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Brushless Motor Speed Controller

Discussion in 'General Electronics' started by Steve Fox, Jan 11, 2016.

  1. Steve Fox

    Steve Fox Veteran
    Builder

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    I know this is off topic, but I'm stumped. I've looked everyplace I can find and can't come up with the answer.
    I have a 7" lathe that I replaced the motor with a brushless, sewing machine motor. It is quiet with lots of speed and lots of low end torque. It works great, but the speed controller leaves a lot to be desired. It is a foot pedal and goes from zero to max in short order, which may be fine for a sewing machine, but that's not desirable for a lathe. I want to replace it with a potentiometer.

    I drew up the schematic and attached it to this thread. I apologize in advance for the quality.

    There are five wires in the connector. Two have something to do with needle position(1 and 3), so they don't apply. That leaves three wires, power (5), ground (2) and signal (4). I don't exactly understand why they use two separate Hall Effect sensors with the outputs tied in parallel, but I think that is irrelevant, too. The entire circuit consists of a voltage regulator, a quad OP amp and two Hall Effect sensors. When a magnet is moved between the Hall effect sensors (2 circled and 3 circled), the signal is amplified through all four OP amps. When I increase the speed, the voltage on the signal wire increases from .65v to 4.98v.

    What little I can find says to use a potentiometer across two of the wires, I am assuming signal and ground, but that doesn't make sense to me. I think I should hook up the pot with ground on one side, power on the other and signal in the middle, if it is going to work at all. I'm assuming it would just replace the variable voltage out of the last OP amp.

    Has anyone ever done this?
    I would appreciate any help or insight I can get.

    Thanks,
    Steve Fox
    Schematic.jpg
     

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