How Do Brushed Motors Work? Commutator, Brushes, and Torque Curve Explained

The Short Answer: The Coil's Current Reverses as It Turns

A brushed motor has two magnetic parts and one rotating switch. The field is stationary, produced by permanent magnets in the housing or by a separate field winding. The working coils sit in the armature, wound around an iron core that is fixed to the shaft and free to spin.

Direct current reaches those coils through the brushes. Each brush presses against the commutator, a cylinder of insulated copper segments. Current enters by the positive brush, crosses into the segment beneath it, travels through one armature coil, and leaves through the opposite segment and brush. Because that coil now carries current inside a magnetic field, each side of it feels a force at right angles to both the field and the current, and the two forces act on opposite sides of the shaft. The armature turns.

The catch is that a coil with fixed current would swing into alignment with the field and stop there. The commutator prevents it. As the armature rotates, each brush slides from one segment to the next, reversing the current in that coil as it crosses the neutral plane. The coil's polarity flips, so the force keeps pushing the rotor the same way round. For every coil, that reversal happens twice per revolution, and the timing is set by geometry alone: segment width, brush width and brush position.

What Each Part Does, and Which Symptom Points Back to It

When a brushed motor underperforms, the fault usually sits in one of five places, and each one leaves a different fingerprint.

The contact surface matters as much as the parts themselves. A thin, even, dark film on the commutator keeps arcing low; bright dry copper usually means the brush grade is too abrasive, or that the motor runs too lightly for long periods.

Why the Current Reverses Twice per Revolution

1. Supply current enters through the positive brush and into the commutator segment touching it.
2. That segment routes current through one armature coil, which becomes an electromagnet with a north and a south end.
3. Its north end is repelled by the stator north pole and attracted to the stator south pole, so the armature turns.
4. At the neutral plane the brush crosses the insulated gap between segments, and current to that coil is briefly cut.
5. The next segment takes over, and the direction of current through the coil is now reversed.
6. The coil's poles have swapped, so the force keeps pushing in the same rotational direction instead of braking the shaft.

A brushed motor is a rotating switch with a magnet bolted around it. Brush dust, brush life, radio-frequency noise and the practical ceiling on speed all come from that switch, and so does the low price.

Step four is where the engineering effort goes. During the brief interruption, the inductance of the coil tries to keep current flowing, and that energy has to go somewhere: a small arc at the brush face, a little heat, or a voltage spike that a suppression capacitor absorbs. Brush grade, spring pressure and segment spacing are chosen to keep the arc small enough to survive thousands of hours.

Speed, Torque and Back EMF: Reading the Numbers Correctly

A spinning coil inside a magnetic field is also a generator. The voltage it produces opposes the supply and is called back EMF, and it grows with speed. The arithmetic is easy to see.

Take a 12 V brushed motor with 1.2 ohms of armature resistance. At the instant the switch closes, the rotor is stationary, back EMF is zero and the winding can pull about 10 A. A fraction of a second later, running near 3,000 rpm, back EMF may have risen to roughly 10.5 V, leaving only 1.5 V across the resistance, or about 1.25 A. Start current and running current are different quantities, and a supply sized on the running figure alone will brown out on every start.

• Free-run speed is measured with nothing on the shaft; the speed quoted on a catalogue page is usually the value at rated load, which is lower.
• Tolerances are real. A plus or minus 10 percent window on no-load speed is normal, and it shifts airflow, current draw and noise in the finished appliance.
• Stall torque is a peak figure for a few seconds at most. It is useful for sizing a fuse, not for describing continuous duty.

Where Brushed Motors Still Win in Appliance Design

Electronics-free commutation buys a specific set of advantages, and they still matter in cost-sensitive products:

• Lowest unit cost per watt of shaft power, particularly between 5 W and 150 W.
• Reliable starting torque; the motor pulls hard from zero speed without a controller.
• Speed control with one PWM stage, a series resistor or a variable supply.
• Direction change by swapping two leads.
• Graceful behaviour as a battery voltage sags.

That combination keeps brushed motors in desktop cooling fans, evaporative air coolers, small fan heaters, value-range kitchen extractors, water pumps and a long list of automotive accessories.

Appliance makers comparing drives for air movement usually evaluate both types before locking a design. Forced-convection coolers and desktop fans, for instance, now have DC alternatives that run at variable speed.

Brush Wear, Heat and Electrical Noise: The Honest Trade-offs

Brush life is the first thing to quantify, because it sets the maintenance promise you can make. Small carbon brushes in appliance-grade motors commonly last somewhere between 500 and 2,000 running hours, and the range is wide for good reason: a motor that starts and stops all day wears faster than one that runs continuously, because much of the brush loss happens in the starting arc.

Heat is the second issue. Losses appear in the copper, in the iron and at the brush face, and the rotor has the worst path to the outside air. Winding insulation class, not housing temperature, sets the real limit on continuous duty. Electrical noise is the third: every arc at the brush face is a small broadband transmitter, which is why brushed motors often ship with suppression capacitors or chokes already fitted.

Specifying a Brushed Motor: A Practical Buyer's Checklist

Two suppliers can both quote "12 V, 3,000 rpm" and still deliver motors that behave differently in the same appliance, because one figure is quoted at no load and the other at a specific torque. Ask for the load point in writing.

• Rated voltage, plus the range the application will actually see.
• No-load speed and no-load current.
• Speed, torque and current at the intended operating point.
• Stall torque, with a time limit for how long it may be held.
• Brush life target in hours at your real duty cycle.
• Continuous or intermittent duty, and expected starts per hour.
• Insulation class and maximum winding temperature.
• Suppression components fitted, and the emission standard they satisfy.
• Shaft diameter, length and tolerance, plus flange and screw pattern.
• Acoustic noise in dB(A) at the load point, at a stated distance.
• Certifications required in the destination market.

It also helps to work with a small electric motor supplier that builds to a drawing or sample instead of only shipping catalogue items, because two millimetres of shaft length or a different brush grade is often the difference between a motor that fits and one that does not. How the commutator and brush assembly behaves under real load is worth reviewing before a drawing is signed off.

Brushed or Brushless: Choosing Without Regret

The decision usually comes down to duty cycle and control requirements rather than raw performance.

Kitchen ventilation shows the shift clearly: range hoods run for long and irregular periods and are judged on noise at low speed, so variable-speed DC drives have taken a growing share of that market.