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26/12/2021

Permanent Magnet Stepper Motor

 

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

Operation : Single Phase ON

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.



permanent-magnet-stepper-motor-single-phase-on.png


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

 

Operation : Two Phase ON

  • 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.

pm-stepper-motor-two-phase-on.png




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


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Operation : 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


pm-stepper-motor-half-step-operation.png


 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

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

 


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