Bipolar Stepper Motors: How They Work and How to Control Them

A stepper motor is a synchronous brushless DC motor whose driver moves the stator magnetic field through a sequence of discrete positions. The rotor follows that field in steps while the load remains within the motor’s torque-speed limit. Many systems run open-loop, but feedback is optional rather than forbidden. A position sensor can detect errors in a closed-loop design. The controller commands steps while the driver regulates winding current.

Two common winding and drive arrangements are unipolar and bipolar. The terms describe how each phase is connected and how the driver establishes magnetic polarity. They do not by themselves specify rotor construction, step angle or control interface.

A unipolar stepper motor has a centre-tapped phase windings arrangement. A simple unipolar driver energises one half of a phase winding at a time, so current through each half flows in one direction. The centre tap connects to the supply and a low-side transistor circuit selects the active half. Some six-wire motors can instead be used as bipolar motors by leaving the centre taps disconnected and driving the winding ends.

A bipolar stepper motor normally has one winding per phase with no centre tap. A two-phase motor usually has four leads. Reversing current through a winding reverses that phase’s magnetic polarity, so each phase needs a full H-bridge or an integrated equivalent. The driver must also provide a safe path for inductive current during switching.

This article explains the bipolar winding arrangement, the driver interface, phase-current sequences, full stepping, half stepping and microstepping. It also covers limits that determine whether the rotor follows the commanded position.

What is a Bipolar Stepper Motor?

A bipolar stepper motor is a stepper motor whose phase current must reverse direction to reverse magnetic polarity. A typical two-phase unit has one continuous winding per phase and four external wires, one for each winding end. Verify phase pairs with the manufacturer wiring diagram or a resistance measurement before connecting a driver.

Because bipolar drive uses the full phase winding, a comparable motor can produce more torque than the same frame operated through half of a centre-tapped winding. This is not a universal size-only guarantee. Torque depends on copper, magnetic design, winding current, temperature and the driver. The trade-off is a dual-H-bridge power stage with current regulation, recirculation paths and fault protection.

The following simplified diagram shows a two-phase bipolar stepper motor:

The drawing represents a permanent-magnet rotor and four stator poles labelled A, B, C and D. A-B forms one phase and C-D forms the other. Real hybrid steppers often have toothed rotors and stators with many pole interactions, so the diagram explains polarity rather than all mechanical detail.

Current in each phase creates a magnetic axis. Reversing the current reverses that axis. The driver selects a sequence of phase-current vectors, and the rotor moves towards each new equilibrium position. Step rate sets commanded speed. Sequence order sets direction. Current magnitude and the motor torque-angle relation determine available torque.

How to Control a Bipolar Stepper Motor?

A bipolar motor needs two independently controlled phase currents. A basic controller can command the four H-bridge polarities directly. Many integrated drivers instead accept one STEP input, one DIR input and mode settings. Each STEP edge advances an internal indexer to the next phase-current state, while DIR selects the order. STEP and DIR are driver-level commands, not separate direction and magnitude signals for each phase. Current magnitude is normally set by a reference and regulated with PWM current chopping.

The required current sequence depends on the selected control modes. The common choices are:

• Full-step mode
• Half-step mode
• Micro-step mode

Full-Step Mode

Full-step drive advances the commanded magnetic field by one basic motor step. One-phase-on and two-phase-on sequences both exist. The sequence below is a two-phase-on example, so both windings carry current at every state. Some integrated drivers set each phase to about 70.7% of the full-scale current so the resultant current vector has the intended magnitude.

The following sequence is one example of clockwise two-phase-on full stepping:

Step 1: phase AB positive, phase CD negative; Step 2: AB negative, CD negative; Step 3: AB negative, CD positive; Step 4: AB positive, CD positive. Repeating the sequence continues rotation. Reversing its order reverses direction.

Positive and negative signs state the current direction in each phase. Neither entry is blank in this two-phase-on sequence. A STEP/DIR driver generates these phase commands internally; a direct-drive controller must also enforce safe bridge switching and current limits.

The full-step vectors are separated by the motor’s basic step angle.

Full stepping is simple and provides strong incremental torque. Its larger position changes can excite mechanical resonance, audible noise and vibration. The available torque still falls with speed as winding inductance and back EMF prevent current from reaching its set value.

Full-step resolution equals the specified basic step angle. A 1.8° motor has 200 commanded full steps per revolution, while a 0.9° motor has 400. This command count is not a guarantee of absolute shaft accuracy. Load torque creates position lag, and an overloaded open-loop motor can miss steps.

Half stepping or microstepping adds commanded current vectors between the full-step positions.

Half-Step Mode

Half-step drive alternates one-phase-on states with two-phase-on states. This inserts one commanded position between adjacent full steps and halves the nominal command increment. If the same phase current is used in both kinds of state, resultant torque alternates. A current-regulating driver can reduce each current to about 70.7% in the two-phase states to make the current-vector magnitude more nearly constant.

The following sequence is one example of clockwise half stepping:

Step 1: AB positive, CD off; Step 2: AB positive, CD negative; Step 3: AB off, CD negative; Step 4: AB negative, CD negative; Step 5: AB negative, CD off; Step 6: AB negative, CD positive; Step 7: AB off, CD positive; Step 8: AB positive, CD positive.

Half stepping doubles command resolution and normally reduces low-speed vibration compared with full stepping. Torque ripple depends on whether the driver scales current in the two-phase states. Power loss depends on RMS winding current, bridge loss and operating time rather than the mode name alone.

A 1.8° motor has a nominal 0.9° half-step increment and 400 command positions per revolution. A 0.9° motor has a nominal 0.45° increment and 800 positions. Mechanical accuracy remains limited by detent torque, load, friction, magnetic nonlinearity and manufacturing tolerance.

Microstepping divides the basic step into more current-vector increments.

Micro-Step Mode

Microstepping regulates both phase currents to intermediate reference values, usually approximating sine and cosine over an electrical cycle. This rotates the commanded magnetic field in smaller increments. It can reduce low-speed vibration, resonance and acoustic noise, but each microstep has less incremental restoring torque than a full step.

The following 16 current vectors cover one electrical cycle. The approximately 22.5° electrical spacing provides four microstep intervals between adjacent 90° full-step vectors:

1: AB 100%, CD 0%; 2: 92%, -38%; 3: 71%, -71%; 4: 38%, -92%; 5: 0%, -100%; 6: -38%, -92%; 7: -71%, -71%; 8: -92%, -38%; 9: -100%, 0%; 10: -92%, 38%; 11: -71%, 71%; 12: -38%, 92%; 13: 0%, 100%; 14: 38%, 92%; 15: 71%, 71%; 16: 92%, 38%.

The following diagram illustrates bipolar phase-current directions:

Microstepping improves command resolution and smoothness, but command resolution is not the same as positioning accuracy. Friction, load torque, detent torque, current-regulation error and magnetic harmonics can prevent the rotor from settling at every ideal microstep angle. The driver also needs enough supply voltage and a suitable decay mode for winding current to track the reference at the required speed.

A 1.8° motor driven at 1/32 step receives 6400 microstep commands per revolution. That number describes electrical command increments, not 6400 equally accurate mechanical positions. Confirm usable accuracy with the motor, driver, supply, load and motion profile.

Conclusion

A bipolar stepper motor has no centre tap and needs current to reverse through each phase. A two-phase motor therefore uses two H-bridges, usually inside a current-regulating driver. Compared with a centre-tapped unipolar arrangement, full-winding use can improve torque for a comparable design. It also requires more capable switching and protection circuitry.

Full-step, half-step and microstep modes select different phase-current vectors. Full stepping gives the largest command increments. Half stepping alternates one-phase and two-phase states. Microstepping uses regulated intermediate currents for smoother motion. None of these modes removes the need to check winding current, supply voltage, decay mode, thermal limits, torque-speed curves and acceleration.

Bipolar stepper systems serve printers, scanners, CNC axes, robots, camera mechanisms, pumps, valves and actuators. An open-loop design is reliable only while the commanded acceleration, speed and load remain within available torque. Use feedback or fault detection when a missed step creates an unacceptable position error.