Stepper Motor Basics
Stepper Motor Basics
Learn more about stepper motors and their use in RE electricity generation
wind | educationIn this article we will attempt to explain some of the basics of
stepper motors and their use in
renewable energy applications, primarily in basic
Stepper Motor Wind Turbines with DIY
PVC rotor blades.
If you want to read more in depth information about the workings of stepper motors then there are links to a selection of online resources at the bottom of this article.
Inside a Stepper Motors
Inside a stepper motor are
four coils of wire located 90 degrees away from each other - i.e. at positions 12, 3, 6, and 9 o'clock. In the middle is the
rotor which spins and has
permanent magnets fitted around its circumference. As the rotor spins each magnet in turn approaches, passes, and moves away from each of the four coils in turn. A magnet passing a coil of wire causes electricity to flow through that coil and so each of the four coils will have different amounts of electricity flowing through it either one way or the other -
alternating current.
Since a
stepper motor has four coils of wire, it is said to be a
four-phase motor. (read more about
three-phase electricity here to better understand
multi-phase electricity). The advantage of this multi-phase set-up for
electricity generation projects is that when one coil has no electricity flowing through it, the next coil will have reached its maximum. When the four-phases are brought together and
rectified (more on rectification later) into direct current (DC), the total electricity generated therefore has a near constant voltage and current.
Stepper Motor Wiring
Most stepper motors have 6 wires, however there are motors with 4, 5, or 8 wires also. Each of the four coils is made up of one length of wire with two ends. One end is called
live and the other end is called
common. In a five-wire stepper motor all four
commons are joined together, in a six-wire stepper motor two pairs of
common wires are joined together, and in an eight-wire stepper motor none of the four
common wires are joined together.
Identifying the Wires in a Stepper Motor
If you do not have a schematic diagram for your stepper motor - for example if it was salvaged from an old printer - it is very easy to work out which wire is which.
Systematically use a
multimeter to measure the resistance between different pairs of wires. All four coils will have near identical resistances - if they did not the motor would not function properly. Therefore if the pair of wires being measured are both
live, the resistance measured will be double that measured if one of the wires is a
common. Why is this? Because two live wires have two coils between them whereas a common and a live have just one coil between them. (see diagram above)
When you have identified the
common wires, be sure to label them.
NEW Click here to view our article on
Examining a Stepper Motor in which we use the resistance between the different wires to work out which wire is which in a small 6-wire 4-phase
stepper motor.
How to Rectify the Output from a Stepper Motor
Having identified the four live wires emerging from the
stepper motor it is now very easy to
rectify the four-phase AC output into more useful direct current (DC) which can be used to power LEDs, charge batteries, and so on.
Basic Rectification with Diodes
The simplest way to get started to to connect each of the four
live wires to a
diode, connecting the other end of each diode to act as the postive, and connect the
common wires together
as the negative. An example of this is pictured above (image from the Campaign for Real Events Website). The positive pulses of electricity are added together and the negative pulses are blocked by the diodes (wasting half of the generated electricity).
Most stepper motors are labelled with their rated voltage and current per phase. Therefore the diodes used must be rated above (ideally at least double) the stated current per phase. (Standard
1N4001 1 Amp 50 Volt diodes are available from the
REUK shop for a few pence each.)
nb. To reduce the amount of voltage
dropped in the diodes,
Germanium or
Schottky diodes can be used though they cost more.
Rectifying with Bridge Rectifiers
To get a higher voltage and current
bridge rectifiers should be used. These will add together the
magnitudes of the pulses of electricity, so +3V stays as +3V and -3V becomes +3V. One
bridge rectifier is required for each of the four phases with each of the
live wires joined to the two AC (~) labelled legs of one bridge rectifier. The
common wires of the stepper motor are not used. The four positive legs of the bridge rectifiers are joined together to make the positive and the four negative legs of the bridge rectifiers are also joined together to make the negative.
As with diodes,
bridge rectifiers should be selected with spare capacity - for example, if the stepper motor is rated at 1 Amp per phase, 2 or 3 Amp rated bridge rectifiers at least should be used to prevent over-heating.
Bridge Rectifiers are available very cheaply - for example we sell
1.5 Amp, and
35 Amp bridge rectifiers in the
REUK Shop, or you can very easily
make a bridge rectifier with diodes if you are using a low current
stepper motor.
A
1000uF Capacitor can be used in the circuit to smooth the output DC voltage from the bridge rectifier.
Further Reading
Here are some links to
stepper motor resources online.
Stepper Motor Voltage Doubler Circuit - double the voltage of a stepper motor with a couple of
bridge rectifiers.
Wikipedia Stepper Motor article.
Working with Stepper Motors.
Stepper Motor Experiments.
Generating Electricity with Stepper Motors.
Stepping Motors Resources.
Lessons in Electric Circuits - stepper motors discussed one-third of the way down the page.
Article Last Modified: 09:54, 4th Apr 2007
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