Hello I have an motor from an old hard drive has four wires, but I can not exactly figure out what kind of motor it is, believe it is a stepper motor, the measured resistance to the 4 wires and they are L = 4.9 Ohm H = 8.2 Ohm

1 2 3 4 1 H H L 2 H H L 3 H H L 4 L L L Is there anyone who can explain a little closer

Peter sorry for my english

How old of a hard drive? Do you know if this was the spindle motor or the r/w head positioner (for the latter, only if the drive is -very- old)?

If it came from a drive less than say, 15 years old, it is likely a spindle motor, and in that case it is likely a 3-phase brushless motor - not a stepper.

Based on your readings, it is likely a "wye" configuration, and pin "4" is the "common" connection (because pin 4 to any other pin is a lower resistance - approx 1/2). The other connections are for the coils, and given the "Y" configuration (imagine the "center" of the "Y" is the common pin, and the "ends" are the other pins, with the coils on the "arms" of the "Y" - see?), coil to coil would be approximately "double" the resistance of "coil" to "common".

In order to drive such a motor, you need to provide 3 phases to each coil, each phase shifted in time by 120 degrees - see:

These waves can be square waves - each shifted by 120 degrees (and ideally centered on 0 volts - you really want an AC signal on the coils), but it won't be very efficient; the motor is actually expecting a sine wave.

If you do some more googling (search on combos of "three phase" "hard drive" and "spindle motor") you will find more info on these motors and how to get them spinning in several areas; some with more advanced information than others, of course. You can get one to work with an Arduino - maybe not at it most efficient point, but you can get it spinning.

Good luck.


The hdd is from 1999 see here:

From the age, it's a voice coil driven head assembly with a three phase Brushless DC spindle motor. Probably Wye wound with your lead 4 being the common tap for the three windings.

Thanks, this is a 3-phase brushless motor