Prototyping
A dc motor consists of a multipole electromagnet coil, called the armature, or sometimes the rotor, that spins inside a constant magnetic field called the stator field. In the small dc motors typically found in model electric cars and toys, permanent magnets are used to create the stator magnetic field. In larger, industrial- type motors, such as an automobile starter or windshield- wiper motors, the stator field is produced by a second coil winding.
Current is sent through the rotating armature coil via a set of contact pads and stationary brushes called the commutator. Each set of commutator pads on the rotor connects to a different portion of the armature coil winding. As the rotor rotates, brush contact is made to different pairs of commutator pads so that the portion of the armature coil receiving current from the brushes is constantly changed. In this way, the magnetic field produced by the rotating armature coil remains stationary and is always at right angles to the stationary stator field. The north and south poles of these fields constantly seek each other; because they are always kept at right angles by the action of the commutator, the armature experiences a perpetual torque (rotational force). The strength of the force is proportional to the value of armature current; hence, the speed of the motor under constant mechanical load is also proportional to armature current.
a. Obtain a small dc motor from a hobby or electronic parts store. Connect two D-cell batteries in series with the motor without regard to polarity. Observe the direction of rotation, and then reverse the polarity of the battery connections. Observe the results.
b. Using a double-pole, double-throw switch like the one described in the previous problem, design a circuit that can reverse the direction of the motor using a single switch.
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