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Need help setting up multi potentiometers.

  1. May 20, 2012 #1
    Hello,
    I have a small $450 frequency drive running a 1/4hp 3ph motor that I am not wanting to burn up. On this drive there is a 5k pot with 5 volts across it, that controls the frequency output/speed.

    I wanting to connect two more remote mounted pots to control the speed with a switch to select which one is in control.

    I was thinking of connecting the three pots in parallel with the center legs selectable via a
    sp3t switch.

    Is the actual resistance of the pots that important since all they are doing is acting as a voltage divider?

    Do I need three 15k pots to maintain the initial 5k load of the original pot, or can I simply add 2 more 10k pots that I happen to have handy.

    Any advice and thoughts would be greatly appreciated.
     
  2. jcsd
  3. May 20, 2012 #2

    AlephZero

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    With a multi pole rotary switch you could easily switch 2 or 3 wires to each pot, so the unused pots had no effect on the circuit.

    Pots are not usually "close tolerance" design components, but without a circuit diagram of the controller you are just guessing what the effect of changing the value might be.
     
  4. May 20, 2012 #3

    vk6kro

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    You need to know the input impedance of the circuit, because this is effectively across the lower part of the pot. It will affect the operation of the pot.

    Also, the switching may be fast, but it isn't instantaneous, and damage may happen while the switching is happening.
    For a few milliseconds, the input could be open circuited. Maybe it is OK to do this, but probably it isn't.

    Maybe you could ring the manufacturer and talk to someone technical. This could be difficult if they are in China.
     
  5. May 20, 2012 #4

    AlephZero

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    You could use a make-before-break switch to get rid if that problem. But the speed controller might not take kindly to "instantaneous" changes in control voltage when you switch between controls.

    Possibly a safer way would be to replace the existing 5K pot with a 5K motorized pot, and use the remote controls to drive it. That way the controller doesn't know you changed anything.
     
  6. May 20, 2012 #5

    vk6kro

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    It would probably be OK to put a 10 uF capacitor across the input, after the switch, so that it would hold the previous voltage while switching happened.

    To be absolutely safe, you can just use three 5K pots and only switch between them when the power was off.
     
  7. May 21, 2012 #6
    Just a side note, this pot is acting as an anolog input on a digital drive. It is a user installed option. One leg of the pot is connected to the common terminal, the middle leg is connected to a programmable input and the last leg is connected to a 5V dc terminal.
    Thanks for the replies.
     
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