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Monday, December 3, 2007

Stepper motors and their use in RE electricity generation

Stepper Motor

In 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.

Inside a Stepper Motor

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. 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.

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 Stepper Motor 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. To reduce the amount of voltage dropped in the diodes, Germanium or Schottky diodes can be used though they cost more.

(*Source - - Stepper Motor Basic)

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