A stepper motor, also known as step motor or stepping motor, is a brushless DC electric motor that converts digital pulses into mechanical shaft rotation incrementally (around 1.8 degrees for a typical stepper) rather than continuously rotating as DC motors do. The stepper motor works on the principle of electromagnetic induction, where a rotating magnetic field is used to turn the motor's shaft incrementally. It is one of the most commonly used motion control components in industrial systems due to its simplicity of operation, accuracy, and smooth movement. 

In this article, we will learn about the Stepper motor, including it's working principle, construction, different types, advantages and applications  to gain a deep understanding of this versatile device. Also we will learn the difference between stepper motor and servo motor. The information in this article helps you extensively in your SSC JE Electrical, RRB JE and GATE Electrical preparation journey.

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What is a Stepper Motor?

A stepper motor is essentially a synchronous motor that divides a full rotation into a number of equal steps (around 200 steps). It is an electromechanical device which converts digital pulses into mechanical shaft rotation. 

Some key point about stepper motor include:

• It has a rotor with permanent magnets and a stator that contains winding. The winding is divided into phases (often two or more).
• Each step is caused by the energizing of the motor's stator winding in a particular sequence. The rotor aligns itself to the nearest stator pole.
• Step angle is defined as the angular rotation produced per pulse applied to the motor. A typical step angle is 1.8 degrees.
• Speed and distance covered by the motor are directly proportional to the rate at which pulses are applied.
• They are available in different NEMA frame sizes denoted by their frame diameter in inches like NEMA 17, 23, 24, etc.
• Stepper motors can hold their position without power, which makes them suitable for open-loop control applications like CNC machines.

Stepper Motor Working Principle 

The basic working principle of a stepper motor can be explained as follows:

• It consists of a stator (stationary outer cylinder) and a rotor (inner cylinder attached to the shaft).
• The stator contains electromagnets which are organized into groups known as "phases". The most common types have two or four phases.
• The rotor contains permanent magnets or pole pieces. It is attached to the motor shaft which extends outside for connection to a device.
• When one phase is energized, it rotates the rotor slightly in one direction. As soon as the first phase is energized, the second one is powered to continue the motion.
• Repeating the process of energizing the phases in sequence causes the rotor to rotate in steps.
• By controlling the sequence of energizing the electromagnets, the direction of rotation can be controlled.
• Varying the number of steps per pulse gives finer position resolution. Higher number of steps per pulse means smaller angular rotation per step.

Stepper Motor Diagram

A stepper motor converts digital pulses into mechanical rotation incrementally by energizing its electromagnetic stator coils in a sequence. This makes it suitable for open-loop precision motion control applications.

Fig-  Stepper Motor Diagram

Stepper Motor Step Angle 

Step angle is defined as the angular rotation produced by the stepper motor per pulse given as input. A standard step angle varies between 1.8° to 0.9°. Smaller the step angle, better the positional accuracy and higher the number of steps per revolution.

Common step angles of 1.8° give a resolution of 200 steps/revolution while 0.9° gives 400 steps/revolution. Microstepping techniques can produce step angles less than 1° for high precision applications.

Types of Stepper Motor

There are mainly three types of stepper motors based on the rotor arrangements. They are as follows:

Stepper motor types:

Fig- Block Diagram Of Types Of Stepper Motor

Variable Reluctance Stepper Motor

In variable reluctance (VR) stepper motors, the rotor contains soft iron teeth while the stator contains electromagnets only. It works on the principle of minimum magnetic reluctance. When one electromagnet is powered, the rotor teeth align to reduce reluctance. When the next electromagnet is powered and the first one turned off, the rotor rotates slightly to align again, thus producing step motion. VR stepper motors are less efficient but simpler in construction compared to permanent magnet types.

Fig- Stator of Stepper Motor

• Construction: VR stepper motors consist of a rotor with teeth and stator windings. The rotor aligns itself with the stator poles when energized, causing rotation.
• Advantages:
  • Simple construction leads to low manufacturing costs.
  • High torque output compared to size.
  • Suitable for applications requiring moderate precision.

Permanent Magnet Rotor

Permanent magnet (PM) stepper motors use strong permanent magnets in the rotor. The stator contains electromagnets only. The rotor aligns itself to the nearest stator pole depending on which electromagnet is powered. It generates higher torque compared to variable reluctance type. PM steppers are more efficient and produce precise stepping but the rotor magnets increase its cost.

• Construction: PM stepper motors feature a rotor with permanent magnets and stator windings. The interaction between the magnetic fields generates rotational motion.
• Advantages:
  • Higher efficiency and torque compared to VR motors.
  • Precise positioning and control.
  • Lower power consumption.

Hybrid Stepper Motor(Combination of VR and PM type)

Hybrid stepper motors combine features of both PM and VR motors to achieve high performance in small form factor.
The rotor contains both soft-iron teeth and permanent magnet segments. This utilizes the advantages of high torque due to magnets as well as wide polar arc due to salient rotor poles. Hybrid steppers are most commonly used due to their efficiency, precision, and reasonable cost.

Construction: Hybrid stepper motors integrate elements from both VR and PM designs, featuring a rotor with permanent magnets and toothed stator poles.

Advantages:

• Enhanced torque output and precision.
• Reduced step size for smoother motion.
• Wide range of applications, from CNC machines to 3D printers.

Stepper Motor Driver Types

Stepper motor drivers come in various types, each designed with specific features to suit different applications. One of the most important characteristics of a stepper driver is its input interface. The most commonly used input types include:

• Step/Direction Interface: In this method, a pulse on the Step pin causes the driver to move the motor one step. The Direction pin determines whether the step is in the clockwise or counterclockwise direction. This is the most common and intuitive control interface.
   
• Phase/Enable Interface: Here, each phase (or winding) of the motor is controlled separately. The Phase input sets the current direction in the winding, while the Enable input determines whether the winding is energized or not.
   
• PWM Control: Pulse Width Modulation (PWM) is used to directly manage the gate signals of both the high-side and low-side FETs (field-effect transistors). This method allows for very fine-grained and efficient control over motor operation.

Fig-Stepper Motor Control Drivers

Voltage vs. Current Control

Another critical distinction among stepper motor drivers is whether they control voltage or current:

• Voltage-Controlled Drivers: These drivers regulate only the voltage applied across the motor windings. The resulting motor behavior—including torque and stepping speed—largely depends on the inherent characteristics of the motor and the mechanical load. This method offers less precision, especially in variable load conditions.
   
• Current-Controlled Drivers: These are more advanced and commonly used in modern applications. They actively monitor and control the current flowing through each motor coil. By regulating current, the driver can maintain consistent torque output and provide better dynamic response, even under changing load conditions. This leads to improved efficiency, precision, and system stability.

Stepper Motor Step Modes

Stepper motors operate in various step modes, primarily defined by the driver design. The most common modes include:

• Full Step: In this mode, both motor windings are energized in sequence, producing 200 steps per revolution (1.8° per step). It offers maximum torque and a straightforward control approach.
   
• Half Step: Alternating between one and two energized windings, this mode doubles the resolution to 400 steps per revolution (0.9° per step). It provides smoother motion but slightly reduced torque (about 30% less than full-step).
   
• Microstep: Advanced drivers divide each full step into many finer steps by precisely controlling winding currents. OMEGA microstepping drives can achieve up to 256 microsteps per full step, offering smoother motion and ultra-fine positioning (as precise as 0.007° per step), though with reduced torque.
• Linear Motion Conversion: Stepper motors can produce linear motion using lead screws or worm gears. The screw's lead (e.g., 1 inch/rev) determines the linear travel per motor revolution. With 200 full steps per revolution, a 1-inch lead yields a resolution of 0.005 inches per step. Microstepping can significantly enhance this resolution.

Series vs. Parallel Motor Connections

• Series Connection: Higher inductance yields greater torque at low speeds but limits high-speed performance.
   
• Parallel Connection: Lower inductance supports higher speed operation with improved torque, but may draw more current.

Advantages of Stepper Motor

Some key advantages of using stepper motors include:

• Precise positioning over full or part rotation with high resolution.
• Excellent speed and position control in open loop systems without feedback devices.
• Ability to start, stop and operate at constant angular velocity with high degree of precision.
• Hold their position efficiently without power due to highly detented nature.
• Easy control using pulses which makes programming simpler.
• Less maintenance required as there are no brushes.
• Suitable for applications like CNC machines where position must be accurately controlled.

Disadvantages of Stepper Motor

Some disadvantages include:

• produce less torque compared to brushed DC motors of similar size.
• Vibration and resonance are likely if not designed properly.
• Require driver circuit to function properly which increases cost.
• Feedback devices may be required for applications requiring high level of precision.
• Speed variations at higher output speeds due to limitations in response time.

Applications of Stepper Motor

Some common uses of stepper motors include:

• CNC machines like routers, lathes, mills, 3D printers.
• Peripheral equipment like printers, scanners, plotters, tape drives, and CD/DVD drives.
• Industrial robotics, material handling, wafer handling equipment.
• Medical equipment like blood analyzers, imaging equipment.
• Automated production machinery like labeling machines.
• Scientific instruments like spectroscopy equipment.
• Desktop CNC machines, laser cutters, laser engravers.
• CCTV camera systems - for focus/zoom lens control.
• Radio-controlled vehicles, quadcopters, cranes for positioning.

Difference between Stepper Motor and Servo Motor

While stepper motors and servo motors share some similarities, they exhibit distinct differences in terms of performance and control:

Conclusion

Stepper motors are versatile actuators that excel at applications requiring precise positioning control. Their ability to divide a rotation into increments, hold position without feedback and ease of operation makes them well suited for open-loop motion control. With the advancements in motor design and driver technologies, their performance is ever improving while costs are declining. Stepper motors will certainly continue dominating precision motion applications like 3D printing, robots, CNC machines etc. 

With their unique working principles, diverse types, and wide-ranging applications, stepper motors continue to drive innovation across various industries. Understanding the intricacies of stepper motors is essential for engineers and enthusiasts alike, as they navigate the complexities of modern automation and robotics.

This article summarises all the information related to Stepper Motor, which helps in propelling your preparation for various AE/JE and ESE examinations. You can visit the Testbook app to keep yourself updated with all the exam-oriented information related to the upcoming examination, like SSC JE, GATE, ESE, RRB JE, and state AE/JE Exams.