting
  • Electrical arcing damage

Regular inspection helps ensure reliable motor operation and prolongs service life.

(Illustration: Comparison of a new commutator and a worn commutator surface.)

Operating Temperature

Every brushed DC motor has a recommended operating temperature range.

Excessive temperature can lead to:

  • Insulation degradation
  • Reduced magnet performance (in PMDC motors)
  • Faster brush wear
  • Bearing lubricant deterioration
  • Shortened service life

Proper cooling and avoiding prolonged overload conditions are important for maintaining reliable operation.

(Illustration: Thermal image of a brushed DC motor under normal and overloaded operating conditions.)

Reading a Brushed DC Motor Datasheet

When evaluating a motor datasheet, focus first on the specifications that directly affect your application.

These typically include:

  • Rated voltage
  • Rated speed
  • Rated torque
  • Rated current
  • Output power
  • Efficiency
  • Duty cycle
  • Shaft dimensions
  • Brush life (if provided)

Comparing motors using only one specification—such as speed or power—can lead to poor selection decisions.

A complete evaluation considers the motor as a system rather than focusing on a single parameter.

(Illustration: Sample datasheet with the most important specifications highlighted for selection.)

Engineering Note

One of the most common mistakes when selecting a brushed DC motor is comparing no-load speed instead of rated operating performance.

A motor may advertise a very high no-load speed, but once connected to a real mechanical load, its speed decreases while current and temperature increase.

For engineering applications, rated operating conditions provide a much more meaningful basis for comparison than maximum or no-load values.

Key Takeaways

  • Voltage, speed, torque, current, power, and efficiency are the primary specifications used to evaluate brushed DC motors.
  • Brushed motors also require consideration of brush life and commutator wear.
  • Rated values are generally more useful than no-load or maximum values when selecting a motor.
  • Duty cycle and operating temperature have a major impact on motor reliability and service life.
  • Reading a motor datasheet correctly is essential for choosing the right motor and avoiding performance or durability issues.

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