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3 phases motor

  1. Oct 18, 2009 #1
    Hello Forum,
    I have a 3 phases DC brushless motor from a Hard Disk Drive, when energize one coil and turning by hand the rotor i can feel 4 steps per rotation, is this means that this motor has 4 poles of north and south?
    Thank you in advance,
  2. jcsd
  3. Oct 18, 2009 #2


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    I saw a similar motor in a hard drive.

    I couldn't open it but I think it was as in the attached diagram.

    If you activated coil 3 it would attract the N pole to the left of it and the disk would rotate counterclockwise. Then activate coil 2 then coil 1. Then activate coil 3 again to attract the next North pole.

    I made a switching circuit for it and got it rotating but it was a bit jerky at slow speeds.

    Maybe your motor is similar.

    Attached Files:

  4. Oct 18, 2009 #3
    There are two basic types of BLDC (brushless dc motors). One type has a HALL EFFECT sensor that can be used for commutating the dc voltages. The other type uses the induced back emf to commutate the dc voltages. How many wires/terminals come out (not counting ground)? Do you think the coil inputs are wired as a delta or wye (use resistance meas)?
    Bob S
  5. Oct 19, 2009 #4


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    The one I played with was part of the mechanism that rotated the hard drive platter.

    It was DC and brushless, but actually a 3 coil unipolar stepper motor used to rotate the platter at a high but constant speed.
  6. Oct 19, 2009 #5
    hello , it has 3 wires and one common , it's a brushless DC motor Y connected. the questions is how i can know the number of poles ? in this way i can know : when energize one coil and turning by hand the rotor i can feel 4 steps per rotation, is this means that this motor has 2 poles of north and 2 poles of south?
  7. Oct 19, 2009 #6


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    Was it part of the turning mechanism for the part of the hard drive that has the data on it?
    This is a shiny grey disk. Mine actually had two disks one above the other.

    I think mine also had 4 poles and they would be permanent magnets with all their south poles all facing inwards and unused but the north poles facing outwards and being attracted to the 3 coils as they were turned on in order.

    It had some screws holding it together but I didn't have anything that would fit the head of the screws to take the mechanism apart.

    It seemed to work as in the above diagram, but I wasn't too sure about it. It worked if I fed it like that, so I guess it was close enough. Yet, I am surprised they would have strong magnets so close to the hard drive data.
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