Spinning a motor.

Thread Starter

samh93

Joined Oct 4, 2011
36
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
 

beenthere

Joined Apr 20, 2004
15,819
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.
 

Roto

Joined Jul 13, 2011
23
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.
 

Thread Starter

samh93

Joined Oct 4, 2011
36
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
 

#12

Joined Nov 30, 2010
18,210
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.
 

Adjuster

Joined Dec 26, 2010
2,148
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!
 

Thread Starter

samh93

Joined Oct 4, 2011
36
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
 

#12

Joined Nov 30, 2010
18,210
Still havent said exactly what kind of motor. Results are: Still can't say how to control it.
 

Audioguru

Joined Dec 20, 2007
11,249
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.
 

wayneh

Joined Sep 9, 2010
16,384
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.
 

Audioguru

Joined Dec 20, 2007
11,249
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.
 

Thread Starter

samh93

Joined Oct 4, 2011
36
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
 

Attachments

#12

Joined Nov 30, 2010
18,210
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.
 

wayneh

Joined Sep 9, 2010
16,384
Why are there 3 battery connections??
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.
 

Thread Starter

samh93

Joined Oct 4, 2011
36
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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