Stepping Motor Operation & Theory

8-2 Excitation Methods

In Table 8-1 are descriptions and features of each method.

8-3 Bipolar and Unipolar Operation

All SKC stepper motors are available with either two coil bipolar or four coil unipolar windings. Bipolar Winding - the stator flux is reversed by reversing the current in the winding. It requires a push-pull bipolar drive as shown in Fig. 8-3. Care must be taken to design the circuit so that the transistors in series do not short the power supply by coming on at the same time. Properly operated, the bipolar winding gives the optimum performance at low to medium step rates.

Stepping Motor Operation & Theory

Unipolar Winding - has two coils wound on the same bobbin per stator half. Flux is reversed by energizing one coil or the other coil from a single power supply. The use of a unipolar winding, sometimes called a bifilar winding, allows the drive circuit to be simplified. Not only are one-half as many power switches required (4 vs. 8), but the timing is not as critical to prevent a current short through two transistors as is possible with a bipolar drive. Unipolar motors have approximately 30% less torque at low step rates. However, at higher rates the torque outputs are equivalent.

Step Motor Load Calculations and Selection

To select the proper step motor, the following must be determined:

1. Load Conditions
1-a. Friction Load
1-b . Load Inertia

2. Dynamic Load Conditions
       2-a. Drive Circuit
       2-b . Maximum Speed (PPS/Frequency)
       2- c. Acceleration/Deceleration Pattern
       With the above load information the
       proper step motor can be selected.

9-1 Load Inertia

The following is an example for calculating the inertia of a hollow cylinder.

Stepping Motor Operation & Theory

9-2

Linear systems can be related to rotational systems by utilizing the kinetic energy equations for the two systems. For linear translations:

Stepping Motor Operation & Theory

1) Gear drive system

When gears are used to drive a load, the inertia reflected to the motor is expressed by the following equation:

Stepping Motor Operation & Theory