Monday, 6 June 2016

Basics of Stepper Motor

A stepper Motor is a special form of AC Motor. The input of a Stepper Motor is in the form of electric pulse for which the shaft of Stepper Motor turns through a specified degree, called Step. The Stepper Motor’s shaft turns one step for each input pulse. The Step size of a Stepper Motor may vary from 0.72° to 90°.

Operating Principle of Stepper Motor:

Stator of Stepper Motor is salient pole over which concentrated winding is wound. The windings of Stator may be properly connected to get two, three or four phase windings. The Rotor of Stepper Motor carries no winding rather it is either Permanent Magnet or Ferromagnetic Material. The Stepper Motors are manufactured with steps per revolution of 12, 24, 72, 144, 180, and 200, resulting in stepping angles of 30, 15, 5, 2.5, 2, and 1.8° per step.

Step angle = 360°/No of steps per revolution

                  = 360°/mP

Where m = Number of Phase winding on the Stator

            P = Number of Rotor Teeth or Poles

A Stepper Motor is fed through an external drive circuit. The main purpose of the drive circuit is receives the input voltage pulse and deliver appropriate current pulse to the Stepper Motor Stator winding. This current set up air gap flux which moves through one angular step for each input pulse. Likewise the Rotor follows the air gap flux and turns through the step.

For understanding the working principle of Stepper Motor, let’s assume that there are four winding on Stator and Rotor has two poles.

There are 4 coils with 90° angle between each other fixed on the stator. The way that the coils are interconnected, will finally characterize the type of Stepper Motor connection. In the above drawing, the coils are not connected together. The above motor has 90° rotation step. The coils are activated in a cyclic order, one by one as per the input pulse. The rotation direction of the shaft is determined by the order that the coils are activated. The above animation demonstrates this motor in operation. The coils are energized in series, with about 1sec interval. The shaft rotates 90° each time the next coil is activated.

Based on the type of Rotor and basic principle of operation, a Stepper Motor is classified into three types as

  • Variable Reluctance Stepper Motor
  • Permanent Magnet Stepper Motor
  • Hybrid Stepper Motor

We will discuss each of them in next post, so be there.

Meaning of 6/2 Pole Stepper Motor:

Stepper Motor is generally designated as Stator Pole / Rotor Poles or Teeth. Therefore 6/2 Pole Stepper Motor means that the Stepper motor has 6 salient pole on Stator and 2 poles on Rotor.

Difference between Stepper Motor and Conventional Motor:

Following are the major differences between the Stepper Motor and Conventional Motor:

  • Input to Stepper Motor is in the form of pulse while the input to Conventional Motor is from constant voltage source.
  • A Conventional Motor has free running shaft while a Stepper Motor shaft truns through a specified angle.
  • In Control System, no feedback loop is required when Stepper Motor is used but if Conventional Motor is used then we need to have feedback loop for position control.
  • A Stepper Motor is a digital electromechanical device while  a Conventional Motor is an analog electromechanical device.
Thank you!

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