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Chapter: Special Electrical Machines - Stepping Motor

Terminologies Used in Stepper Motor

1. Step motor 2. Resolution 3. Stepping rate 4. Hold position 5. Detent position 6. Stepping error 7. Position Error

TERMINOLOGIES USED IN STEPPER MOTOR

 

1. Step motor

 

2. Resolution

 

3. Stepping rate

 

4. Hold position

 

5. Detent position

 

6. Stepping error

 

7. Position Error

 

1. Step angle (θs or β)

 

It is the angular displacement of rotor of a stepper motor for every pulse of excitation given to the stator winding of the motor. it is determined by the number of teeth on the rotor and stator, as well as the number of steps in the energisation sequence. It is given by

 


Where

 

m = Number of phases (m and q)

 

Nr- number of teeth on rotor.

 

Also,         Θs=((Ns~Nr)/(Ns.Nr))*360

 

2. Resolution

 

It is the number of steps per revolution. It is denoted as S or  Z. it is given by

 

Z=360/(Θs)

 

For variable reluctance motor Z=(q Nr) or (m Nr)

 

For PM motor and hybrid motor Z=2q Nr

 

Also ,        Z=(Ns.Nr)/(Ns~Nr)

 

Where Ns-number of teeth/poles on stator.

 

3. Stepping Rate

 

The number of steps per second is known as stepping rate or stepping frequency.

 

4. Hold Position

 

It corresponds to the rest position when the stepper motor is excited or energized(this corresponds to align position of VR motor)

 

5. Detent Position

 

It corresponds to rest position of the motor when it is not excited.

 

6. Stepping Error

 

Actual step angle is slightly different from the theoretical step angle. This is mainly due to tolerances in the manufacture of stepper motor and the properties of the magnetic and other materials used.

 

The error in the step angle is expressed as a percentage of the theoretical step angle.

 

%error= ((step angle – theoretical step angle)/theoretical step angle)*100

 

Percentage error is restricted to ± 5%.In some cases it is restricted to ±2%. The cumulative error between the actual angular displacement and theoretical angular displacement is expressed as a percentage of theoretical angular displacement. It is usually considered for one complete cycle.

 

7. Positional Error

 

The maximum range of cumulative percentage of error taken over a complete rotation of stepper motor is referred to as positional accuracy as shown in fig below.




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