Brushed DC Motor
A DC motor that reverses the winding current mechanically through a commutator and brushes; driven on two cables, it turns the other way on polarity.
A brushed DC motor is a direct current motor that reverses the direction of the winding current mechanically by means of a commutator and carbon brushes. It is driven with two cables; when the polarity of the voltage applied to its terminals changes, it turns the other way. This two-terminal arrangement is directly compatible with a DC motor driver that has an H-bridge output.
How it works
A permanent magnet or a field winding produces the stationary field, and the windings on the rotor turn inside that field. The rotor has to be pushed the other way every half turn; the commutator and the brushes rubbing on it make this change of direction mechanically. As the voltage rises the speed rises, and as the load rises the current rises. Thanks to this linear behaviour the speed can be set without a separate feedback element, simply by changing the average armature voltage.
Why the driver side is simple
On brushless types the rotor position has to be known and the phases driven in sequence. On a brushed motor the commutator handles that job, so all that is left for the driver is to apply voltage to two terminals; an H-bridge made up of four switches covers both the change of direction and the speed setting. The AXI ST42, KS250 and PT500 modules are designed for motors of this type; all three have speed and ramp adjustment, and because the ST42 can also drive two motors independently it is often used in on/off and reversing applications.
Advantages and drawbacks
- Advantage: The driver side is simple; two power cables and one H-bridge are enough, and no phase order or rotor position information is needed.
- Drawback: The brushes and the commutator are wearing parts; the commutation spark produces broadband electrical noise.
The noise can reduce the range of RF receivers working on the same vehicle. A capacitor across the motor terminals, a ferrite ring and a twisted motor cable are the first three measures; how brush arc interference is suppressed is dealt with separately.
The three currents to look at when choosing
| Value | What it tells you | Its counterpart at the driver |
|---|---|---|
| No-load current | Friction and losses with no load | A lower bound, not decisive in the choice |
| Rated (continuous) current | The steady current under load | The continuous current capacity of the driver |
| Stall current | The current drawn while the rotor is held | The peak window of the driver |
The KS250 is listed with a continuous 15 A and a peak of 45 A, the PT500 with a continuous 30 A and a maximum of 40 A. A choice made without reading the three currents together produces a system that fits on paper and cuts out in the field.
Where a brushed motor does not fit
The brush is a wearing part: in applications that run without a break for most of the day, that are hard to reach or that are expensive to maintain, the brushed type is a poor choice. Because it produces sparks it is not suitable in an explosive atmosphere, and because it can produce brush dust it is not suitable in a clean environment either. Against that, in winch, hatch, slide and arm applications that run intermittently, want high torque and are expected to have simple wiring, it is still a practical and economical option.
Knowing the signs of wear also helps: as the brush shortens the spring pressure falls and the contact becomes unstable. Increasing sparking at the commutator, a drop in speed that becomes marked under load, and a dark, irregular track on the commutator surface are the signs of this process. If the motor cover can be taken off, check the brush length periodically; taking a motor that a brush change would save for a driver fault and replacing the module is a mistake often made in the field. Once you have chosen the motor, find out its stall current and compare it with the peak value of the driver.
The context in which this term is used: DC motor and driver troubleshooting matrix