Examining a Stepper Motor

Read how easy it is to understand the wiring of a stepper motor

In our article Stepper Motor Basics (1) we explaine how a stepper motor can be used to generate electricity and how to use a multimeter (2) to identify the wires which emerge from one.

The commonest type of stepper motor is a six-wire four-phase unit with four live AC outputs and two common outputs. In this article we will look at a small stepper motor of this type and identify the wires and build a picture of its inner workings.

The Stepper Motor

Our stepper motor is an old MB 11 sized Philips model with a 7 1/2 degree step angle. That means that it has 360/7.5 = 48 steps which means it would not be ideal for electricity generation and battery charging. As mentioned in our article Electricity with Stepper Motors (3) it is best to choose a stepper motor with the most steps possible - usually 200 - since these motors generate higher voltages at lower RPMs; perfect for small wind turbine generators.

Labelling the Wires

6 wires emerge from inside the stepper motor

There are six wires emerging from the stepper motor: two red, two yellow, and two grey. One red, one grey, and one of the yellow wires have a dark mark on them, the other wires are clean. Therefore we can use that to individually identify the wires. If the wires all looked the same, then we would have individually identified them with small pieces of labelled tape. We will call the wires R1, Y1, G1, R2, Y2, and G2 where the letter is the first letter of the wire's colour, and the number 1 or 2 identifies if the wire was clean (1) or marked (2).

Measuring Resistances

Systematically we connected the testing leads of the multimeter to pairs of wires coming out from the stepper motors and put the measured results into the following simple table:

	R1	R2	G1	G2	Y1	Y2
R1		-	117	-	117	-
R2	-		-	117	-	117
G1	117	-		-	234	-
G2	-	117	-		-	236
Y1	117	-	234	-		-
Y2	-	117	-	234	-

All of the above results are measurements taken in Ohms. Infinite resistance is represented by a '-'.

Analysing the Stepper Motor Resistances

Where the resistance between two wires is infinite we know there is no connection between those two wires within the stepper motor - for example, between R1 and R2 or G1 and G2. We have two different values of resistance between the other wires - 117 Ohms and 234 Ohms with one being half of the other. This is because this stepper motor has four phases and therefore four identical coils. When the resistance measured between two wires is 117 Ohms, the wires are connected across one coil, and when the resistance is 234 Ohms the wires are connected across two coils.

R1 is connected to G1 and Y1 across one coil. R2 is connected to G2 and Y2 across one coil. G1 and Y1 are connected across two coils, and G2 and Y2 are connected across two coils. None of the 2's are connected to any of the 1's. Therefore we can draw the following simple diagram of the wiring of this stepper motor:

Our four live wires are G1, G2, Y1, and Y2, and there are two common wires R1 and R2. We now know everything we need to know to rectify (4) the output from this small stepper motor into DC electricity for battery charging or to light some LEDs.

Web Link References

(1) http://www.reuk.co.uk/Stepper-Motor-Basics.htm
(2) http://www.reuk.co.uk/buy-MULTIMETER.htm
(3) http://www.reuk.co.uk/Electricity-with-Stepper-Motors.htm
(4) http://www.reuk.co.uk/Bridge-Rectifier.htm