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A brushed DC motor is a type of electric motor that runs on direct current (DC) and uses brushes to transfer electricity to the rotating part of the motor. Its simple operation, relatively low cost, and straightforward speed control make it useful in automotive systems, consumer products, and industrial equipment. This article explains how brushed DC motors work, their main components and types, performance characteristics, control methods, advantages, limitations, and common maintenance issues, and will also introduce brushless motors.
Figure 1. An Overview of Brushed DC Motor
A brushed DC motor is an electric motor that uses mechanical commutation to control the current flowing through its rotating armature windings. When DC power is applied, current in the armature interacts with the magnetic field produced by the stator, generating torque that turns the rotor and its connected shaft. Brushes maintain electrical contact with a segmented commutator, which switches the winding connections as the motor rotates.
Brushed DC motors are available with permanent magnets or electrically excited field windings. They operate without an external electronic commutation circuit, making basic drive arrangements relatively simple. In contrast, brushless DC (BLDC) motors use electronic commutation rather than brushes and a mechanical commutator. This fundamental difference affects the motors’ drive requirements, maintenance needs, and operating characteristics.
In a permanent-magnet brushed DC motor, the stator magnets provide a stationary magnetic field. The rotor carries the armature windings and turns the output shaft. The following four steps explain how electrical current produces continuous rotation.
Figure 2. Exploded View of a Brushed DC Motor
Step 1. Electrical Current Enters the Armature
When DC voltage is applied across the motor terminals, current flows through one stationary brush, the rotating commutator, and the armature windings before returning through the opposite brush.
Step 2. Magnetic Interaction Produces Torque
Current-carrying conductors in the armature experience electromagnetic forces within the stator’s magnetic field. These forces act at a distance from the rotational axis and produce torque, causing the rotor to turn. The resulting torque depends on the armature current and magnetic flux, as well as the motor’s construction. The shaft transfers this rotational motion to the connected mechanical load.
Step 3. The Commutator Switches the Winding Current
As the rotor turns, the commutator segments move beneath the stationary brushes. Their changing contact positions switch the current between armature winding connections and reverse the current in the appropriate conductors at the required points in the rotation. This mechanical switching maintains the electromagnetic torque in the intended rotational direction instead of allowing it to reverse as the rotor moves through different positions.
Step 4. The Rotor Continues to Rotate
The repeated commutation process allows the motor to continue rotating while electrical power is supplied. As the armature rotates through the magnetic field, it generates a voltage known as back electromotive force (back EMF), which opposes the applied armature voltage. Back EMF influences the current flowing through the windings and therefore the torque produced. At steady speed, the developed electromagnetic torque balances the load torque and the motor's internal mechanical losses.
Brushed DC motors are classified by how their stator magnetic field is produced and how the field circuit is connected to the armature. The five main types have different torque, speed, and control characteristics.
Permanent magnet DC motors use magnets in the stator to establish a magnetic field without requiring a separate field winding. This simplifies the motor’s construction and eliminates electrical power consumption in a stator excitation winding. With approximately constant magnetic flux, electromagnetic torque is proportional to armature current, and motor speed changes predictably with voltage and load.
In a series-wound DC motor, the field winding and armature are connected in series, carrying the same current. This arrangement produces strong starting torque, but speed varies significantly with load. As the load decreases, weakening field flux can cause excessive acceleration. Therefore, conventional series-wound motors must not operate without an appropriate mechanical load.
A shunt-wound DC motor has its field winding connected in parallel with the armature across the supply. The field current is therefore largely independent of armature load current when the supply voltage remains constant. This provides relatively stable magnetic flux and better speed regulation than a series-wound motor under changing loads.
Compound-wound motors combine series and shunt field windings. In cumulative compound designs, the magnetic fields reinforce each other, providing strong starting torque and better speed regulation than series-wound motors. Differential compound connections produce opposing fields and have different, often undesirable load characteristics.
Separately excited DC motors use independent electrical supplies for the armature and field winding. This allows separate adjustment of armature voltage and field current, providing flexible speed and torque control. Field weakening also permits operation above base speed within the motor’s specified electrical and mechanical limits.
The performance of a brushed DC motor is described by its speed, torque, voltage, current, efficiency, and thermal limits. These characteristics determine how the motor responds to mechanical loading and whether it operates within its rated conditions.
For a permanent-magnet brushed DC motor operating at a fixed voltage, speed decreases approximately linearly as load torque increases. No-load speed is the speed reached without an external mechanical load, while stall torque represents the torque developed when the shaft is prevented from rotating under specified electrical conditions.
Manufacturer speed–torque curves illustrate this relationship and help identify suitable operating conditions. However, stall torque is not a continuous operating rating. The allowable continuous operating region depends on current and thermal limits. Wound-field motors have different speed–torque characteristics because their magnetic flux varies with excitation.
Figure 3. Typical Speed–Torque Characteristic of a PMDC Motor at Fixed Voltage
At startup, the rotor is stationary and back electromotive force (back EMF) is zero. When full voltage is applied directly to the armature, the initial current is limited mainly by winding resistance, brush contact effects, and supply characteristics, producing high starting torque.
Stall current occurs when the energized motor cannot rotate. Prolonged stalling causes substantial winding heating and risks motor damage. Current limiting and overload protection are therefore important in motor drive systems.
Motor efficiency is the ratio of mechanical shaft output power to electrical input power. Losses arise from winding resistance, brush contact, magnetic effects, and mechanical friction. Copper losses increase with the square of armature current, making high-current operation a major source of heating. Continuous operation must remain within the manufacturer's specified current, temperature, and duty-cycle limits. Actual efficiency varies with motor design, load, and operating speed, so maximum torque does not necessarily correspond to maximum efficiency.
Applied voltage influences motor speed, while armature current determines electromagnetic torque under approximately constant magnetic flux. For a permanent-magnet brushed DC motor, the basic relationships are:
Te=ktIa
Eb = keω
Where:
Te = developed electromagnetic torque
kt = torque constant
Ia = armature current
Eb = back electromotive force
ke = back EMF constant
ω = angular velocity
In a simplified steady-state model, the armature voltage relationship is:
V≈ Eb + IaRa
Here, V is the applied armature voltage and Ra is the armature resistance. As speed increases, back EMF rises, reducing armature current at a fixed voltage. When load increases and speed decreases, lower back EMF allows more current to flow, increasing electromagnetic torque.
These equations assume approximately constant magnetic flux and neglect brush voltage drop and other nonideal effects. They should not be applied unchanged to series-wound motors.
Brushed DC motors switch armature connections mechanically, so an external commutation controller is unnecessary. An external drive can set the voltage or PWM duty cycle, use an H-bridge to select direction and braking behavior, and use speed feedback to adjust the drive command as the mechanical load changes. Protective functions address overloads and electrical faults.
At a given load, changing the applied armature voltage changes the motor’s operating speed. Pulse width modulation (PWM) switches the drive voltage on and off and varies the duty cycle to adjust the effective armature voltage. PWM sets the drive command, but it does not, by itself, maintain a fixed speed when the load changes. Holding a target speed under varying loads requires speed measurement and a controller that adjusts the command.
PWM performance depends on switching frequency, motor inductance, and current ripple. Excessive ripple increases losses and brush wear, while switching and commutation contribute to electromagnetic interference (EMI). The switching frequency should therefore follow the motor and driver specifications.
An H-bridge uses four switches to control the direction of current through a permanent-magnet brushed DC motor. One diagonal switch pair drives the motor in one direction; the opposite pair reverses the current and rotation. The bridge also supports braking or coasting through appropriate switch states, depending on the driver design.
PWM can be applied through the H-bridge for speed-command adjustment. In a coasting state, the drive does not intentionally apply a braking connection; electrical braking uses a current path that opposes rotation. Switch timing must prevent simultaneous conduction of the high-side and low-side MOSFETs in the same leg, which would short the supply. Appropriate freewheel paths handle winding current during switching.
For wound-field motors, rotation depends on the relative directions of armature current and field flux. Reversing both together leaves the torque direction unchanged, so their excitation connections require separate consideration.
Figure 4. H-Bridge Circuit for Direction Control
Open-loop voltage or PWM control does not measure actual shaft speed, so speed can change when load torque varies. With closed-loop speed control, an encoder or tachometer measures rotation and a controller adjusts voltage or PWM duty cycle to correct the error between measured and target speed, within the motor’s available torque and driver limits.
Current sensing and thermal protection help prevent damage from overloads and prolonged stalls. The driver must also handle switching transients, short circuits, and supply disturbances to maintain safe operation.
Brushless DC (BLDC) motors typically use permanent-magnet rotors and wound stators. Instead of mechanical commutation, an electronic controller switches the winding currents using rotor-position information from sensors or sensorless estimation.
Figure 5. Brushed vs. Brushless DC Motor: Mechanical vs. Electronic Commutation
| Feature | Brushed DC Motors | Brushless DC Motors |
| Commutation | Mechanical brushes and a commutator; no electronic commutation circuit needed | Electronic commutation through a motor drive circuit |
| Control complexity | Relatively simple basic drive and reversal; fixed-speed regulation still needs feedback | Requires an electronic controller and rotor-position sensing or estimation |
| Efficiency | Brush contact and friction cause losses; high current increases heating | Often higher in comparable designs, depending on motor and driver |
| Maintenance | Brush and commutator wear, sparking, and electrical noise/EMI require consideration | No brush replacement; drive switching can still create EMI |
| Cost | Often lower initial system cost, especially for basic DC drives | Often higher initial cost due to more complex drive electronics |
| Best suited for | Cost-sensitive, widely available, intermittent drives with useful starting torque; observe thermal and duty limits | Long service life, high efficiency, reduced brush maintenance, and continuous-duty applications |
Brushed DC motors are used in equipment that requires electrically controlled rotary motion. Their application depends on motor size, torque capability, operating time, and the complexity of the associated drive system.
Brushed DC motors are used in power windows, windshield wipers, seat adjusters, and mirror positioning mechanisms. Many of these systems operate intermittently and use gear reductions to deliver the required output torque and movement.
Small permanent-magnet brushed motors power battery-operated toys, compact actuators, and selected household appliances. Their compact construction and straightforward electrical connections suit products requiring basic rotary motion without electronic commutation.
Certain cordless drills, electric screwdrivers, and portable tools use brushed motors to provide starting and operating torque. Gearboxes adjust the output speed and torque to match the tool's mechanical requirements, while brushless alternatives are increasingly used in higher-performance products.
Industrial applications include small pumps, geared actuators, and positioning mechanisms. Brushed motors support adjustable-speed operation and, when paired with encoders and suitable controllers, closed-loop positioning. Brush wear and thermal limits require particular attention in demanding-duty applications.
The electrical and mechanical contact between brushes and the commutator is a common source of deterioration in brushed DC motors. Identifying operating symptoms early and maintaining suitable operating conditions helps reduce unplanned failures.
Several common motor problems are associated with electrical supply conditions, mechanical loading, brush contact, and thermal stress. The following table outlines typical symptoms, possible causes, and initial checks.
| Common Problem | Possible Cause | Basic Troubleshooting |
| Motor fails to start | Missing supply voltage, poor electrical connections, worn brushes, or blocked rotor | Check the supply and connections; inspect for mechanical obstruction and serviceable brush wear |
| Reduced speed or unstable rotation | Low supply voltage, excessive load, poor brush contact, or driver faults | Verify the supply under load, check the driven mechanism, and inspect electrical contacts |
| Motor overheating | Excessive current, prolonged stalling, overloaded operation, or inadequate cooling | Measure operating current, check load conditions, and verify ventilation and duty cycle |
| Excessive sparking | Brush wear, damaged commutator surfaces, or unfavorable commutation conditions | Inspect the brush–commutator assembly according to the manufacturer’s service procedure |
| Unusual noise or vibration | Bearing wear, shaft misalignment, loose mounting, or mechanical damage | Check mounting, shaft alignment, and mechanical condition |
These symptoms have multiple possible causes and do not identify a fault on their own. Abnormal arcing, smoke, or excessive heating requires the motor to be stopped and inspected before further operation.
Regular maintenance helps limit brush wear, prevent overheating, and maintain reliable motor operation. The following practices apply according to the motor manufacturer’s service instructions:
• Inspect Brushes and Commutator: Check wear and contact condition at manufacturer-specified intervals, replacing serviceable components when required.
• Maintain Proper Operating Conditions: Keep voltage, current, temperature, and duty cycle within specified limits to prevent excessive heating and wear.
• Keep the Motor Clean: Remove dust from accessible surfaces and ventilation openings using approved cleaning methods.
• Follow Lubrication Requirements: Lubricate bearings only where specified, keeping lubricant away from electrical contacts.
• Follow Safe Servicing Procedures: Disconnect power and secure the mechanism before maintenance. Replace sealed, non-serviceable motors when internal wear prevents reliable operation.
Brushed DC motors remain an established solution for converting DC electrical power into rotary motion. Their mechanical commutation system provides straightforward operation and relatively simple drive requirements, while different excitation arrangements offer distinct torque and speed characteristics.
Although brush wear, electrical noise, and maintenance requirements limit their suitability for some applications, brushed DC motors continue to serve automotive mechanisms, consumer products, portable tools, and industrial equipment. Compared with brushless DC motors, they trade some efficiency and service life for lower cost and simpler drive electronics, which keeps them a practical choice wherever those trade-offs are acceptable. Understanding their working principles, performance characteristics, and operating limitations is essential for reliable and efficient use.
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