Editing Stepper motor (section)

== Phases ==

=== Two phase ===
There are two basic winding arrangements for the [[electromagnetic coil]]s in a two phase stepper motor: bipolar and unipolar.[[Image:Unipolar-stepper-motor-windings.png|thumb|right|Unipolar stepper motor coils]]
==== Unipolar motors ====
A unipolar stepper motor has one winding with [[center tap]] per phase. Each section of windings is switched on for each direction of magnetic field. Since in this arrangement a magnetic pole can be reversed without switching the polarity of the common wire, the [[Commutator (electric)|commutation]] circuit can be simply a single switching transistor for each half winding. Typically, given a phase, the center tap of each winding is made common: three leads per phase and six leads for a typical two phase motor. Often, these two phase commons are internally joined, so the motor has only five leads.[[File:28BYJ-48 unipolar stepper motor with ULN2003 driver.jpg|thumb|The 28BYJ-48, accompanied by a [[ULN2003A|ULN2003]] driver, is one of the most popular stepper motors among hobbyists.]]

A [[microcontroller]] or stepper motor controller can be used to activate the drive [[transistor]]s in the right order, and this ease of operation makes unipolar motors popular with hobbyists; they are probably the cheapest way to get precise angular movements.
For the experimenter, the windings can be identified by touching the terminal wires together in PM motors. If the terminals of a coil are connected, the shaft becomes harder to turn. One way to distinguish the center tap (common wire) from a coil-end wire is by measuring the resistance. Resistance between common wire and coil-end wire is always half of the resistance between coil-end wires. This is because there is twice the length of coil between the ends and only half from center (common wire) to the end. A quick way to determine if the stepper motor is working is to short circuit every two pairs and try turning the shaft. Whenever a higher-than-normal resistance is felt, it indicates that the circuit to the particular winding is closed and that the phase is working.
[[File:Unipolar stepper motor with reduction gear mechanism.jpg|thumb|Stepper motors like this are often accompanied by a reduction gear mechanism to increase the output torque. The one shown here was used in a [[Image scanner|flatbed scanner]].]]
==== Bipolar motors ====
[[File:A bipolar stepper motor used in DVD writers.jpg|thumb|A bipolar stepper motor used in [[Optical disc drive|DVD drives]] for moving the laser assembly.]]
Bipolar motors have a pair of single winding connections per phase. The current in a winding needs to be reversed in order to reverse a magnetic pole, so the driving circuit must be more complicated, typically with an [[H-bridge]] arrangement (however there are several off-the-shelf driver chips available to make this a simple affair). There are two leads per phase, none is common.

A typical driving pattern for a two coil bipolar stepper motor would be: A+ B+ A− B−. I.e. drive coil A with positive current, then remove current from coil A; then drive coil B with positive current, then remove current from coil B; then drive coil A with negative current (flipping polarity by switching the wires e.g. with an H bridge), then remove current from coil A; then drive coil B with negative current (again flipping polarity same as coil A); the cycle is complete and begins anew.

Static friction effects using an H-bridge have been observed with certain drive topologies.<ref>See "Friction and the Dead Zone" by Douglas W Jones https://homepage.divms.uiowa.edu/~jones/step/physics.html#friction</ref>

Dithering the stepper signal at a higher frequency than the motor can respond to will reduce this "static friction" effect.
[[File:A bipolar stepper motor with a reduction gear mechanism.jpg|thumb|A bipolar stepper motor with gear reduction mechanism used in a [[Image scanner|flatbed scanner]].]]
Because windings are better utilized, they are more powerful than a unipolar motor of the same weight. This is due to the physical space occupied by the windings. A unipolar motor has twice the amount of wire in the same space, but only half used at any point in time, hence is 50% efficient (or approximately 70% of the torque output available). Though a bipolar stepper motor is more complicated to drive, the abundance of driver chips means this is much less difficult to achieve.

An 8-lead stepper is like a unipolar stepper, but the leads are not joined to common internally to the motor. This kind of motor can be wired in several configurations:
* Unipolar.
* Bipolar with series windings. This gives higher inductance but lower current per winding.
* Bipolar with parallel windings. This requires higher current but can perform better as the winding inductance is reduced.
* Bipolar with a single winding per phase. This method will run the motor on only half the available windings, which will reduce the available low speed torque but require less current

=== Higher-phase count ===
Multi-phase stepper motors with many phases tend to have much lower levels of vibration.<ref>{{cite web|url=http://www.electricmotors.machinedesign.com/guiEdits/Content/bdeee4/bdeee4_1-1.aspx|title=electricmotors.machinedesign.com|access-date=2011-04-05|archive-date=2014-01-22|archive-url=https://web.archive.org/web/20140122182509/http://www.electricmotors.machinedesign.com/guiEdits/Content/bdeee4/bdeee4_1-1.aspx|url-status=dead}}</ref> While they are more expensive, they do have a higher [[power density]] and with the appropriate drive electronics are often better suited to the application{{Citation needed|date=July 2010}}.