Windings
There are different methods for winding stepper motors. A standard three-lead motor is shown in Figure 35 – 5. The common terminal of the two windings is connected to ground of an above- and below-ground power supply. Terminal 1 is connected to the common of a single-pole double-throw switch (switch #1) and terminal 3 is connected to the common of another single-pole double-throw switch (switch #2). One of the stationary contacts of each switch is connected to the positive or above-ground voltage, and the other stationary contact is connected to the negative or below-ground voltage. The polarity of each winding is determined by the position setting of its control switch.
Stepping motors can also be wound bifilar as shown in Figure 35 – 6. The term bifilar means that there are two windings wound together. This is similar to a trans- former winding with a center tap lead. Bifilar stepping motors have twice as many windings as the three-lead type, which makes it necessary to use smaller wire in the windings. This results in higher wire resistance in the winding, producing a better inductive-resistive (L/R) time constant for the bifilar wound motor. The increased L/R time constant results in better motor performance. The use of a bifilar stepper motor also simplifies the drive circuitry requirements. Notice that the bifilar motor does not require an above- and below-ground power supply. As a general rule, the power supply voltage should be about five times greater than the motor voltage. A current-limiting resistance is used in the common lead of the motor. This current-limiting resistor also helps to improve the L/R time constant.