When selecting a two-phase stepper motor, you will encounter two fundamental terms: “Bipolar” and “Unipolar.”
This terminology doesn’t describe the motor itself, but rather the electrical structure of its internal windings (coils). This structural difference determines what kind of driver is required and directly impacts the motor’s performance.
1. Bipolar Stepper Motors
Bipolar motors are the most efficient and popular choice in modern applications.
Structure & Principle
- Internal Structure: There is one winding (coil) per phase.
- How it Works: To make the rotor turn (i.e., reverse the magnetic pole), the driver must reverse the direction of the current flowing through the entire coil (e.g., from +A to -A).
- Driver Requirement: This requires a more intelligent driver circuit, known as an “H-Bridge,” which can precisely control and reverse the current. Fortunately, H-bridges are built into virtually all modern microstepping driver chips.
How to Identify
A bipolar motor is electrically simple, with two leads per phase.
- Wire Count: Only 4 wires.
Advantages
- High Efficiency and Torque: At any given moment, current is flowing through the entire coil. 100% of the copper is being used. This allows a bipolar motor to deliver significantly more torque for its size compared to a unipolar motor.
- Compact: Simpler internal winding structure.
2. Unipolar Stepper Motors
Unipolar motors are an older design, famous for their extremely simple drive circuitry.
Structure & Principle
- Internal Structure: There are two windings per phase (or, more accurately, one center-tapped winding per phase). This “center-tap” wire is the common wire.
- How it Works: The driver does not need to reverse the current. It simply switches the current from the center-tap to one half of the coil or the other half to reverse the magnetic pole.
- Driver Requirement: The drive circuit is very simple, requiring only a basic switch (like a transistor) for each half-winding.
How to Identify
Because of the “center-tap” common wires, it has more leads.
- Wire Count: Typically 5 wires or 6 wires.
Limitation
- Low Efficiency and Torque: At any given time, only half of the phase winding is being used. The other half of the coil (copper) sits idle, which results in much lower torque output compared to a bipolar motor of the same size.
3. Key Comparison: How to Choose
| Feature | Bipolar | Unipolar |
| Winding Structure | 1 Winding per Phase | 1 Center-Tapped Winding per Phase |
| Drive Principle | Current Reversal (H-Bridge) | Current Switching (Simple Transistors) |
| Performance | High Torque, High Efficiency | Lower Torque, Lower Efficiency (Only 50% of winding used) |
| Common Wires | 4 Wires | 5 or 6 Wires |
| Driver Complexity | Driver (chip) is complex, but standard | Driver (hardware) is very simple |
A Special Note: The “Universal” 6-Wire Motor
The most interesting case is the 6-wire motor. It is a unipolar motor that offers excellent flexibility:
- As Unipolar: Connect the two “center-tap” wires and the four end wires to a simple unipolar driver.
- As Bipolar (Recommended): Completely ignore the two “center-tam” wires. By using only the four “main” end wires, it becomes a standard, high-performance bipolar motor. It can then be driven by a modern bipolar driver to achieve its full torque potential.
Conclusion:
- If your motor has 4 wires, it must be driven as Bipolar.
- If your motor has 6 wires, you can drive it as a low-torque Unipolar motor, or (more commonly) treat it as a Bipolar motor to get the best performance.
In modern applications, because the cost of bipolar driver chips has dropped significantly, bipolar driving is the absolute standard, as it delivers the maximum performance from the smallest possible motor.