Problems amps and powering a motor

Discussion in 'The Projects Forum' started by emc2k99, May 28, 2011.

  1. emc2k99

    Thread Starter New Member

    Mar 12, 2011
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    I’m working on a home project where I’m trying to build a circuit where the motor (24v 10A max at 3,000rpm) is controlled by a 555 timer (PWM). I ran the motor as a test using a wall power supply (24v 550mA). It ran well for a while until it burned the power supply out. Live and learn I suppose. I suspect I was getting some bad ‘feedback’, rather reversed voltage, from the motor and should’ve used a diode to protect the power supply. But I’m not exactly sure. I’m very uneasy with working with such high voltages especially at my experience level. What I’d like to do is run the motor off say 12 or so volts but raise the amps. I’ve read that, in simple terms, voltage is more closely related to the speed at which the motor will turn, while amps are related to the torque. How could I raise the amps without going crazy with the voltage? I would imagine somehow charging capacitors in parallel from a single source. Then “pulsing” the caps output to the motor… Could someone point me in the right direction?
    Thanks in advance!
    P.S. I’ve drawn a schematic of a simple pwm circuit that I’ve actually tested, it used 9v DC fed to the 555 which controlled an LED and small motor. But the schematic isn’t very different from anything you may have seen online. I can still post if someone wants me to…

    PPS: I'm not on here expecting to just "get all the answers". I've never been opposed to doing my own homework. I certainly appreciate any help, though, and am only posting here because my own research isn't getting me very far. Again...thanks.
     
  2. Adjuster

    Well-Known Member

    Dec 26, 2010
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    The first thing that occurs to me is that your motor requires a power supply properly rated to supply the necessary current. Trying to operate it from a supply rated at almost twenty times less than its maximum demand is not a good idea. Back-emf may or may not have been relevant to the power unit failure, but clearly in any case you will need a much higher rated supply.

    It is true that for some types of motor the speed is related roughly to the voltage, while the current is more dependant on the torque, but these are not independent variables. You cannot sensibly speak of "raising the amps" as if the current can be determined separately from the voltage. What you may be able to do is to operate the motor at some relatively low voltage, and to set the mechanical loading conditions so that a given current level will be achieved.
     
  3. emc2k99

    Thread Starter New Member

    Mar 12, 2011
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    Thank you for the quick reply. :)
    I probably should've noted that I understand ohms law in that I is directly related to V and inversely to R. It's because of this that I'm having such trouble with understanding how to supply the motor with the proper power.

    when you said:

    " What you may be able to do is to operate the motor at some relatively low voltage, and to set the mechanical loading conditions so that a given current level will be achieved."

    It was rated for 24v but I was hoping to operate it at 12 or so volts. However, I don't understand what you mean by "set the mechanical loading conditions so that a given current level will be achieved" Are there any links to something I can read online that would describe that further?
    I've been looking for books that will explain the electromechanical factors and formulae of dc motors further than what something like howstuffworks.com supplies. But I couldn't find anything.

    So i guess what I need to know (above all else right now) is how do I get 10 amps to that motor? What's the best way?

    Oh... BTW here's a link to the motor: http://www.allelectronics.com/make-.../24-VDC-135W-MOTOR-BELT-DRIVE-SPROCKET/1.html
     
    Last edited: May 28, 2011
  4. BillB3857

    Senior Member

    Feb 28, 2009
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    Current provides torque. If you don't supply enough current, the motor can't supply enough torque to run properly. When it tries to draw more current than the power supply can give, it will burn out the power supply. Ohms law will apply at stall conditions (motor shaft not being allowed to turn) Otherwise, current draw is primarily the result of load on the motor. (Remember, current develops torque). As a DC motor comes up to speed, it actually begins to act as a generator, producing a voltage that tends to oppose the source, hence called Counter EMF. As this voltage rises, current will drop to a level that is required to maintain enough torque to overcome losses such as bearing friction, air motion (fan loading if fan equipped), as well as any other loading.
    As an answer to your question as to how to get 10A, have you thought about using a lead/acid battery? One used for lawn mowers may work for you along with a small battery charger.
     
  5. emc2k99

    Thread Starter New Member

    Mar 12, 2011
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    Unfortunately, using batteries isn't an option. It has to use a power supply. Is there no way to use this particular motor with a power supply? Should I try a smaller motor with less of an amp requirement? I found this link online yesterday: Build A 10 Amp 13.8 Volt Power Supply is this feasible? I've never been able to find anything close to a 10amp transformer, much less anything over 4 volts that wasn't insanely expensive.
     
  6. GetDeviceInfo

    Senior Member

    Jun 7, 2009
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    at the power level you'd like to work with, lead acid batterys are common, cheap, and in abundance. Alternatively, AC/DC converters would be employed, but not something you'd play without experience.

    Why are batteries not an option when motor size is? Sounds like homework or a missapplication.
     
  7. shortbus

    AAC Fanatic!

    Sep 30, 2009
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  8. Adjuster

    Well-Known Member

    Dec 26, 2010
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    Motors are not resistors, and don't behave as such, except that as someone has mentioned a stalled DC motor approximates to a low resistance. When I suggested adjusting the mechanical load, I meant to change the torque required of the motor, e.g. by using gearing. That would of course also affect the final operating speed.

    What you really need to have to estimate the input power are are at least approximate figures for the mechanical power requirement, and the efficiency of your motor. Suppose you needed 140W of mechanical effort, and your motor was 70% efficient. Your electrical input would be 140W/0.7 = 200W. At 24V this would require 200W/24V = 8.33A.

    Unless you have a big enough power supply to cope with the demand, you have no chance of getting this to work. Forget about small "wall-warts" designed for only a few watts. You are looking for something much heavier duty - and a large(ish) mains supply is not really a project for a novice.

    I wonder if a 12V supply from a defunct computer would be any good?
     
  9. emc2k99

    Thread Starter New Member

    Mar 12, 2011
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    Misapplication perhaps, but i'm definitely not trying to get someone to do my homework for me. The problem is, i'm learning on my own. If I were in a class, I'm sure I wouldn't be having to ask this in a forum. I'm kind of alone and sorta in the dark. If you're talking about ac/dc converter...I can put one together, but I have yet to come to an understanding of how to provide the proper amperage.

    Off the top of my head: I could buy a transformer that's rated for 14v at 5amps, feed that through a bridge rectifier to the caps and 7812 voltage regulator sitting against a large heatsink which will provide power to the 555 timer outputting to transistors etc... I had the idea of charging an array of parallel lower voltage capacitors then control their output via pin 3 of the 555 and the transistors. My (untested) theory is that, though the power supply will only output 5 amps max, I could multiply that by pulsing the output of the capacitors. These pulses wouldn't need to last longer than a 50% duty cycle of the PWM. I'm thinking I could run the motor at a max of half-speed, still providing 24v, from the capacitors at a higher amount of current. The capacitors can charge when the PWM output is set to off. I fully understand that there's a trade off in that you can't just get amps from nowhere...Also at the moment, I don't know if I could even get enough amps from parallel capacitors...I have a ton of math to do, I just want to know generally what direction to take. If you looked at my original post, i mentioned (albeit in less detail) the idea of using capacitors...Am I definitely heading in the wrong direction?

    Basically, all I need is to find a way for the motor to output enough force while being able to do so continuously without having to worry about replacing or recharging batteries. I understand I'm asking for something that isn't very simple. I appreciate all the replies I've gotten so far, but if it's too much to answer in a forum, please simply suggest a book I can buy or an article I can read and I'll put the pieces together myself. I should point out that while waiting for responses, I've been searching online for more information so I'm not just sitting around asking you all to solve this for me, just please point me to what I need to read. I'll figure it out.
    Thanks again.
     
  10. emc2k99

    Thread Starter New Member

    Mar 12, 2011
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    @Shortbus Thanks for the link! Great info there. And here I thought I was damn good at google searching, I have no idea where they came across those links, I was only able to find such things at sites like allelectronics.com, digikey.com,mouser.com and still couldn't find what I was looking for.

    Also, @Adjuster Thank you for that explanation. It was very clear. As for gearing, the ratio may be 1:4 where the latter is the former in a second 1:4 set. Effectively dropping 3000 rpm down to 187.5rpm.

    I'd like to take this opportunity for me and those who may be looking for something similar in the future to ask for some links to valuable info, or the names to some books dealing with dc motors or electromechanics in general... I have bought and read Electronics for Computer Technology (so far 3:4 through) and Make: Electronics by Charles Platt. If anyone can suggest something I'd be forever grateful.
     
  11. emc2k99

    Thread Starter New Member

    Mar 12, 2011
    10
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    I found this link http://www.powersupplydepot.com/Power_Supplies/Transformers/24V_CENTER_TAP_%2812-0-12%29_10A_TRANSFORMER_7846_TR.asp
    The price isn't too bad either. Again, shortbus, thanks! Adjuster, there's no way of me knowing the efficiency of the motor I'm using because I bought it from allelectronics, however it's sold by an electric scooter company and unfortunately, i haven't been able to find any sort of datasheet for it. The only info I have is that which is on the page I brought if from (link above). BTW, I can forget about the capacitors...I may play around with that Idea in the future when I get some time.
     
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