Stepping Motor

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the static stiffness of the system, in other words; how the shaft position changes when a load is applied to a stationary motor. The shaft must deflect until the torque generated matches the applied load. Therefore, the static position varies with the load.

Static load angle
The static load angle is defined as, the angle between the actual rotor position and the stable end position for a given load. Figure 14 illustrates (whether for full or half step) that as the torque increases so does the shaft deflection from the stable position.



Figure 14

The static load angle can be calculated using the formula:

configuration and for their size provide excellent torque at low speed.

The benefits offered by stepping motors include:
• a simple and cost effective design
• high reliability
• maintenance free (no brushes)
• open loop (no feed back device required)
• known limit to the 'dynamic position error'

Although various types of stepping motor have been developed, they all fall into three basic categories.
1. variable reluctance (V.R)
2. permanent magnet (tin can)
3. hybrid
The variable reluctance or V.R. (fig 1) motor consist of a rotor and stator each with a different number of teeth. As the rotor does not have a permanent magnet it spins freely i.e. it has no detent torque. Although the torque to inertia ratio is good, the rated torque for a given frame size is restricted. Therefore small frame sizes are generally used and then very seldom for industrial applications.


Figure 1. cross section through a variable reluctance stepping motor

The permanent magnet (PM) or tin can (fig. 2) motor is perhaps the most widely used stepping motor in non-industrial applications. In it's simplest form the motor consists of a radially magnetized permanent magnet rotor and a stator similar to the V.R. motor. Due to the manufacturing techniques used in constructing the stator they are also sometim...