Help requested on AC motor control project

Discussion in 'The Projects Forum' started by legomyeggo, Jun 18, 2009.

  1. legomyeggo

    Thread Starter New Member

    Jun 18, 2009
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    0
    Hi -

    I'm an architect and structural engineer but very new to electrical and electronics - been reading a lot and hoping to learn more.

    The project...

    I've designed and built a custom staircase for a limited space situation in an aircraft hangar - to access the second floor loft area. The requirement was to not take up any of the first floor space. So, the staircase hinges at the first landing and the lower section (14 step strait run) is pulled up and down using a rope/pully system from the lower floor so it completely disappears against the ceiling when not in use.

    The staircase has it's weight offset by a set of heavy duty springs (from a garage door) so there's very little weight to lift/lower. I haven't put a strain guage on it yet but I'd guess it's the equivalent of a 20-30 lb lift.

    The goal...

    I'd like to automate this raising and lowering of the staircase using an electric motor, wall switch (up/down/stop), with upper and lower limits to turn off the motor automatically when the stairs reach the floor when lowered and reach the ceiling when raised. So, I guess I'm looking to build something very similar to a garage door opener for the staircase.

    So far, I've fashioned a spare 1hp 110v AC motor to a sprocket and hooked it to the stairs using aircraft cable. When I plug the motor in, it lifts the stairs quitely, evenly, and at a nice speed.

    So - can anyone help a newbie with an A/C motor control circuit to make a 1 hp motor go forward, backward, stop, and automatically stop when it reaches it's upper and lower limits? I'd prefer the limits to be "felt" by the motor instead of having actual switches mounted to the stairs - meaning have the motor shut off when the weight of the stairs is supported by the floor (lower limit), and also shut off when the resistance of the stairs against the ceiling is felt (so the motor pulls the stairs snug against the ceiling but doesn't try to yank it through...). Again, similar to a garage door opener. I suppose these limits could also be programmed using an encoder as well. Anyway - thanks in advance for any ideas or suggestions.
     
  2. Bernard

    AAC Fanatic!

    Aug 7, 2008
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    397
    Is the motor reversable by reversing the start winding on the motor terminal board? If so a heavy duty DPDT center OFF toggle switch will raise-lower-and stop. All that remains is the soft stops.
     
  3. CDRIVE

    Senior Member

    Jul 1, 2008
    2,223
    99
    Welcome to the forum.

    If you don't want limit switches then there are other options. One option is sensing motor current, as it will increase when it reaches end of travel and is stalled. I don't like this method because you would have to ignore the very high start current, when the motor is turned on, so they're not confused with each other.

    I think your best bet would be using a 555 as a missing pulse detector. The way it would work is like this:

    You will need to interface an optical sensor to the rotor shaft of the motor. This optical device will generate one pulse for every revolution of the motor. If the 555 doesn't see a pulse the motor circuit is then opened.
     
  4. jpanhalt

    AAC Fanatic!

    Jan 18, 2008
    5,699
    905
    I built a bifold hangar door that used cable and a winch attached to the rafter for lifting. It was probably similar to your folding stairs. A rotary limit switch was attached to the end of the drum axil. It could be set for multiple turns and had adjustable stops for open and closed (i.e., down). I let the cables go just a little slack in the down position to take strain off the winch.

    Just did a quick check of Grainger (where I got mine about 25 years ago) and it had 16 pages of limit switches, but only 3 rotary ones. They did not appear to be the right type. Sorry that I cannot give you a real part number, but devices are made for that application. Grainger prices were about $150.

    John

    Being a pack rat helps. The rotary limit switch was made by GE and was, I believe, a CR115E121112. The shaft had a 20:1 built in reduction; thus, I had 20 turns + or - before the limit switches activated. Different gear ratios were available
     
    Last edited: Jun 19, 2009
  5. CDRIVE

    Senior Member

    Jul 1, 2008
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    Nice find John! I didn't know that they existed.
     
  6. beenthere

    Retired Moderator

    Apr 20, 2004
    15,815
    282
    With a 1 horse motor pulling a light load, I would want at least one mechanical (current interrupting) limit switch at either extreme of travel. Garage doors have current limiters, but the open and closed position is determined by a mechanical limit switch.
     
  7. BillB3857

    Senior Member

    Feb 28, 2009
    2,400
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    I would just go out and get a garage door opener. It has adjustable limits built in to accommodate various height doors. If the wheel has already been invented, why not use it?
     
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