Stepper Motor - Basics, Working Principle, Application| SSC JE Electrical

What is a Stepper Motor?

A stepper motor, also known as a stepping motor or step motor, is a type of electromechanical device that converts electrical pulses into discrete mechanical movements.

The name stepper comes from its unique ability to rotate in fixed angular steps in response to each input current pulse received from its controller.

In recent years, stepper motors have gained immense popularity due to the rapid growth of the computer and automation industries.
One of the key feature use is that they can be directly controlled by computers, microprocessors, and programmable logic controllers (PLCs) without requiring complex feedback systems.
Unlike conventional industrial motors that convert electrical energy into mechanical energy but need a closed-loop feedback system for precise motion, stepper motors excel in precise positioning and speed control. This makes them ideal for applications in automation systems, robotics, CNC machines, and 3D printers, where accurate and repeatable motion is essential.

Working Principle of Stepper Motor

A stepper motor operates in discrete intervals or steps, with each step corresponding to a command pulse received from the controller.
Every time a pulse is applied, the motor’s shaft rotates by a fixed and known angle.
By supplying a specific number of pulses, the motor can be made to rotate through a precise angle, making it highly suitable for open-loop position control systems where no feedback from the output shaft is required.
Stepper motors are available in a wide range of torque and power ratings.
The torque of these motors can vary from as low as 1 µN·m (in miniature motors used in devices like wristwatches of about 3 mm diameter) to as high as 40 N·m (in motors of around 15 cm diameter used in machine tools and industrial applications).
The power output of stepper motors typically ranges from 1 watt up to 2500 watts, depending on their size and application.

Step Angle in Stepper Motor

The step angle (β) is defined as the angle through which the motor shaft rotates for each input pulse. In simple terms, it represents how much the rotor moves per command pulse received by the controller.
A smaller step angle means the motor will take more steps per revolution, resulting in higher accuracy and better resolution in positioning applications.
Step angles in stepper motors can vary from as small as 0.72° to as large as 90°, but the most common values are 1.8°, 2.5°, 7.5°, and 15°.

Step Angle Formula

The value of step angle can be determined using the relationship between the number of stator poles (Ns) and rotor poles (Nr), or in terms of the number of stator phases (m) and rotor teeth (Nr).

$β = \frac{N_{s} - N_{r}}{N_{s} \times N_{r}} \times 360^{\circ}$

$β = \frac{360^{\circ}}{m \times N_{r}}$

$β = \frac{(8 - 6) \times 360}{8 \times 6} = 15^{\circ}$

$Resolution of Stepper Motor$

The resolution of a stepper motor indicates how many steps are required for one complete revolution of the rotor shaft.

$Resolution = \frac{360^{\circ}}{β}$

A smaller step angle gives a higher resolution, which means more precise control of position and motion.

High-Speed Operation (Slewing) of Stepper Motor

Stepper motors have an extraordinary ability to operate at very high stepping rates, sometimes up to 20,000 steps per second, while maintaining synchronism with the input pulses. At such high rates, the motion of the shaft appears continuous—a condition known as slewing.
During slewing, the motor often emits a high-pitched whine, whose frequency equals the pulse rate (f). The speed of the stepper motor can be expressed as:

$n = \frac{β \times f}{360} rps$

where
n = rotor speed (revolutions per second)
β = step angle (in degrees)
f = pulse frequency (pulses per second or pps)

If the stepping rate increases too rapidly, the motor may lose synchronism and stop suddenly. Similarly, when operating in the slewing range, if the command pulses are stopped abruptly instead of being gradually reduced, the motor may stall.

Stalling Characteristics of Stepper Motor

Unlike other motors, stepper motors can safely operate while stalled. They are designed to hold the rotor in a fixed position even when rated current flows through the stator windings. This means that stalling does not damage a stepper motor, whereas in other types of motors, stalling can cause a collapse of back EMF and lead to excessive current flow resulting in burnout.

Applications of Stepper Motor

Stepper motors are widely used in applications that require precise motion control, accurate positioning, and repeatable movement. Their ability to convert electrical pulses into exact mechanical movements makes them ideal for automation, robotics, and computer-based control systems.

1. Industrial and Engineering Applications

Computer peripherals such as printers, plotters, and disk drives
Textile industry for controlling yarn tension and weaving operations
Semiconductor and IC fabrication for precise wafer positioning
CNC machines and numerically controlled tools for accurate cutting, drilling, and shaping
Process control systems where stepwise operation and accurate movement are required

2. Office and Peripheral Devices

Typewriters and line printers
Tape drives and floppy disk drives
X–Y plotters and laser engravers
In these systems, position control can be easily achieved by simply counting the number of input pulses sent to the motor, thereby eliminating the need for costly position sensors or feedback mechanisms.

3. Robotics and Automation

Stepper motors are an essential component in robotic arms, pick-and-place machines, and automated production systems, where precise angular movement and repeatability are critical.

4. Commercial, Military, and Medical Applications

Medical equipment such as infusion pumps, imaging devices, and robotic surgical tools
Military systems requiring precise control of radar or weapon positioning
Commercial equipment performing tasks like mixing, cutting, blending, metering, striking, and purging

They are also widely used in food packaging industries, consumer products, and even in the production of animated and science fiction movies, where controlled motion is required for special effects and automation.

Types of Stepper Motors

There are several types of stepper motors used in industrial and automation applications. Based on their construction and working principle, stepper motors can be broadly classified into three main categories:

Variable Reluctance Stepper Motor (VR Stepper Motor)

The variable reluctance stepper motor has wound stator poles, while the rotor poles are made of a soft ferromagnetic material (non-permanent magnet).
The reluctance (magnetic resistance) of the magnetic circuit formed between the rotor and stator varies with the angular position of the rotor — hence the name variable reluctance.
It can be designed as a single-stack or multi-stack type. The multi-stack design offers smaller step angles and higher precision.
The direction of rotation in a VR stepper motor is independent of the polarity of the stator current.
These motors are simple in construction, cost-effective, and ideal for low-torque, high-speed applications.

Permanent Magnet Stepper Motor (PM Stepper Motor)

In a permanent magnet stepper motor, the stator poles are wound, and the rotor is permanently magnetized. The rotor usually has a cylindrical shape with alternating north and south poles.
The direction of rotation depends on the polarity of the stator current.
These motors provide better torque characteristics than variable reluctance types.
They are widely used in low-speed, high-torque applications, such as printers, medical instruments, and small positioning systems.

Hybrid Stepper Motor

The hybrid stepper motor combines the features of both variable reluctance and permanent magnet types. It has wound stator poles and a permanently magnetized rotor.
This combination improves torque, resolution, and speed characteristics.
Hybrid stepper motors are designed for applications where very small step angles such as 1.8°, 2.5°, or 0.9° are required.
They are the most commonly used type in CNC machines, robotics, and automation systems due to their high precision and reliability.

Stepper Motor Important MCQs and Numerical for SSC JE Electrical

Prepared for SSC JE / RRB JE and other electrical competitive exams. Each question shows the correct option and a step-by-step explanation to help revision.

1. For a stepper motor, which torque has the highest numerical value?

Options: Pull-out torque / Pull-in torque / Detent torque / Holding torque

Answer: Pull-out torque (a)

Explanation:

Pull-out torque (also called maximum torque or breakdown torque) is the highest torque the stepper motor can develop when running at a given speed before it loses synchronism.
Pull-in torque is the maximum torque at start-up (synchronous engagement) and is smaller than pull-out torque.
Holding torque is the static torque when the motor is energized but not moving — typically less than pull-out torque.
Detent torque is the torque due to the rotor's permanent-magnet detent and is the smallest among listed. Therefore the highest numerical value is the pull-out torque.

2. The step angle of the stepper motor is 2.5°. If the stepping frequency is 3600 pulses per second (pps), then the shaft speed will be:

Options: 3600 rps / 144 rps / 25 rps / 2.5 rps
Answer: 25 rps (c)
Explanation (step calculation):
Use the relation between step angle (β), stepping frequency (f) and shaft speed (n):
n (rps) = (β × f) / 360
Substitute β = 2.5° and f = 3600 pps:
n = (2.5 × 3600) / 360 = 9000 / 360 = 25 rps
So the correct speed is 25 rps.

3. A 3-stack (three-phase), variable reluctance step motor has 20 poles on each rotor and stator stack. The step angle of this step motor is:

Options: 6° / 30° / 18° / 90°
Answer: 6° (a)
Explanation:
For a multi-stack (m-stack) VR motor where each stack has the same number of rotor teeth/poles, the step angle β is given by:
β = 360° / (m × N_r)
Here m = 3 (three stacks/phases), N_r = 20 rotor poles per stack. So
β = 360° / (3 × 20) = 360° / 60 = 6°

4. Which type of motor is most suitable for a computer printer device?

Options: Reluctance motor / Hysteresis motor / Shaded pole motor / Stepper motor
Answer: Stepper motor (d)
Explanation: Computer printers require precise open-loop position control (accurate incremental positioning of paper and print heads) and easy digital interface. Stepper motors provide precise discrete steps, good holding torque and simple drive electronics — making them ideal for printers. Therefore the best choice is stepper motor.

5. The stepper motor has six-phase winding on its stator and has 12 teeth on the rotor. The stepping angle is:

Options: 5° / 10° / 2.5° / 30°
Answer: 5° (a)
Explanation:
For a single-stack motor where the stator has m phases (or effectively m stator poles excited sequentially) and the rotor has N_r teeth, the step angle β can be found using:
β = 360° / (m × N_r)
Here m = 6 (six-phase), N_r = 12. So
β = 360° / (6 × 12) = 360° / 72 = 5°

6. A 4-phase, 4-pole permanent magnet stepper motor has a step angle of:

Options: 90° / 45° / 22.5° / 30°
Answer: 22.5° (c)
Explanation:
For many permanent magnet (PM) stepper designs the generalized formula is
β = 360° / (m × N_r)
Here, "4-phase, 4-pole" commonly means m = 4 phases and the rotor has N_r = 2 pole-pairs (4 poles). However the conventional interpretation for simple step calculation is:
If rotor has 4 poles, N_r = 4, m = 4 → β = 360° / (4×4) = 360° / 16 = 22.5°
So correct answer is 22.5°.

7. The step angle for a 3-stack, 15-tooth variable reluctance stepper motor is:

Options: 8° / 4° / 10.5° / 7.5°
Answer: 8° (a)
Explanation:
Using β = 360° / (m × N_r) for multi-stack VR motors. Here m = 3 and N_r = 15:
β = 360° / (3 × 15) = 360° / 45 = 8°

8. The step angle for a 3-phase, 24 pole permanent magnet stepper motor is:

Options: 2° / 5° / 10.5° / 15°
Answer: 5° (b)
Explanation: Use β = 360° / (m × N_r) where m = 3 phases and N_r = 24 poles.
β = 360° / (3 × 24) = 360° / 72 = 5°

10. A permanent magnet stepper motor with 8 poles in the stator and 6 poles in the rotor will have a step angle of:

Options: 7.5° / 15° / 10.5° / 60°
Answer: 15° (b)
Explanation:

For a simple single-stack PM stepper, a useful relation for the step angle β is:
β = |N_s − N_r| × 360° / (N_s × N_r)

Where N_s = number of stator poles (or effective excited positions) and N_r = number of rotor poles (teeth).
Substitute N_s = 8 and N_r = 6:

β = (8 − 6) × 360° / (8 × 6) = 2 × 360° / 48 = 720° / 48 = 15°