types of stepper motors?
What are the different types of stepper motors?
To classify stepper motors correctly, distinguish between rotor construction, winding arrangement and control system. Hybrid describes the motor's construction; bipolar describes how its windings are driven. One motor can therefore be hybrid, bipolar and closed-loop at the same time.
1. Three basic types by rotor construction
Permanent-magnet stepper motor (PM): The rotor contains a permanent magnet. These motors are used in compact, economical positioning systems, but the step angle depends on the design. When assessing suitable applications, consider more than the motor's physical size.
Variable-reluctance stepper motor (VR): The rotor uses a toothed ferromagnetic structure rather than a permanent magnet. It aligns with the energised stator poles to reduce the magnetic circuit's reluctance. Motion comes from sequential phase excitation; the number of poles does not change during operation. “Variable reluctance” refers to the magnetic path, not an adjustable pole count.
Hybrid stepper motor: Combines a permanent magnet with a toothed rotor and stator. It is not a combination of bipolar and unipolar drive arrangements. Common in CNC axes and 3D printers, hybrid motors often have full-step angles of 1.8° or 0.9°.
2. Bipolar and unipolar winding arrangements
Bipolar connection: The driver reverses the current through each phase winding. A typical two-phase motor has four motor leads for phases A and B. An H-bridge is commonly used to reverse the current. Select the stepper driver to suit the motor's phase-current rating and winding connection.
Unipolar connection: Uses centre-tapped windings, switching between their two halves to change magnetic polarity. Common arrangements have five or six leads. Less winding copper may be active than with bipolar operation of the same winding arrangement, but “unipolar is always less accurate” is not a valid general rule. Assess torque behaviour and the drive mode separately.
Animation: Reversing current or selecting a half-winding?
On the left, current reverses through the same winding. On the right, it flows from the centre tap (CT) through alternate half-windings. Moving dots represent conventional current direction. This simplified view shows one phase; the second phase and switching circuitry are omitted. It is not a wiring diagram.
3. What does the number of wires tell you?
| Lead count | Typical winding arrangement |
|---|---|
| 4 wires | Two separate phase windings; a typical bipolar connection. |
| 5 wires | The centre taps may be internally joined; a typical unipolar arrangement. Do not assume compatibility with a standard bipolar driver. |
| 6 wires | Each phase has its own centre tap. Bipolar operation may be possible with the taps left disconnected, subject to the manufacturer's instructions. |
| 8 wires | The half-windings are accessible separately. Suitable wiring allows unipolar, bipolar series or bipolar parallel operation. |
Wire colours are not universal. Identify phases and centre taps using the manufacturer's diagram. When matching a six- or eight-wire motor to a driver, do not assume that series, parallel and half-winding connections use the same current setting. Never connect or disconnect motor leads while the driver is powered.
4. Small step angles and microstepping are different
A small full-step angle is a motor design feature. Microstepping is a drive method that sets intermediate phase-current values. Calculate full-step resolution as 360° / step angle: a 1.8° motor has 200 full steps per revolution; a 0.9° motor has 400.
With a 1.8° motor set to 1/16 microstepping, an interface where each pulse commands one microstep requires 200 × 16 = 3,200 pulses per revolution, giving a 0.1125° command increment. This does not guarantee positioning accuracy of 0.1125°. Load, driver current regulation and mechanical backlash also affect the result.
5. Open-loop, closed-loop and mechanical options
An open-loop stepper system does not continuously measure rotor position; missed steps may go undetected if the load or speed exceeds its limits. A closed-loop stepper system uses an encoder or position sensor to provide feedback. The response to a detected error depends on the system. When comparing stepper and servo motors, encoder presence alone is not enough.
Two-phase and five-phase motors are classified by phase count; their drivers are not directly interchangeable. Geared, lead-screw and brake-equipped versions describe mechanical options. A gear mechanism changes the output motion, while STEP/DIR signals describe the driver's command interface. A PLC output must also be electrically compatible with the driver input.
6. Which stepper motor should you choose?
Define the motion first: Required speed and positioning, acceleration, load inertia and duty cycle determine the motor requirements. Holding torque is not the torque available at operating speed. Use the motor-and-driver combination's torque-speed curve for sizing.
Check electrical compatibility next: Evaluate phase current, winding inductance, connection method and supply voltage together. Power-supply selection and thermal management are part of the system design. NEMA 17 or NEMA 23 alone does not establish torque, step angle or driver compatibility.
Assess the complete application: Consider advantages and limitations, maintenance requirements, common faults and motor-and-driver cost. Answer “stepper or servo?” using the required motion, feedback needs and complete system.
Technical references: Oriental Motor — motor construction · winding arrangements · Texas Instruments — microstepping and accuracy