## 26/12/2021

### Permanent Magnet Stepper Motor

In this article, Single Phase ON operation, Two Phase ON operation, Half Step Operation and Features of Permanent magnet ( PM ) Stepper Motor is given.

# Permanent magnet ( PM ) Stepper Motor

• The stator of the PM stepper motor consists of two project poles which is wound by separate windings.
• The rotor is made of permanent magnet.
• The stator is magnetized north pole and south pole alternately according to supply given to the stator winding.

Let us consider that the stator has 4 poles and rotor has 2 poles

Step angle θ = ( NS – Nr ) × 3600 / NSNr

= ( 4 – 2 ) × 3600 / 4 × 2

= 900

## Permanent magnet ( PM ) Stepper Motor: Single Phase ON Operation

Mode 1

• The operation of the permanent magnet stepper motor is explained here.
• The stator winding X is energized such that positive current passes through X ( + ) and negative current passes through X’ ( - ).
• The stator winding Y is kept deenergized during that time.
• The rotor magnetic poles attracts towards stator magnetic poles as shown in the Figure.

Mode 2

• The stator winding X is deenergized and stator winding Y is energized such that positive current passes through Y(+) and negative current passes through Y’(-).
• The rotor try to align its position along X axis and rotates clockwise through 90 degree because there is no current passes through XX’ winding.

Mode 3

• Again, the stator winding X is energized in the negative direction such that X (-) and X’( + ).
• The stator winding Y is kept de-energized during that time.
• The rotor magnetic poles try to align its position along Y axis and rotates through angle 90 degree.
• The rotor rotation is 180 degree from its initial position.

Mode 4

• The stator winding X is deenergized and stator winding Y is energized such that positive current passes through Y(-) and negative current passes through Y’(+).
• The rotor try to align its position along X axis and rotates clockwise through 90 degree because there is no current passes through XX’ winding.
• The rotor rotation is 270 degree from its initial position.
Operation of PM Stepper Motor : Single Phase ON

 Step Energization of winding Step Angle X winding X’ winding Y    winding Y’   winding 1 + - Not energized Not energized 00 2 Not energized Not energized + - 900 3 - + Not energized Not energized 1800 4 Not energized Not energized - + 2700 5 + - Not energized Not energized 00  OR 3600

## Permanent magnet ( PM ) Stepper Motor: Two Phase ON Operation

• When two phase are energized simultaneously, the operation of permanent magnet is explained here.
• The resultant magnetic field is combination of two magnetic field produced by X winding and Y winding.
• The rotor rotates through 45 degree.

Mode 1

• Both windings are energized in the same direction X ( + ) and Y ( + ), the rotor rotates through 45 degree.

Mode 2

• One winding is energized in the positive direction Y ( + ) and other winding is energized in the negative direction X ( - ), the rotor rotates through 90 degree further, resulting 135 degree from its initial position.

Mode 3

• Both winding is energized in the negative direction X ( - ) and  Y ( - ) resulting rotor rotates through 90 degree further resulting rotation of rotor 225 degree from its initial position.

Mode 4

• One winding is energized in the negative direction  Y( - ) and other winding is energized in the positive direction X ( + ), the rotor rotates through 90 degree further, resulting 315 degree from its initial position.

Operation of PM Stepper Motor : Two Phase ON

 Step Energization of winding Step Angle X winding X’ winding Y    winding Y’   winding 1 + - + - 450 2 - + + - 1350 3 - + - + 2250 4 + - + - 3150

## Permanent magnet ( PM ) Stepper Motor: Half Step Operation

• In the half step operation, once – single phase ON operation is done then two phase ON operation is done.
• The rotor rotates at 45 degree ( half that of full step on ). The half step operation is explained in the table.
• Higher the resolution as step size decreases

Operation of Permanent Magnet : Half step.

 Step Energization of winding Step Angle X winding X’ winding Y    winding Y’   winding 1 + - Not energized Not energized 00 2 + - + - 450 3 Not energized Not energized + - 900 4 - + + - 1350 5 - + Not energized Not energized 1800 6 - + - + 2250 7 Not energized Not energized - + 2700 8 + - - + 3150 + - Not energized Not energized 3600

## Features of Permanent magnet ( PM ) Stepper Motor

The following is features of the half step permanent magnet stepper motor.

• The number of rotor teeth is half that of stator teeth.
• The number of poles in the stator half is generally 12 or 24 resulting step angle 15 degree or 7.5 degree.
• The rotor requires low operating power in the due to permanent magnet in the PM stepper motor as compared to VR stepper motor.
• Higher detent torque generated as compared to VR stepper motor
• Low acceleration due to higher inertia of rotor