Stepper Motor

Introduction

The motor which moves in discrete steps is known as the stepper motor. A stepper motor is an electromechanical device it converts electrical power into mechanical power. Also, it is a brushless, synchronous electric motor that can divide a full rotation into an expansive number of steps. Stepper motors operate differently from DC brush motors, which rotate when voltage is applied to their terminals. Stepper motors, on the other hand, effectively have multiple toothed electromagnets arranged around a central gear-shaped piece of iron. The electromagnets are energized by an external control circuit, for example, a microcontroller. The motor’s position can be controlled accurately without any feedback mechanism, as long as the motor is carefully sized to the application. Stepper motors are similar to switched reluctance motors. The stepper motor uses the theory of operation for magnets to make the motor shaft turn a precise distance when a pulse of electricity is provided. The stator has eight poles, and the rotor has six poles. The rotor will require 24 pulses of electricity to move the 24 steps to make one complete revolution. Another way to say this is that the rotor will move precisely 15° for each pulse of electricity that the motor receives. The stepper motors are generally two-phase motors like unipolar otherwise bipolar. For each phase in a unipolar motor, there are two windings. Precise positioning and repeatability of movement since good stepper motors have an accuracy of 3 – 5% of a step and this error is noncumulative from one step to the next. Very reliable since there are no contact brushes in the motor. Therefore the life of the motor is simply dependant on the life of the bearing.

Construction

The construction of a stepper motor is fairly related to a DC motor. It includes a permanent magnet like Rotor which is in the middle & it will turn once force acts on it. This rotor is enclosed through a no. of the stator which is wound through a magnetic coil all over it. The stator is arranged near to rotor so that magnetic fields within the stators can control the movement of the rotor. The stepper motor can be controlled by energizing every stator one by one. So the stator will magnetize & works like an electromagnetic pole which uses repulsive energy on the rotor to move forward. The stator’s alternative magnetizing as well as demagnetizing will shift the rotor gradually &allows it to turn through great control.

Working Principle

The stepper motor working principle is Electro-Magnetism. It includes a rotor which is made with a permanent magnet whereas a stator is with electromagnets. Once the supply is provided to the winding of the stator then the magnetic field will be developed within the stator. Now rotor in the motor will start to move with the rotating magnetic field of the stator. So this is the fundamental working principle of this motor. In Stepper motor, there is a soft iron that is enclosed through the electromagnetic stators. The poles of the stator as well as the rotor don’t depend on the kind of stepper. Once the stators of this motor are energized then the rotor will rotate to line up itself with the stator otherwise turns to have the least gap through the stator. In this way, the stators are activated in a series to revolve the stepper motor. To make the motor shaft turn, first one electromagnet is given power, which makes the gear’s teeth magnetically attracted to the electromagnet’s teeth. At the point when the gear’s teeth are thus aligned to the first electromagnet, they are slightly offset from the next electromagnet. So when the next electromagnet is turned ON and the first is turned OFF, the gear rotates slightly to align with the next one and from there the process is repeated. Each of those slight rotations is called a step, with an integer number of steps making a full rotation. In that way, the motor can be turned by a precise. Stepper motor doesn’t rotate continuously, they rotate in steps. There are 4 coils with a 90° angle between each other fixed on the stator. The stepper motor connections are determined by the way the coils are interconnected. In a stepper motor, the coils are not connected. The motor has a 90° rotation step with the coils being energized in a cyclic order, determining the shaft rotation direction.

Types of Stepper Motor

Permanent magnet stepper
Hybrid synchronous stepper
Variable reluctance stepper

Permanent Magnet Stepper Motor

Permanent magnet motors use a permanent magnet (PM) in the rotor and operate on the attraction or repulsion between the rotor PM and the stator electromagnets. This is the most common type of stepper motor as compared with different types of stepper motors available in the market. This motor includes permanent magnets in the construction of the motor. This kind of motor is also known as tin-can/can-stack motor. The main benefit of this stepper motor is less manufacturing cost. For every revolution, it has 48-24 steps.

Hybrid Synchronous Stepper Motor

Hybrid stepper motors are named because they use a combination of permanent magnet (PM) and variable reluctance (VR) techniques to achieve maximum power in small package sizes. The most popular type of motor is the hybrid stepper motor because it gives a good performance as compared with a permanent magnet rotor in terms of speed, step resolution, and holding torque. But, this type of stepper motor is expensive as compared with permanent magnet stepper motors. This motor combines the features of both the permanent magnet and variable reluctance stepper motors. These motors are used where less stepping angle is required like 1.5, 1.8 & 2.5 degrees.

Variable Reluctance Stepper Motor

Variable reluctance (VR) motors have a plain iron rotor and operate based on the principle that minimum reluctance occurs with minimum gap, hence the rotor points are attracted toward the stator magnet poles. The stepper motor like variable reluctance is the basic type of motor and it is used for the past many years. As the name suggests, the rotor’s angular position mainly depends on the magnetic circuit’s reluctance that can be formed among the teeth of the stator as well as a rotor.

Advantages

Simple construction
Ruggedness
Can work in an open-loop control system
Maintenance is low
It works in any situation
Reliability is high
The rotation angle of the motor is proportional to the input pulse.
The motor has full torque at standstill.
Excellent response to starting, stopping, and reversing.
The motor’s response to digital input pulses provides open-loop control, making the motor simpler and less costly to control.
It is possible to achieve very low-speed synchronous rotation with a load that is directly coupled to the shaft.
A wide range of rotational speeds can be realized as the speed is proportional to the frequency of the input pulses.

Disadvantages

As compared with DC motors, it uses more current
Efficiency is low
The Torque of a motor will declines fast with speed
Accuracy is low
Feedback is not used for specifying potential missed steps
Small Torque toward Inertia Ratio
Extremely Noisy
If the motor is not controlled properly then resonances can occur
Operation of this motor is not easy at very high speeds.
The dedicated control circuit is necessary

Applications

Stepper motors are used inside medical scanners, samplers, and also found inside digital dental photography, fluid pumps, respirators, and blood analysis machinery.
Stepper motors are used in automotive gauges and machine tooling automated production equipment.
new surveillance products for the security industry.
Stepper motors are used inside medical scanners, samplers, and also found inside digital dental photography, fluid pumps, respirators, and blood analysis machinery.
Stepper motors in cameras for automatic digital camera focus and zoom functions.

Stepper Motor Working Principle advantages, disadvantages and applications