STEPPER MOTOR

Stepper motor:-

A stepper motor can be considered as a digital electro-mechanical device where each electrical pulse input results in a movement of shaft by a discrete angle called as step angle of the motor.

                                

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Stepper motor is a brushless Dc electric motor that divides a full rotation into a number of equal steps.

As we all know, industrial motors are used to convert electrical energy to mechanical energy but they cant be used for precision positioning of an object or precision control of speed without using closed-loop feedback, that is where stepper motor comes into picture.Stepping motors are ideally suitable for situations where either precise positioning or precise speed control or both are required in the automation system.

A steppers main purpose is to rotate through a precise angler and remains at standstill condition.The speed and torque of the rotation are secondary concerns in stepper motor.Stepper motor rotates through the exact angle and stops. The angle through which the motor turns or shaft moves for each pulse is known as the step angle, expressed in degree, common step angles include 30⁰ ,2⁰,2.5⁰ , etc. At low command pulse rate, the motor moves in steps but when the pulse rate is made sufficiently high because of inertia the rotor moves smoothly, as in case of dc motors.As motor speed is proportional to the rate of command pulses, it can be used for speed control.thus motor is ideally suitable to open loop position and speed control i.e there is no need of feedback sensor.

Let K= step angle(the angle through which the motor shaft rotates for each command pulse)

       Then K= 360/ (no of stator phases * no of rotor teeth)

The stepper motor works on the simple basic principle of magnetic interaction that takes place between the rotor and stator, which makes the rotor move.The rotor has no windings and of salient pole type whereas stator has windings on it.The rotor may or may not have permanent magnets depending upon types of the stepper motor.There is a one-one relationship between an input pulse and step movement of shaft i.e each pulse input actuates one-step movement of the shaft.

Stepper motors are often constructed with a 3-phase stator winding similar to the conventional electrical machines.The rotors are either of the permanent magnet type or the variable reluctance type.Stepper motors are actuated by means of an external drive logic circuit.Series of the pulse voltage is applied to the input of drive circuit and as result it supplies suitable currents to stator windings of the motor to make the axis of magnetic field step around in synchronism with the input pulses.

Stepper motors can be broadly classified into three type:

1.   Variable reluctance stepper motor

2.   Permanent magnet stepper motor

3.   Hybrid type stepper motor

Variable reluctance stepper motor:-

It works on the principle that when a magnetic material placed in a magnetic field experiences a force to align it in minimum reluctance path.

In a variable reluctance motor, excitation of the stator phase gives rise to a torque in a direction which minimizes the magnetic circuit reluctance.The reluctance torque depends upon the square of the phase current and its direction is independent of the polarity of the phase current .this type of motor can be single-stack or multi-stack motor.

Permanent magnet stepper motor:

Permanent magnet stepper motors are widely used in accurate positioning/controlling application due to their small step size.

The stator is of salient pole structure, while rotor shaft consists of a permanent magnet with toothed end caps. The stator poles are provided with windings that are energized sequentially to produce the stepping action.

The major advantage of permanent magnet stepper motor is that it doesn’t require any external exciting current, hence it has low power requirement but possesses high detent torque as compared to variable reluctance stepper motor.this type of motor has high inertia and hence lower acceleration.the step size of this type of motor is relatively large ranging from 30 to 90 as it is difficult to manufacture small permanent magnet rotor with large no of poles.

Hybrid Stepper motor:-

It combines the features of the variable reluctance and permanent magnet stepper motor.the direction of its torque depends upon the polarity of stator current.

It has wound stator poles and permanently-magnetized rotor poles.it is best suitable for small angle steps are required, like 1.8 degree,3.1 degrees etc.This type of motor achieves small step sizes easily and with a simpler magnet structure.As compared to variable reluctance motor hybrid motor requires less excitation to achieve a given torque.

Applications:-

Stepper motors are widely used in computer peripherals and office machines.In printer,graph plotter ,floppy/hard disk drives stepper motors are used.stepper motors are also used in Automobiles for closing and opening of glass windows,door latching and wiper movement.

The most common application of stepper motor is a wristwatch, synchronize clocks in railway platforms.

Advantages :-

1.   Highly precise.

2.   Steeper motor can produce high output torques at lower angular velocities.

3.   The angular displacement can b precisely controlled without any feedback arrangement.

4.   No sensor is needed for the position and speed sensing.