Stepper Motor Drives

Stepper motors are special DC motors that rotate in discrete angular steps in response to digital pulses. Each pulse moves the rotor by a fixed step angle, making stepper motors inherently suitable for open-loop position control without encoders or feedback systems. They are widely used in 3D printers, CNC machines, printers, and robotic systems. The drive circuit determines how the motor windings are energized, and the choice between unipolar and bipolar drive significantly affects torque, efficiency, and complexity.

Core Concept of Stepper Motor Operation

A stepper motor works on the principle of variable reluctance or magnetic attraction between the rotor teeth and the stator poles. In a permanent magnet stepper motor, the rotor has alternating north and south poles. When a stator winding is energized, the rotor aligns itself to minimize reluctance (or to align with the field), moving to a specific position. When the next winding is energized in sequence, the rotor steps forward to the next position. The step angle determines the minimum angular movement per pulse.

The most common stepper type used in drives is the hybrid stepper motor, which combines a permanent magnet with toothed rotor and stator poles. A typical hybrid stepper has a step angle of 1.8 degrees, giving 200 steps per revolution. The step angle depends on the number of rotor teeth Nr and the number of phases. For a 2-phase motor: step angle = 360 / (4 * Nr). With Nr = 50 teeth, step angle = 360 / 200 = 1.8 degrees.

Microstepping is a technique where the current in the two phases is varied sinusoidally in small increments instead of switching abruptly. This creates intermediate positions between full steps, greatly reducing torque ripple and mechanical noise. Microstepping can divide one full step into 8, 16, 64, or even 256 microsteps, enabling smooth and quiet motion at the cost of increased drive circuit complexity.

Unipolar and Bipolar Drive Comparison

In a unipolar drive, each winding has a center tap that is connected to the supply rail. Current flows from the supply through one half of the winding through a transistor switch to ground. The direction of the flux is reversed not by reversing current but by switching between the two halves of the winding. Since only half the winding carries current at any time, the effective number of turns contributing to the magnetic field is halved, which reduces the torque output.

A bipolar drive uses the entire winding without a center tap. Current is reversed in the winding by using an H-bridge circuit per phase. An H-bridge consists of four transistors arranged so that the current can be made to flow in either direction through the winding. This utilizes the full winding all the time, giving approximately 40 percent more torque than a unipolar drive with the same motor. The trade-off is that bipolar drive requires twice as many transistors and a more complex gate drive circuit.

Mathematical Expression

The step angle of a stepper motor is defined by its construction. For a 2-phase hybrid stepper motor, the step angle is 360 / (2 * N_s) where N_s is the total number of steps per revolution. The number of steps per revolution equals 4 * Nr for a 2-phase motor, where Nr is the number of rotor teeth. The holding torque of a stepper motor is the maximum static torque it can exert without losing synchronism. The pull-out torque is the maximum torque the motor can develop while maintaining step synchronism at a given stepping rate.

Given:
2-phase hybrid stepper motor, Nr = 50 rotor teeth
Winding resistance R = 2.5 ohm per phase
Supply voltage V = 12 V
Unipolar drive: only half winding used (N_eff = N/2)

Why this formula applies:
Step angle uses rotor teeth and phase count.
Current determines holding torque via I = V/R.

Formula:
Step angle = 360 / (4 * Nr) degrees

Substitution:
Step angle = 360 / (4 * 50) = 360 / 200

Calculation:
Step angle = 1.8 degrees
Steps per revolution = 200

Phase current (bipolar): I = V / R = 12 / 2.5 = 4.8 A
Phase current (unipolar, half winding): I = 12 / 2.5 = 4.8 A but N_eff = N/2, so MMF = N_eff * I = 0.5 * N * 4.8

Final Answer:
Step angle = 1.8 degrees, 200 steps/rev.
Bipolar drive produces ~41% more torque than unipolar for same V and R.

Exam Tip: Step angle for a 2-phase hybrid stepper = 360 / (4 * Nr). For GATE, remember that bipolar drive gives more torque than unipolar because the full winding is used. Microstepping improves smoothness but does not increase the maximum torque at high speed.

Mechanism Explained

• Full step drive energizes one phase at a time, giving maximum step angle (1.8 degrees for a standard motor). Torque is moderate and vibration is higher.
• Two-phase-on full step drive energizes two phases simultaneously at every step. This gives higher holding torque (approximately 40 percent more) and the same step angle.
• Half step drive alternates between one-phase-on and two-phase-on states, effectively halving the step angle to 0.9 degrees and doubling steps per revolution to 400.
• Microstepping applies proportional sinusoidal currents to both phases simultaneously, allowing the rotor to hold at intermediate positions between steps. It reduces torque ripple and audible noise significantly.
• Unipolar drive uses center-tapped windings and 4 switches per 2-phase motor. Bipolar drive reverses current using H-bridges (8 switches per 2-phase motor) but delivers about 40 percent more torque because the full winding is used.
• Stepper motors can lose steps (miss a pulse) if the load torque exceeds the pull-out torque. Open-loop control is only reliable within the safe torque-speed range of the motor.

Quick Revision

• Stepper motor step angle = 360 / (4 * Nr) for a 2-phase hybrid motor. Standard: Nr = 50, step angle = 1.8 degrees, 200 steps/rev.
• Unipolar drive: center-tapped winding, 4 switches per 2-phase motor, only half winding used, lower torque.
• Bipolar drive: H-bridge per phase, 8 switches per 2-phase motor, full winding used, approximately 40 percent more torque than unipolar.
• Half stepping halves the step angle and doubles resolution. Microstepping further subdivides steps using sinusoidal current profiling.
• Stepper motors are open-loop controlled. No encoder needed for position, but step loss occurs if load exceeds pull-out torque.
• GATE trap: Microstepping improves smoothness and reduces vibration but does NOT increase the rated maximum torque of the motor.
• Holding torque is the static torque at standstill when phase is energized. Pull-out torque is the dynamic maximum torque at a given stepping rate.