Review: Stepper Motor

We'd to do a review on the stepper motor for the subject Electrical Technology. We rushed to prepare the information for our report & the presentation. The presentation went smoothly just now, since the information is fresh in my mind, I see no harm in posting a brief review about this special motor :)

What is the stepper motor?
  • Electromechanical device: converts electrical pulses into discrete mechanical movements.
  • The shaft/spindle (rotor) rotates in discrete step increments when electrical pulses are applied in the proper sequence.
  • 3 main types: variable-reluctance (VR), permanent-magnet (PM) & hybrid.
  • Used for controlled movement: control rotation angle, speed, position & synchronism.

VR stepper motor

PM stepper motor

Hybrid stepper motor
1. Rotating magnetic field
  • Current flows through phase winding - energizes the phase - develop magnetic flux in the stator.
  • Rotor aligns itself to minimize flux opposition - creates torque.
  • Adjusting the sequence of winding energizing will manipulate the magnetic flux which the rotor will follow.
2. Torque generation
  • Depends on 3 factors: step rate, the drive current in the windings, the drive design/type.
  • Torque generated when magnetic fluxes of rotor & stator are displaced from each other.
  • Stator: made from high permeability magnetic material - concentrates flux at the stator poles.
  • Basic relationship: H = (N x i)/l; H: intensity of magnetic flux, N: no. of winding turns, i: current, l: magnetic flux path length - shows that the same frame size stepper motor can have different torque output capabilities simply by changing the winding parameters.

Magnetic flux path through a 2-pole stepper motor 
with a lag between the rotor & stator
3. Phases, poles & stepping angles
  • Phase: on stator, can be unipolar/bipolar.
  • Pole: region where magnetic flux is concentrated - found on both rotor & stator. 
  • Step angle = 360/(NPh x Ph) = 360/N; NPh: no. of rotor poles, Ph: no. of phases, N: total no. of poles for all phases together.
4. Stepper mode
  • Wave drive: 1 phase on
  • Full step drive: 2 phases on
  • Half step drive: 1 & 2 phases on
  • Microstepping: continuously varying motor currents
  • Excellent response to starting/stopping/reversing.
  • Very reliable: no contact brushes.
  • Able to achieve wide variety of rotation speeds.
  • Accurately controlled in open-loop system: doesn't require expensive detecting equipment.
  • Resonances can occur if not properly controlled.
  • Difficult to operate at extremely high speeds.
  • Industrial: high-precision tools, direct drilling of screws etc.
  • Commercial: printers, hard disk drives etc.
We use the stepper motor for smaller applications as the power generated isn't sufficient for bigger equipment. An interesting piece of technology that comprises electrical, magnetic & mechanical properties to make it work :)

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