Stepping Motor

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Construction
Stepping motors are electromagnetic, rotary, incremental devices which convert digital pulses into mechanical rotation. The amount of rotation is directly proportional to the number of pulses and the speed of rotation is relative to the frequency of those pulses.

Stepping motors are simple to drive in an open loop Static or holding torque - displacement characteristic

The characteristic of static (holding) torque - displacement is best explained using an electro-magnet and a single pole rotor (fig. 12). In the example the electro-magnet represents the motor stator and is energized with it's north pole facing the rotor


Figure 12 Curve illustrating static torque verses rotor position

Assuming there are no frictional or static loads on the rotor, fig. 11 illustrates how the restoring torque varies with rotor position as it is deflected from it's stable point. As the rotor moves away from it's stable position, the torque steadily increases until it reaches a maximum. This maximum value is called the holding torque and represents the maximum load that can be applied to the shaft without causing continuous rotation. If, the shaft is deflected beyond this point, the torque will fall until it is again at zero. However, this zero point is unstable and the torque reverses immediately beyond it back to the stable point. A pendulum (fig. 13) can also be used to demonstrate the effects we observe.




Figure 13 Pendulum effect of static torque verses rotor position

Depending on the number of phases, the cycle in figures 11 and 12 would be equivalent to the following number of full steps:
• 2 phase 4 steps
• 3 phase 6 steps
• 5 phase 10 steps
The torque required to deflect the shaft by a given angle can be calculated using the formula:

Although this static torque characteristic is not a great deal of use on it's own, it does explain some of the effects we observe. For example, it dictates ...