Brushed DC Pumps

The simplest (and cheapest) DC motor is the brushed DC motor. A brushed DC motor is like the simple loop generators we've seen in cartoons (or theJensen generator used with the steam engine), only run backwards: the magnet is stationary and the DC-current-carrying coil (or coils), connected to a shaft, rotates through the xed magnetic eld. Loop generators we've seen use \slip rings" to make electrical contact between xed current-carrying wires and the rotor. But remember, in the simple loop generators, steady rotation in one direction with a slip ring produced alternating current. And a motor is the converse of a generator. So if you drove the slip-ringed generator as a motor with alternating current, you could get steady rotation. If drive it with direct current, though, the loop would just move to a place where force was zero and would stay there. To keep the loop moving and produce rotation in one direction requires not slip rings but some kind of \commutator" that essentially switches the diretion of the current, putting it rst on one side of the loop and then on the other, mimicking alternating current. Only if you manage to continue to kick the loop around will it keep turning.

 The simplest commutator is a \split ring" that makes contact with two separate brushes. In the simple AC loop generator, the rotating and xed conductors are connected with two slip rings, each of which maintains a continuous connection between one end of the rotating loop and a xed wire. The split ring commutators joins both ends of the loop to xed wires, and every half-rotation, the contacts are switched. In a generator, the simple split ring commutator on a single loop gener-ator would produce a voltage (and current) around the loop that is a single rectied sine wave, varying between zero and maximum every quarter rev-solution. In a DC motor with a simple split ring commutator, the torque provide by the motor is similarly irregular, varying from zero to maximum every quarter revolution. The motor doesn't provide steady even rotational force but a jerky force, wich is obviously not ideal for industrial applications. This is termed \torque ripple." To minimize torque ripple, DC motors use the same strategies that DC generators use to minimize voltage ripple: in-creasing the number of windings on the rotor or magnetic poles on the stator. (The windings on the rotor are also called the \armature"). By producing many out-of-sync rectied sine waves superimposed on each other, adding windings or poles begins to approximate a constant torque.

 Brushed DC motors seem like a relatively practical solution, but there are drawbacks. The brushes themselves wear out fairly fast, and it is not wise to put big amounts of current through brushes that make sliding contacts, as they will spark as contacts are made and then broken. If the rotor turns too quickly, the sparking can damage or destroy the commutator. Designing a brushed DC motor involves tradeoffs between power, speed, and repair frequency/cost.