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 = ( N_S – N_r ) × 360⁰ / N_SN_r

                    = ( 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

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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