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138 Build Your Own Electric VehicleFi g u r e 6-2 DC motor s basics—obtaining torque from mo ving conductor s.Now the current through the conductor (which flows into the page—you’re seeingthe back of the arrow) would exert a force that would tend to rotate it in the clockwisedirection. The magnitude of the torque would be given byT 5 Frwhere T is torque in foot-pounds, F is force in pounds, and r is the distance measuredperpendicularly from the direction of F to the center of rotation in feet. Now you havea motor design—on paper. You need to take some steps to make it real.First you need to make it more “force-full.” Since the force varies with the length ofthe conductor, if you make a coil of wire, as shown in the upper right of Figure 6-2,twice as much length is cutting the lines of flux. The force generated on the right-handwire is downward and the force generated on the left-hand wire is upward; they wouldassist one another in producing rotation and result in twice the torque.To further assist rotation, you add the commutator and brushes, shown in themiddle of Figure 6-2. This arrangement allows you to power your motor from a constantsupply of direct current (DC) voltage. Switching the polarity of the coils when theyreach the 12 o’clock or 6 o’clock position (minimum flux point) guarantees that currentwill always flow in through brush number 2 and out through brush number 1, thusalways producing upward force on the left-hand conductor and downward force on theright-hand conductor, and creating a constant rotation.To further increase the motor’s torque abilities, you add additional coils as shownat the bottom of Figure 6-2. In reality, each coil can have many windings, and you can