Working Principle of Stepper Motor
Working Principle of Stepper Motor – A stepper motor is a special type of synchronous motor, designed to rotate through a specific number of degrees for each electrical pulse it receives from its control unit. Typical step angles are 7.5° or 15°. This motor can rotate in both directions, move in precise angular steps, maintain a holding torque at zero speed, and be controlled digitally.
It moves in accurate angular increments, referred to as steps, in response to the application of digital pulses to its drive circuit. These motors are usually manufactured with a specific number of steps per revolution. Depending on the electrical power supply, stepper motors can be classified into:
Unipolar
In this type, the coils are always energised in the same direction using a single power source. Hence, it is called unipolar.
Bipolar
Here, the coils are energised in alternating directions using two power sources. This results in the creation of magnetic poles (North and South) that alternate, hence the term bipolar.
Unlike ordinary DC motors, stepper motors are brushless and can divide a full 360° rotation into a large number of steps — for example, 200 steps per revolution.
Operating Principle of Stepper Motor
Stepper motors function differently from conventional DC motors. While a DC motor rotates continuously when voltage is applied, a stepper motor has multiple “toothed” electromagnets arranged around a central gear-shaped iron rotor.
The electromagnets are powered by an external controller (such as a microcontroller). To rotate the motor shaft:
- One electromagnet is energised, attracting the nearest rotor teeth.
- When aligned, the next electromagnet is powered and the first one is turned off.
- This causes the rotor to shift slightly to align with the next electromagnet.
- This process repeats, resulting in precise rotational steps.
Each such movement is called a step, and a defined number of steps results in a full rotation.
Advantages of Stepper Motors
- Cost-effective solution
- Can operate in open-loop control (no feedback needed)
- Excellent holding torque (eliminates the need for brakes/clutches)
- High torque at low speeds
- Low maintenance due to brushless design
- Rugged and suitable for harsh environments
- Ideal for precise positioning
- No need for tuning like in servo systems
Disadvantages of Stepper Motors
In comparison to servo motors, stepper motors have the following drawbacks:
- Uneven motion at low speeds unless micro-stepping is used
- Constant power consumption, irrespective of the load
- Limited range of available sizes
- Can be noisy
- Torque decreases with increase in speed (may require an oversized motor)
- Can lose position or stall in the absence of a feedback control loop
Applications of Stepper Motors
- Cruise control systems in vehicles
- Automatic air vents
- Headlamp beam leveling systems
- Printers and plotters
- Industrial automation equipment
- Automotive instrument clusters
- Office machines and equipment
- Hard disk and optical drives
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