Stepper motors are actually multiphase motors. The more the coils, the more the phases. The more the phases, the smoother the operation of the motor and the higher the price. The torque is irrelevant to the number of phases. The most common type of stepper motor is the two-phase. Two phases, is the minimum number of phases needed for a stepper motor to operate. What you need to make clear here, is that the number of phases does not necessarily set the number of coils. If for example each phase has 2 coil pairs, and the motor is a 2-phase motor, the number of coils will be 8. That has to do only with the mechanical characteristic of the motor. To simplify things, i will explain the simplest 2-phase motor with one coil pair per phase.

There are basically 3 different connection types for a 2-phase stepper motor. The coils are interconnected and according the connection, a different number of wires are used to connect the motor to the controller.

This configuration is the most simple. 4 wires are used to connect the motor to the controller. The coils are internally connected either in series or parallel. This is an example of a bipolar stepper motor:

The motor has 4 terminals. The two yellow terminals (the colors i use are NOT according to standards!!!) are for powering the horizontal coils, while the two purple terminals are for powering the vertical coils. The problem with this configuration is that, if someone wants to change the magnetic polarity, the only way to do this is by changing the current direction. This means that the driver circuit will have to be complicated, for example with a H-bridge.

In a unipolar motor, a common wire is connected to the point where the two coils are connected together:

A unipolar motor with 5 terminals

With this common wire, the magnetic poles can now easily be changed. Suppose for example that we connect the common wire to the ground. By powering once the first end of the coil and once the other end, the magnetic poles are changed. This means that the circuit for a bi-directional motor application is very simply, usually with only two transistors per phase. A major drawback is that, each time, only half of the available coil windings are used. This is like the motor is driven with single-coil excitation. Therefore, the torque generated is always about half the torque that would have be generated if both coils were powered. In other words, unipolar motors needs double the space as a bipolar motor, to provide the same torque. The unipolar motor can be used as a bipolar motor, simply by leaving the common wire unconnected.

Unipolar motors may have 5 or 6 terminal wires. The drawing above demonstrates a unipolar motor with 6 wires. There are situations though, that the two common wires are internally connected. In this case, the motor will have 5 wire terminals.

This is the most flexible stepper motor in terms of connection modes. All coils have wire terminals for both sides:

This motor can be connected with any connection possible. It can be connected as a 5 or 6 leads unipolar, as bipolar with series windings, as bipolar with parallel windings, or as bipolar with single winding per phase for lower current applications.

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