A direct current (DC) motor is a cheap and simple type of electric powered motor. DC motors have a high number of rotations per minute (RPM) varying from 2000 to 10000 rpm. The rotation direction of a brushed DC-motor can be reversed by reversing the current. You can do that by using a so-called H-bridge motor-driver. A downside of these motors is that they do not have much power or torque. Therefor they are not suitable for moving objects without gears or another type of transmission.
Brushed DC-motors are the oldest kind of DC motor around. When people talk about ‘a DC-motor’ they usually mean a brushed DC-motor. Brushed DC-motors comprise a rotor; the rotating part of the motor. A stator; the non-rotating part of the motor. And the brushes. The stator basically contains permanent magnets. The rotor contains copper wires wound around a spindle, with the wire ends connected to a so-called commutator. That is the copper strips at the end of the spindle enabling the electrical contact with the individual wires (coils) of the rotor. The electrical terminals of the motor directly lead to the brushes. So current travels through the brushes, which are electrically and physically in contact with the commutator acting as a kind of sliding contact or slip ring. The permanent magnets that surround the rotor interact with the magnetic field generated by the rotor-current, this interaction is causing the rotor to rotate. Resulting in the rotor to move to the next connection of the commutator. This process repeats over and over and is called commutation.
Higher voltages across the motor terminals lead to larger currents in the rotor. Leading to stronger magnetic fields. Increasing again rotational speed. So lowering the motor voltage by using a PWM for example, decreases the speed. Applying PWM to a single transistor (or FET) gives you speed control in one rotation direction. Combining PWM and an H-Bridge, gives you full control over both speed and rotation direction. H-Bridges are also called motor drivers. But not all motor drivers are meant for Brushed DC-motors. Check the datasheet if you have the right one. If you also need to know the position of the motor shaft, you will need external sensors. This is not so straight forward, especially at a high rotational speed.
Brushless DC or BLDC-motor
Another kind of DC-motor is a Brushless DC (BLDC) motor. As the name suggests, BLDC motors have no brushes. Brushes cause friction, resulting in noise, wear and some energy loss. In a BLDC-motor the permanent magnets are located on the rotor. So, no need for brushes. The coils are in the stator, so in fixed positions, surrounding the rotor. In a brushless DC-motor, an electronic control circuit switches the phase (timing) of current through the different windings to accomplish the rotor to rotate. In other words the commutation is done by electronics, not in a mechanical way. brushless DC-motors are more expensive than brushed motors and are more complex to control. But they need less maintenance and are typically more powerful than a brushed motor with the same size. Although you can buy sensor-less BLDC-motors, mostly they have sensors inside to sense speed and position.
- Voltage (V)
- Current (A). Usually specified without load and at optimal efficiency. Higher mechanical load results in higher current.
- Rotation speed (rpm, usually specified without load and at optimal efficiency)
- Torque (Nm, usually specified at optimal efficiency)
- Stall torque(Nm, the torque at which the motor will stop turning)
- Shape and diameter of the axis connection.
Note: since a motor contains coils, always use a flyback diode in reverse direction to protect the remainder of your circuit.
Brushed DC-motors are often used in battery powered toys, power tools and pumps. Brushless motors are used in drones and higher end power tools.