reverse polarity

Discussion in 'The Projects Forum' started by Hexorg, Nov 6, 2009.

  1. Hexorg

    Thread Starter New Member

    Nov 6, 2009
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    Good daytime to you!
    I'm trying to have +O- polarity on a motor when there is an input voltge and -O+ polarity when there is no input voltage... Any possible ways to acheive it without any special IC chip?

    the way I thought of it would require 2 extra voltage sources (see attachment) But is there anyway to do that with one extra sourse?
     
  2. jpanhalt

    AAC Fanatic!

    Jan 18, 2008
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    It can be done with a double pole, double throw relay. Just look at the relay as a reversing switch.

    John
     
  3. thatoneguy

    AAC Fanatic!

    Feb 19, 2009
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    To stay with the relay, a DPDT relay would work, with the battery voltage feeding the two poles of the relay contacts. On Throw 1 (Normally Closed), wire the motor - and +, on Throw 2 (coil energized), wire the motor as + and -.

    The two "poles" are essentially two Dual Throw switches run by the same relay coil. Your schematic above shows a Single Pole, Dual Throw.
     
  4. SgtWookie

    Expert

    Jul 17, 2007
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    Keep in mind that it's VERY hard on a DC motor to simply throw it into reverse while it's running. You should stop the motor completely before running it in the opposite direction.

    Here is a way that you could use two SPST relays and two pushbutton switches to reverse a DC motor. If neither pushbutton is depressed, both ends of the motor are shorted to ground, which acts as a brake. If both pushbuttons are depressed, both ends of the motor are connected to +V, which also acts as a brake:

    [​IMG]

    It would help us to help you if you explained more about what you're trying to do.
     
  5. CDRIVE

    Senior Member

    Jul 1, 2008
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    Funny you should mention this Sgt. I was also under the same impression for many years. That belief went up in smoke about 12 years ago when members of 7 South Bend Lathe (antiques) groups, which I moderate, started fitting instant reverse DC or AC motors to their machines. The manufactures actually tout them as such. Mine still has the original motor but I'm a purist when it comes to anything antique... If I can. Besides that, threaded (non Cam Loc) spindles and instant reverse or stop is a recipe for duck and cover exercises in your shop! :rolleyes:

    BTW, I don't think this negates what you said, as I have no idea how these motors are constructed to handle this abuse. ;)
     
  6. Ron H

    AAC Fanatic!

    Apr 14, 2005
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    Correct me if I'm wrong, but I don't think a lathe motor has a high inertial load. Now, if the load were a large flywheel...:eek:
     
  7. Ron H

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    Come again? are you living in a parallel universe?:D
     
  8. shortbus

    AAC Fanatic!

    Sep 30, 2009
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    Does a 15 pound chuck count as a flywheel?:D They will unscrew, don't ask me how I know:rolleyes:

    Cary
     
  9. Ron H

    AAC Fanatic!

    Apr 14, 2005
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    Seems like instant reversal would be a good (bad?) way to unscrew the chuck (more formally known as the Charles).:D
     
  10. CDRIVE

    Senior Member

    Jul 1, 2008
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    When I purchased my SB Heavy 10 I was a dumb _ss and went on a buying spree of accessories. One of which was a 10 inch 3 jaw chuck. I think it weighs is excess of 30lbs. Yes, it's definitely a flywheel and way to big for a 11" swing lathe.

    Sorry for drifting off topic but I think Sgt. Wookie's solution solved the OP's question anyway. ;)
     
  11. SgtWookie

    Expert

    Jul 17, 2007
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    Dog plates make nice flywheels, too - and yes, they'd unscrew if you suddenly reversed the motor...

    Dad's old South Bend lathe had a capacitive start AC motor with a drum switch. Fortunately, if you switched from Fwd to Rev when it was running, the motor just kept going in the same direction. You had to stop it almost completely for the centrifugal switch to click.
     
  12. CDRIVE

    Senior Member

    Jul 1, 2008
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    Yes, that was the most common setup on those old machines. They also came off the line with 3PH motors. Fortunately VFRs for home use has made these motors usable on 240V/SPH. You can program them for ramp up and ramp down and (I believe) instant stop / rev., which wise operators don't use. :D
     
  13. jpanhalt

    AAC Fanatic!

    Jan 18, 2008
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    I know this is not a lathe thread, but you can drill and tap the chuck rear end and add a grub screw with brass tip to allow easy reverse turning, as in some threading operations.

    Or, get a real lathe: 1962 Monarche 10EE ;)

    John
     
  14. Hexorg

    Thread Starter New Member

    Nov 6, 2009
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    ok ok, thank you all!
    I'll explain what i'm doing - i'mm connected to the LPT port on my computer, having my programm running. When I have an email, i set first bit of LPT to 1, when i read it, LPT is set to 0, so I have 5VDC input to my cirquit. When there is a input - I spin the motor one way (raising a flag), when the flag is vertical it presses a reverce push-button that will stop the motor. when I read the email I need to spin the motor another way(reverce the polarity) to lower the flag. When the flag in horisontal it will press another reverce push-button to make the motor stop.... I just thouht it wasn't relevant to the topic, so I didn't include the reverce pushbuttons on my scheme above.
    But thank you all for the ideas :)
     
  15. SgtWookie

    Expert

    Jul 17, 2007
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    LOL! :D Funny project ;)

    OK, I actually have another version of the schematic I posted that's closer to what you're looking for. I'll have to dig it out.

    [eta]
    Try this:
    [​IMG]
     
    Last edited: Nov 7, 2009
  16. Hexorg

    Thread Starter New Member

    Nov 6, 2009
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    SgtWookie, yea, if you want - here -
    LPT Pinouts:
    Pin 2 = Bit0
    Pin 3 = Bit1
    Pin 4 = Bit2
    Pin 5 = Bit3
    Pin 6 = Bit4
    Pin 7 = Bit5
    Pin 8 = Bit6
    Pin 9 = Bit7
    Pin19 = GND for Bit0 and 1
    Pin20 = GND for Bit2 and 3
    Pin21 = GND for Bit4 and 5
    Pin22 = GND for Bit6 and 7

    it's very useful in case of such little projects :) Currently I juce have an LED and a little resistor in between pin2 and 19 - weorks great especially if you are waiting for a urgent email :)
     
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