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Identifying Stepper Motors

W.G. Jansen (The Netherlands)

There are many different types of stepper

motor. Because there is no documentation

available for stepper motors that have been

removed from old equipment you have to

carry out some measurements to identify the

different wires.

We only need three things for this: an ohm-

meter, an AC voltmeter and a transformer

with an output voltage between 2 and 6 V.

The majority of stepper motors have either

two or four stator coils, which are presented

to the outside world via 4, 5, 6 or 8 dif ferent

coloured wires, see Figure 1.

For a motor with 4 wires we have to find two

wires that have a resistance between them.

We then write down the value of the resistance

and the colour of the wires. In this way, we can

distinguish between the two stator coils and

we know that this is a bipolar motor.

For a motor with 5 wires (unipolar) it is more

difficult to identify the four individual coils.

We start with the measurement of the resist-

ance between all the differently coloured

wires and make a note of them in a list (see

the example in Figure 2). Next, find all pairs

of wires that have the lowest resistance

between them and call those R x ...Ω. The

resistance values of the other combinations

aren’t important.

Measurement s: yellow/red = Rx ...Ω

  blue/red = Rx ...Ω

  white/red = Rx ...Ω

  brown/red = Rx ...Ω

For a motor with 8 wires (both bipolar as

well as unipolar devices) it is quite difficult to

determine the correct order of the four coils

in the two phases. A s for the other motors,

we start w ith the resistance measurements

and put them in a list, which will make clear

what the individual coils are (see Figure 4). In

order to connect the coils in pairs and in the

correct phase, the winding direction of each

coil has to be determined. To do this, con-

nect the transformer to one of the coils and

measure the voltage across the other coils

with the AC voltmeter. The coil that shows

the largest voltage will be the one that forms

one phase in conjunction with the coil con-

nected to the transformer. To find out if the

coils are connected in phase, the coils are con-

nected in series and the transformer is con-

nected across one coil. Fir st measure the volt-

age across the powered coil and then across

both coils in series.

There are two possible outcomes: The volt-

age across the series connec tion is about

twice that across the single coil, or it’s almost

zero. The correct series connection is the one

where the voltage is highest. For bipolar us e

you should connect the two coils for each

phase in series or parallel, since that results

in the maximum torque from the motor.

(090420)

Literature‘Stepper Motors Uncovered’,

Elektor November & December 2003

From this example it appears that the red wire

is the common one (COM). Two pairs of coils

make up the A-B phase and the C-D phase. To

find out which ones belong together we con-

nect a small AC voltage to one of the coils, if

need be via a series resistor to limit the cur-

rent. In this example we chose yellow/red. Now

use the AC voltmeter to measure the voltage

across the remaining coils. The coil where we

measure the largest voltage will be the one

that forms one phase in conjunction with the

yellow/red coil. It’s not important whether we

call this the A- B phase or C-D phase.

For a motor with 6 wires (both bipolar as well

as unipolar devices) it is straightforward to

identify the individual coils. Again, we meas-

ure the resistance between all coloured wires

and put them in a list.

Measurement s: yellow/red = Rx ...Ω

  red/brown = Rx ...Ω

  blue/black = Rx ...Ω

  black/white = Rx ...Ω

  yellow/brown = 2Rx ...Ω

  blue/white = 2Rx ...Ω

We find a low resistance value (Rx ...Ω) four

times and a higher resistance (2Rx ...Ω) twice.

There is no connection between the two

phases (see Figure 3). From this it can be seen

that yellow/red/brown is one phase with red

as common, and blue/black/white is the sec-

ond phase with black as common. For bipo-

lar use the 2Rx connections are used and the

common wire is left unconnected.

4 Lead

Motor

A+

A-

B+ B-

8 Lead

Motor

A

B

A'

C C' D' D

B'6 Lead

Motor

A

      C      O      M      1

B

COM2C D

5 Lead

Motor

A

      C      O

      M

B

C D

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