Re: HDD spindal motor
Please remember that I'm guessing this without having seen the motor.
You need to measure the resistance accurately, there are probably three windings, each having one side joined to the common connection and the other side having it's own connection. The common connection will have equal resistance to the other three because you are only measuring one winding. If you measure between the ends of the windings (in other words, not the common pin) you should measure the resistance of two windings so the value will be twice as high. Be careful because the readings may be very low anyway. If possible use an analog Ohm meter, some digital ones carry signals across the probes which will give false readings when you measure inductors.
Think of the windings being arranged so that when current flows it aligns the rotor at a certain angle. Each winding is positioned so when energized it pulls the rotor slightly in one direction from the last winding. So if you only energize one winding, the rotor will jump but stop in one position, if you energize two windings, with the current alternating from one to the other, the rotor would rock to one side then back again. If you energize all three, the rotor will shift to one side, then a bit more, then a bit more and so on until it completes a rotation. There are two ways these motors can be driven, without knowing the motor design or the original circuit that powered it there is no way of knowing which way it is intended to work. The simplest method is just to put power on the common pin and sequence an electronic switch (ULN2003, transistors or FETs) down to ground on each other pin in turn. Some motors require the polarity of the current to be reversed so the common pin also has to be switched between supply and ground. If you look on the Wiki page for "BLDC motors" there is a good explanation.
Brian.