DC Motor voltage reduction

Discussion in 'The Projects Forum' started by snizbatch, Jul 16, 2012.

  1. snizbatch

    Thread Starter New Member

    Jul 14, 2012
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    Hello all, I understand just enough about electronics to make some pretty simple mistakes, so I'd like some quick input on a project before I purchase some resistors.

    Basis:
    My brother-in-law's water softener stopped working. The valve actuation motor-a 24V motor as far as I can tell-is fried. The casing became corroded and rusted solid when the house sat for a year without use before they bought it. The motor is a MOLON motor in a gear housing that costs $60 to replace. I found an IDENTICAL motor in all respects including mounting holes at Radio Shack except for the voltage rating is 9-18V vs the 24 supplied by the water softener. So, I did a lot of reading and even got a Circuit testing app to try to understand what I am attempting to tackle in converting the 24V down to 12V to run this motor.

    My research:
    I've learned that by creating a voltage divider with two identical resistors I can tap off half the voltage for the motor. My trouble is in choosing the wattage rating and proper value resistor to dissipate some of the voltage. The way I calculate it, two 20 ohm, 20 watt resistors will do the trick, but I'm not 100% certain and don't want to mail order them if they won't work. I made the mistake of putting two 1/4 Watt 20 ohm resistors together across a 12 volt gel-cell battery that I'm using for testing and blowing one of them out. My understanding of the voltage/current/supplied amperage vs drawn amperage relationship has a few holes but I'm really trying to learn through this. If anyone can offer some advice or assistance I would be appreciative.

    Motor: 9-18VDC motor from radio shack rated at 1.98A max
    Water softener supply voltage 24V, current unknown because it gives an error and won't function without the motor--and I can't run the new motor without knowing how to step the voltage down!

    Thanks in advance,
    Jim
     
  2. strantor

    AAC Fanatic!

    Oct 3, 2010
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    A voltage divider is not good for a power application. You need a voltage regulator or a new power supply.
     
  3. wayneh

    Expert

    Sep 9, 2010
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    First I'd look around more to see if you can find the right motor. I had to replace mine a while back and there's a wide range of prices out there. It helps if you can find the part number and search on that. MANY brands use the same parts, and once you figure out the cross references, you have a wider net. That all said, I paid $55 for my delivered motor and gear box. (FWIW, #7133008 from softenerparts.com)

    A resistor divider might be feasible but I can't recommend it. Your motor is not a constant resistance - it will draw more current when it starts or when it encounters a stall. At that moment, it might be just 10Ω (1.8A aT 18V DC). Supplying 24V at that moment might ruin it. While free running, the resistance might be 50Ω or more.

    And anything you use to burn off "excess" power is that much power not delivered to the motor, which it needs to turn the mechanism. I'm not sure your 12V motor - even if it is a mechanical match - will have sufficient torque.
     
  4. strantor

    AAC Fanatic!

    Oct 3, 2010
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    not the way he's thinking (I think):

    Putting a resistor in series with the motor would effectively be a voltage divider and would have a better chance of working, but it's still a kludge IMO.
     
  5. wayneh

    Expert

    Sep 9, 2010
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    Yeah, that's the only part I was considering. Adding a resistor in parallel to the motor would just be a wasteful burden on the power supply. Not a good idea either way.
     
  6. Audioguru

    New Member

    Dec 20, 2007
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    A DC motor uses a high current when it starts.

    When you reduce the voltage to a motor with a series resistor then you reduce its current so the current might not be high enough for it to start running.

    Reduce the supply voltage, not the current.
     
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  7. snizbatch

    Thread Starter New Member

    Jul 14, 2012
    3
    0
    Thank you all for your input. I'm not really sure how I'm going to tackle this one now! All good info and makes what I thought might be a simple solution seem awfully daunting! If I get it working I'll certainly post the solution I utilize.

    Jim
     
  8. shortbus

    AAC Fanatic!

    Sep 30, 2009
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    Have you taken the motor out of the valve yet? You may find that the Radio Shack motor won't even work for you. A lot of things that have a gearbox attached have the end of the shaft cut with gear teeth. The gear is actually part of the shaft, and not easy to get around using that special motor.
     
  9. crutschow

    Expert

    Mar 14, 2008
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    Another consideration is that the 24V supply may not be able to supply the additional current that the lower voltage motor will require unless you use a buck regulator to reduce the voltage.
     
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