Spinning a motor.

Discussion in 'The Projects Forum' started by samh93, Oct 4, 2011.

  1. samh93

    Thread Starter Member

    Oct 4, 2011
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    hey everyone, I'm creating a circuit for my college. It's a circuit with a battery that runs a motor which will eventually be an extractor fan. It has 2 lights in it to indicate when its turned on, these are in a series and have resistors of 240 ohms. The problem is that I want to have a variable speed for the motor but i can't get my head round what I need to do. I've looked at potentiometers but im not sure which one i'd need.
    This is my first circuit so any help would be greatly appreciated.

    Battery = 9.6volts 1600mAh
    LED (x2) = 2 watt
    Resistors (x2) 240 ohms

    Thanks
    Sam
     
  2. beenthere

    Retired Moderator

    Apr 20, 2004
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    The thread does not give nearly enough details to give advice to. Can you supply us with the idea you have so we might be able to see if it sounds practical.
     
  3. mcgyvr

    AAC Fanatic!

    Oct 15, 2009
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    Specs of said motor?
     
  4. Roto

    New Member

    Jul 13, 2011
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    You can use a 555 timer circuit or a small micro controller (PIC?) connected to a power transistor.
    Look up motor control circuits on the net.
     
  5. samh93

    Thread Starter Member

    Oct 4, 2011
    36
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    sorry everyone I was a bit vague about the problem im having. I've already built the circuit, the motor spins perfectly and the lights come on as I want them to. I just need a way to vary the speed, and im not sure about the specs of the motor, It's one from an old airsoft gun.

    Sam
     
  6. #12

    Expert

    Nov 30, 2010
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    If it's just a DC motor, you can use pulse width modulation created with a 555 timer chip. If it has internal circuits to change the DC to AC, you're stuck.
     
  7. Adjuster

    Well-Known Member

    Dec 26, 2010
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    Note that if running with 240 ohm series resistors from a 9.6V supply, your LEDs can't be running at anything near 2W.
    That may be just as well, they would be dazzling!
     
  8. samh93

    Thread Starter Member

    Oct 4, 2011
    36
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    I'm pretty confused here. I have a 9.6 volt battery, and the motor spins perfectly. What resistors do I need for 2x two watt light bulbs? And what variable resistor do I need? I'm kinda lost atm :s.

    Sam
     
  9. #12

    Expert

    Nov 30, 2010
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    Still havent said exactly what kind of motor. Results are: Still can't say how to control it.
     
  10. Audioguru

    New Member

    Dec 20, 2007
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    You do not control the speed of a DC electric motor with a series potentiometer. It would simply burn up. If the pot is huge enough not to burn then it would rerduce the torque of the motor so it will not start running.

    A Pulse-Width-Modulation (PWM) circuit is used to control the speed of a DC motor. It applies full power pulses. Narrow pulses cause slow speed. Wide pulses cause high speed. The torque is always high.
     
  11. wayneh

    Expert

    Sep 9, 2010
    12,128
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    Another thing you should specify is how much variation you want. The PWM approach already suggested is best if you need a wide range of adjustment and good control.

    But if you just need a modest reduction in speed, try running it at say, 6V as provided by 4 AAs or maybe 5V from a USB charger. The motor will lose torque and may have trouble starting, the lower you go in voltage. But if it this works, it'd be very easy to implement. It could be as simple as using those different power sources or putting a light bulb in series with the motor. Putting a switch in parallel with the bulb would allow you to short across it for full power to the motor, or open the switch to reduce power to the motor.
     
  12. Audioguru

    New Member

    Dec 20, 2007
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    A light bulb in series with a DC electric motor is a good way to slow the motor. The light bulb is almost a short when cold so the motor has a lot of torque to start running then the resistance of the light bulb increases as it heats which slows the motor.

    If the motor has a variable load then the light bulb will cause the speed of the motor to change a lot.
     
  13. samh93

    Thread Starter Member

    Oct 4, 2011
    36
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    Wow thanks for all the help everyone. I'm gonna go with the pwm circuit :).
    Thanks again

    Sam
     
  14. samh93

    Thread Starter Member

    Oct 4, 2011
    36
    0
    hey everyone, I've bought some components for my PWM circuit and im starting to understand it a little better. But at the moment i'm stumped. I have a picture of the circuit i've mostly built but im not sure about how to connect it to to the battery. Why are there 3 battery connections??
    Also is grounding always necessary? What exactly is grounding for?

    Thanks

    Sam
     
  15. #12

    Expert

    Nov 30, 2010
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    There are 3 battery connections because all 3 points get connected to the battery. Same with the ground connections. They all get connected to the other end of the battery.
     
  16. wayneh

    Expert

    Sep 9, 2010
    12,128
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    Because it is cleaner and simpler to draw it that way. The alternative would be to bring everything together and draw one line to the battery, but it's really no different. And in reality, you might want a fatter wire to the motor for the higher current it will carry. The other electronics will use very little current, and will operate better if they don't share the same conductor as the motor.

    I don't really understand your question about ground. Electricity requires a complete circuit before current flows. "Ground" simply identifies the common low voltage point for your circuit - the place to which all current flows. No ground, no current, same as the high side.
     
  17. samh93

    Thread Starter Member

    Oct 4, 2011
    36
    0
    Okay I've got a new 8.4v, 1500mAh battery and a new motor. Could someone please tell me what I need to set up a PWM circuit and detailed instructions or maybe diagrams to put them all togethor.
    The motor specs are:

    Supply Voltage: 8.4v
    RPM: 23100 (I think)
    Current: 1500mAh
    I'm not 100% sure on these as the only specs I found were written In Japanese.
    The motor is a universal one which I belive is ac and dc. Is PWM still possible?

    Thanks

    Sam
     
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