Old 5.25" Floppy drive Motor with 3 wires

Hey Guys,

I’ve removed a motor from a 5.25" floppy drive. It has three wires input (might be an ac 3-? stepper motor / spindle motor). It uses an obsolete M51784P motor drive, whose data sheets were nowhere found. The motor turns a step when i randomly give a 12v DC power to two of the three wires.

All I want to do is to turn the motor in either clock wise or anti-clock wise. How can I predict the correct order or sequence of supplying power to those three wires ?

If it is a 3-? AC motor, how can i create the a 12v ac power ?

I’ve attached the images of the motor, driver IC and the internal windings for reference.

Thx !

That is a three phase brushless motor. You need a three phase H-Bridge. 3 input lines - when HIGH the corresponding output is tied to +V, when low the corresponding output is tied to -V

If you were to number the phases 1, 2 and 3

to rotate the motor you would need to feed power this way - for a full step arrangement -

1 2 3 and back to 1

For a half step arrangement - 1, 2 3 4 5 6 and back to the begining.

Reverse the sequence to reverse.

Or use it as the basis for a motor for a model airplane. I have built a number of them and drive them with a brushless motor controller. I personally like the Castle Creations units.

Thank you very much....

I tried the sequence with a DC power supply. It took 48 steps to complete one revolution (~7.5 degrees per step).

What are the maximum voltage and current ratings of motor ? Any Idea ?

I've use normal H-Bridges, but I'm not sure about 3-? H-Bridges... Do you know any good resource about them ?

Also, what about Hall Sensors. How do we take feedback from them ?

Regards, -ART.

If the existing board just has 3 wires then 2 are power and 1 is a control signal, so it might be possible to use the existing driver chip if you can figure out the control protocol (possibly just a clock or an enable line?)

Otherwise you'll need to figure out the position feedback (hall sensors?)

Actually, If you look closely at the pictures 2nd I attached you’ll find a set of 5 solder junctions.

They were initially connected to another main PCB of the floppy drive. I dismantled the floppy some 5 years ago ago and i have no idea where the went.

So, as kf2qd said, they are the connections of the stator windings and I have verified the same.

I don’t have the data sheets of the driver IC either. I’ve already requested “mitsubishichips” to send any info they have on the chip.

Though I have a similar IC, M51785P data sheet.

spindle motor ic.pdf (305 KB)

Normally these motors have 3 hall-sensors on the board to measure the position of the rotor and time the commutation of the windings. You'll have to find the connections to these.

PWM is then employed to control the speed via a PID loop (the actual speed can be derived from the hall-sensor pulses) There are various 3-phase motor driver chips available, for this kind of low-power motor ones with built-in MOSFETs are perfectly adequate. Usually the PID loop is left to the microcontroller to handle.

[ looking at the photos again I suspect the Hall sensors are under the unwound poles ]

Yes, there are a set of Hall sensors beneath the unwound poles (E1, E2 and E3). And using a continuity tester I found the IC pins they are connected to.

I’ll solder additional wires to those pins and get the feedback from hall sensor to a micro-controller.

I’ve found an Atmel Application note on brushless motors (http://www.atmel.in/Images/doc2596.pdf). It gives a relation between the hall sensors and current flow in the windings (image attached).

Is there anything else I need to know to get the most out of the motor ?

Hall + Winding sequence.png

You'll need to know if it uses digital or analog hall sensors or bare hall-elements. The datasheet you found mentions hall-amplifiers which implies bare-hall elements or analog (these days most simple motors use digital hall switches).

Then finding an appropriate driver (or just a 3-phase bridge - use microcontroller to handle hall-signals via pin-change interrupts?)

You'll have to figure out the right phase relationship between hall signals and windings (but there are only a few possibilities so you can simply swap windings over till it works!)

You'll have to figure out the right phase relationship between hall signals and windings (but there are only a few possibilities so you can simply swap windings over till it works!)

How to find the relation between Hall Sensors and windings ?

Any other advantage of using the hall effect sensors ? I know they are used to find the position of the rotor...

Also, any hard and fast rules to find the type of Hall sensors used ?

Please see this links--->




I believe it will give a complete picture of the application of BLDC motors...

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