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 :)