10/21/2008

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
Mechanism...
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
Advantages
  • 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.
Disadvantages
  • Resonances can occur if not properly controlled.
  • Difficult to operate at extremely high speeds.
Applications
  • Industrial: high-precision tools, direct drilling of screws etc.
  • Commercial: printers, hard disk drives etc.
Conclusion
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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