STEPPER MOTOR

Hello friends , welcome to ELECTRICAL ENCYCLOPEDIA. In this article we will study about the varioous aspects of STEPPER MOTOR.

Stepper motor is a special purpose machine , feedback motor in which the rotor of the motor rotates in discrete steps. The angular rotation is determined by the number of pulses fed to the control circuit.
Each input pulse produces the one step of the angular movement. The total rotation of the rotor comprises of various such steps, therefore it is called Stepper Motor.


There are three most popular types of rotor arrangements:

1. Variable Reluctance (VR) type.

2. Permanent Magnet (PM) type.

3. Hybrid type , a combination of VR and PM.

Here we will study only about Variable Reluctance type. Before moving into it , we will focus on a new term called STEP ANGLE.

STEP ANGLE - It is the angle by which the rotor rotates when an input pulse is applied to stator
Resolution can be defined as number of steps it makes in one revolution of the rotor
Smaller the step angle , higher is the resolution of the positioning of the motor.

VARIABLE RELUCTANCE (VR) STEPPER MOTOR

The principle operation of such motor is based on the property that flux lines always travels/moves through the low reluctance path. The stator and rotor are aligned in such a way that magnetic reluctance is minimum. Variable reluctance motor can be single stack or multi-stack type.


SINGLE STACK VARIABLE RELUCTANCE MOTOR

A variable reluctance motor has a salient pole type stator and winding are placed over it. Stator coils are excited  from a dc source with the help of semiconductor switches , a magnetic field is produced. Due to this the rotor occupies the position which represents the path of minimum reluctance. That is, rotor axis aligns itself to the stator field axis.

STEPPER MOTOR

As shown in figure above, consider a variable reluctance motor with 4 poles on stator and 2 poles on rotor. Four stator poles A,B,C and D are connected to the dc source with the help of semiconductor switches. These switches are switched in sequence A,B,C,D,A.....

When the winding A is excited , the rotor aligns with the axis of phase A. The rotor gets stable there and cannot move until A is de-energised. Next phase B is excited and A is disconnected . The rotor moves through 90 degree to align with the axis of pole B and in such way this sequence goes on.
The rotor completes one revolution in the 4 steps and rotor moves a step of 90 degree after each pole transistion.
Direction of the rotor can be reversed by changing the sequence of the switching i.e. A,D,C,B,A... . Direction does not depend upon the magnitude of the current.

The step angle and resolution of the Stepper motor depends upon the number of poles on the stator and rotor.

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