Stepper Motor (4-wire bipolar) Symbol
Definition: The Stepper Motor (4-wire bipolar) symbol represents a precision electromechanical actuator drawn as a circle labeled 'M / STEP' with four coil terminals — A+ and A− for coil A, and B+ and B− for coil B — denoting a brushless motor with two independent electromagnetic winding phases that rotates in precise discrete angular steps when driven with sequenced bipolar current pulses, as referenced in IEC 60617-07 (rotating machines) and used in industrial CNC, automation, and motion-control schematics.
Also known as: 4-wire stepper symbol, bipolar stepper motor symbol, CNC motor symbol, industrial stepper symbol, two-phase stepper symbol.
What the Stepper Motor (4-wire bipolar) symbol means
The Stepper Motor (4-wire bipolar) symbol represents the industrial-grade variant of the stepper motor specifically identified by its four-wire bipolar coil configuration. The four pins — A+ (A-plus), A− (A-minus), B+ (B-plus), and B− (B-minus) — represent the two ends of coil A and the two ends of coil B respectively, confirming that the motor is wired for bipolar operation requiring the driver to reverse current direction in each winding to produce the step sequence. Unlike a unipolar (5- or 6-wire) stepper, a bipolar motor uses all of each coil's copper for torque production, giving higher torque per unit of motor volume.
In industrial and CNC motion-control schematics, this 'Stepper Motor (4-wire bipolar)' symbol specifically distinguishes from the simpler hobbyist 'stepper' symbol by explicitly labeling the bipolar coil polarity (A+/A−, B+/B−), enabling the engineer to document correct wiring to the driver's H-bridge outputs without ambiguity. The symbol is used in CNC router, gantry crane, pick-and-place machine, and servo-equivalent positioning applications where the four-wire connection and current polarity must be explicitly documented.
How to identify the Stepper Motor (4-wire bipolar) symbol
The Stepper Motor (4-wire bipolar) symbol is a circle labeled 'M' in a large serif font and 'STEP' in smaller text below, identical in shape to the hobbyist stepper symbol. The key differentiator is the four terminal pins: A+ (top-left, pin id: a1), A− (bottom-left, pin id: a2), B+ (top-right, pin id: b1), and B− (bottom-right, pin id: b2). The coil polarity labels (+ and −) on both left and right sides are the unique identifier of the bipolar four-wire variant, versus the simpler three-pin stepper symbol (which shows A1, A2, B1 and a Common, suggesting a unipolar-compatible or simplified representation).
Function in a circuit
A 4-wire bipolar stepper motor operates by energising its two winding phases (A and B) in sequence using full bipolar current reversal. A bipolar driver H-bridge applies positive current (A+ to A−) in one state, then reverses to negative current (A− to A+) in the next, while simultaneously shifting phase B through its four current states. Full-step mode uses four states; half-step uses eight; microstepping uses up to 256 intermediate states per mechanical full step. Each full step advances the rotor shaft by the step angle (typically 1.8° = 200 steps/revolution for a standard motor). Holding torque is maintained by keeping one or both phases energised. The bipolar configuration delivers approximately 40% more torque than an equivalent unipolar motor because the full coil cross-section carries current in both directions.
Standards: IEC vs ANSI
| IEC 60617 | IEC 60617-07 provides the rotating machine symbol (circle with 'M' designator) used for all motor types. No unique IEC symbol distinguishes a stepper from an induction or DC motor beyond annotation. IEC 60034-12 governs starting performance of motors; IEC 61800-7 addresses servo and motor interfaces in motion control but does not define a stepper-specific glyph. |
|---|---|
| ANSI/IEEE 315 | ANSI Y32.2-1975 (IEEE 315) uses the same circle-with-letter convention for all motor types. NEMA defines physical frame dimensions (NEMA 17, 23, 34, 42) and holding torque ratings for stepper motors but does not define schematic symbols. The '+'/'-' phase terminal labeling convention is a de facto standard in motion control documentation. |
| Key difference | No glyph difference exists between IEC and ANSI for stepper motor symbols. Both use an annotated circle. The bipolar 4-wire variant is identified solely by the A+/A−/B+/B− terminal label annotation, which is consistent across IEC and ANSI schematic practice. |
Terminals / pins
| Pin | Name |
|---|---|
| a1 | A+ |
| a2 | A- |
| b1 | B+ |
| b2 | B- |
Typical values
Typical NEMA 23 (57 mm frame): step angle 1.8° (200 steps/rev), current 2.0–3.5 A per phase, resistance 0.5–2.5 Ω per phase, inductance 1–8 mH per phase, holding torque 0.5–3.0 N·m. Typical NEMA 34 (86 mm frame): holding torque 3–12 N·m, current 3–8 A per phase. Standard drive supply: 24–80 V DC (NEMA 23/34 with high-performance drivers). Step accuracy: ±5% of one step (non-cumulative).
Where the Stepper Motor (4-wire bipolar) symbol is used
- Industrial CNC milling and routing machines using NEMA 23 or NEMA 34 bipolar steppers for X, Y, Z, and A-axis positioning
- Laser cutting and engraving machines requiring precise bidirectional gantry movement with four-wire stepper motors
- Pick-and-place SMT assembly machines driving X-Y gantry rails and component head mechanisms
- Automated dispensing systems (adhesive, solder paste, conformal coating) requiring accurate volumetric control via lead-screw
- Industrial textile looms and weaving machines using stepper motors for thread feed and pattern control
- Coordinate measuring machine (CMM) probe positioning stages requiring sub-millimetre repeatability
Example
In a CNC router spindle Z-axis schematic, a Stepper Motor (4-wire bipolar) symbol shows A+ and A− connecting to the 1A and 1B outputs of a DM542 digital stepper driver, and B+ and B− connecting to 2A and 2B. The driver's STEP and DIR inputs come from the breakout board connected to the PC's parallel port running Mach3 CNC software. The 48 V DC motor supply (VMOT) is provided by a dedicated switching power supply, delivering the high voltage needed for adequate torque and response speed on the 3 N·m NEMA 23 motor.
Key facts
- The Stepper Motor (4-wire bipolar) symbol is a circle labeled 'M STEP' with four terminals: A+ (pin id: a1, top-left), A− (pin id: a2, bottom-left), B+ (pin id: b1, top-right), and B− (pin id: b2, bottom-right), explicitly showing bipolar coil polarity.
- A 4-wire bipolar stepper motor delivers approximately 40% more torque per unit volume than an equivalent unipolar motor because the full coil winding carries current in both directions, making all copper useful for torque production.
- The standard step angle of 1.8° (200 steps/revolution) applies to most industrial NEMA 23 and NEMA 34 bipolar stepper motors; some high-resolution motors use 0.9° (400 steps/rev).
- Step accuracy is typically ±5% of one step angle (non-cumulative) — errors do not accumulate over multiple steps, meaning a 1.8° motor steps within ±0.09° on any single step without needing encoder feedback.
- Bipolar stepper drivers must reverse the current direction in each coil to produce the step sequence; this requires full H-bridge circuitry (4 MOSFETs per phase), unlike unipolar drivers (2 MOSFETs per phase with a center-tap return).
- NEMA 17 (42 mm) is common in 3D printers (holding torque 40–60 N·cm); NEMA 23 (57 mm) is common in CNC routers (0.5–3 N·m); NEMA 34 (86 mm) is used for large CNC and industrial axes (3–12 N·m).
- The four wires of a bipolar stepper motor can be identified by resistance measurement: the two wires of each coil pair show the coil resistance (e.g., 1.5 Ω) while wires of different coils show open circuit.
- Microstepping (1/8, 1/16, 1/32) divides each 1.8° full step into finer increments by proportionally splitting current between A and B phases, improving motion smoothness and reducing resonance at the cost of reduced peak torque.
Diagrams that use this symbol
- stepper motor wiring diagram
- stepper motor pinout
- stepper motor circuit diagram
- 4 wire stepper motor wiring diagram
Frequently asked questions
What does the stepper motor (4-wire bipolar) symbol look like in a circuit diagram?
The Stepper Motor (4-wire bipolar) symbol is a circle labeled 'M' and 'STEP'. Four terminal pins are arranged at the corners: A+ (top-left) and A− (bottom-left) for coil A, and B+ (top-right) and B− (bottom-right) for coil B. The plus/minus labeling on both sides confirms the bipolar four-wire configuration, distinguishing it from a unipolar motor symbol.
What is the difference between a 4-wire bipolar and a 5-wire or 6-wire stepper motor?
A 4-wire bipolar motor has two separate coils with no center tap — the driver must reverse current polarity in each coil to produce step motion, requiring a full H-bridge. A 5-wire unipolar motor has two coils sharing a common center tap — current flows in only one half of each coil at a time, simplifying the driver to two transistors per phase but reducing torque. A 6-wire motor has two coils each with a separate center tap (equivalent to two 5-wire motors combined), offering the choice of bipolar or unipolar operation.
How do I identify the coil pairs of a 4-wire stepper motor?
Use a multimeter in resistance mode: measure resistance between all pairs of wires. The two pairs that each show the coil resistance (typically 0.5–10 Ω) are coil A (A+/A−) and coil B (B+/B−). Pairs from different coils show open circuit. To determine polarity within each pair, temporarily connect a 1.5 V battery across one coil and feel for the rotor to lock — reversing the battery connection reverses the holding direction.
What driver is used with a 4-wire bipolar stepper motor?
Common drivers for bipolar stepper motors include the A4988 (2 A, 35 V), DRV8825 (2.2 A, 45 V), and DM542 (4.2 A, 50 V) for industrial applications. All these drivers accept STEP and DIR digital inputs and internally generate the full H-bridge current-reversal sequence required by bipolar windings. The motor supply voltage should typically be 10–20× the coil resistance × rated current for good torque at speed.
What is the holding torque of a bipolar stepper motor?
Holding torque is the maximum torque a fully energised (stationary) stepper motor can resist before the rotor moves. For a NEMA 17 bipolar motor it is typically 40–90 N·cm (0.4–0.9 N·m); for NEMA 23 it is 0.5–3.0 N·m; for NEMA 34 it is 3–12 N·m. Holding torque is reduced to approximately 70% of rated value when microstepping (due to reduced peak phase current in intermediate positions).
What voltage supply do bipolar stepper motors need?
Bipolar stepper motors are current-controlled rather than voltage-limited — the driver regulates coil current regardless of supply voltage. Using a higher supply voltage (within the driver's VMOT range) improves high-speed torque because the higher voltage overcomes coil inductance faster, reducing the time constant for current rise. NEMA 17 motors in 3D printers typically use 12–24 V; NEMA 23/34 industrial motors use 24–80 V with industrial drivers such as the DM542 or DM860H.
What standard defines the 4-wire bipolar stepper motor symbol?
IEC 60617-07 defines the general rotating machine symbol (circle with 'M' designator) applicable to stepper motors. No IEC or ANSI/IEEE standard defines a unique symbol specifically for 4-wire bipolar stepper motors. The A+/A−/B+/B− terminal labeling convention is an industry standard used in motion control documentation, CNC machine schematics, and CAD tool libraries (KiCad, Altium, Eagle).
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