Practical DC Motors

A simple dc motor consists of coils of wire wound on an iron armature in a magnetic field as shown below. A set of brushes and a commutator allow the current always to flow in the same sense.

When a current flows through the coil, Flemings Left Hand Rule laws tells us that the coil rotates in the sense shown, with side 1 moving up and two moving down. After half a turn 1 and swap positions but the current flows in the same sense to the coil continues to rotate. The dc motor above has the distadvantage that the output is not steady. When the armature nears the vertical, the torque nears zero and the motor slows. We can make the motor turn at a more nearly constant speed by having more coils of wire on the armature as shown below. The armature receives a push from each pair coil when the coil is horizontal or four pushes per rotation, and the speed is more nearly constant.

A rapidly varying electric current (AC or DC) is unsuitable, however, for most electrical appliances and uses. Many such applications require a steady almost constant emf. If your lights were supplied by a 50 Hz AC power supply for example, you would see them flickering and the current rose and fell. The emf/current can be smoothed by using three or more coils in a generator, offset from each other by 120°. Each coil produces single phase AC which is transmitted over power lines separately from the other two phases. When the current from all three phases is superimposed, a smother output is produced (either AC or DC, depending on whether a rectifier has been used). This three-phase supply is commonly used in industrial and domestic power supply and uses generators with three sets of stator coils as shown below.

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