Types of Stepper Motors

A general schematic of stepper motors is shown in figure 3.2.

Figure 3.2: Stepper Motor Schematic

$$\includegraphics{stepper}$$

In this picture, the light blue ``magnet'' rotates at the center of the four coils. A, B, C, D, X and Y are electrical nodes that are connected to wires. There are different classes of stepper motors, depending on the configuration of point X and point Y in the figure.

• X and Y are both available and independent: most unipolar stepper motors are like this. This is the most general configuration as you can use bipolar or unipolar drive, with or without half stepping.
• X and Y are available but they are connected as one node: less common unipolar stepper motors are like this. This is much less general than the previous one. You can still use bipolar drive, but you cannot half step when the motor is bipolarly driven.
• X and Y are not available: this is the configuration for motors designed for bipolar drive.

By switching current through the coils, the freely rotating magnet follows the changing magnetic field and rotates. The magnet is connected to the drive axle of a stepper motor to deliver torque.

In unipolar full-step drive, only one coil is active an any time. In order to turn the magnet clockwise, the following current sequence can be repeated:

1. AX
2. BY
3. CX
4. DY

The beauty of unipolar drive is that the direction of current through the coils do not change. A controller merely needs to switch current on and off. Switching is easy when current direction does not change.

In bipolar full-step drive, points X and Y are not used. Instead, two coils in series are used at the same time. The current sequence to turn the magnet clockwise is as follows.

1. AB
2. BD
3. BA
4. DB

In this case, the controller needs to switch on and off as well as the direction of current. We will explain how to do this later. Why would anyone want to switch current direction when there is an easier solution (from the perspective of circuit design)?

It turns out bipolar drive ideally saves 50% energy while delivering the same amount of work. The strength of magnetic field is proportional to both the current and the number of winding of a coil. Let us use the following assumptions:

1. the resistance of each of the four coils is 30
2. the number of winding of each of the four coils is 50
3. the supply voltage is 12V

In unipolar drive, the current is $\frac{12}{30}=0.4$A, and there is a total of 50 winding for the active coil. In bipolar drive, the current is $\frac{12}{30+30}=0.2$A, and there is a total of 50+50=100 winding. You can see that in bipolar drive, even though the current is dropped by 50% compared to unipolar drive, the number of winding is increased by 100%. The developed magnetic field is equally strong in both cases.

From the perspective of power consumption, unipolar drive consumes $12\times \frac{12}{30}=4.8$W, whereas bipolar drive consumes $12\times \frac{12}{30+30}=2.4$W. Hey, bipolar drive saves 50% energy!