11 April 2019

Stepper Motor Terminology - Holding torque, Detent Torque, Step angle and Resolution

The terminology associated with stepper motor is given as below.

Holding torque

  • It is torque required by the rotor in order to rotate at full step angle from its standstill condition when one or two phases of the stator winding is energized by Direct current.

Residual or Detent torque

  • It is torque developed by the stepper motor rotor without slipping when supply is off. 
  • The detent torque is developed by the residual magnetism and its value is approximately 5 – 10 % of holding torque. 
  • It is developed only in the Permanent Magnet and Hybrid Stepper Motor.
  • It is also called as Cogging Torque.

Pull in torque

  • It is maximum torque developed by the stepper motor when it starts, stop and rotates in the reverse direction without step out at constant frequency.

Pull out torque

  • It is maximum torque developed by shaft of the stepper motor at constant frequency without step out it.

Step rate

  • It is defined as the number of steps requires in order to rotation of shaft in specific time.

Step angle

  • It is defined as the specific rotation angle of rotor for each input pulse in the stepper motor. 
  • Its value depends upon number of stator poles or teeth, rotor poles or teeth and number of stator phases.
Step angle = 360o / S
Where S = mNr
         Nr = Number of rotor teeth
           m = Number of Phases
                                               OR
Step angle = [ ( Ns – Nr ) / ( Ns * Nr ) ] * 360
Where Ns = Number of stator poles or teeth
            Nr = Number of rotor poles or teeth
  • The number of steps requires in order to rotate one revolution of rotor depends upon step angle. 
  • Higher the step angle, lower the steps in order to rotor one revolution of rotor and vice versa.

Resolution of Stepper Motor

  • It is defined as the number of steps requires in order to complete one revolution ( 2π ) of rotor shaft. 
  • It is measured in steps per revolution.
Resolution = ( 360o / Step angle )
  • The stepper motor even works at 20,000 steps per revolution. 
  • Higher the resolution, lower the pulsating torque and rotor rotates continues smoothly without any jerk.   
Example
Find out step angle and resolution if number of the stator pole is 8 and rotor poles is 6.
Solution :
Step angle =  [ ( Ns – Nr ) / ( Ns * Nr ) ] * 360
                 = [ ( 8 – 6 ) / 8 * 6 ] * 360
                 = 15o
Resolution = 360o / Step angle
                   = 360o / 15o
                   = 24 Steps / revolution
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