DC Motor Speed

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bzblues

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Hello!

I've build a DC Motor speed control with a NPN transistor. It works fine, but the problem is that when I turn the potentiometer up, the motor doesn't start until half way and, to slow down the speed I turn the knob back very slowly, but at certain low speed the motor stops. So, the question is, how can I make the speed control more accurate ? should I solder a resistor to the motor lugs? Or maybe use another kind op potentiometer? I don't need the motor going super fast, actually all the contrary, I would like to it to mantain a certain low speed.

Thank you!
 

If your speed control circuit provides rather constant current than constant voltage, the motor is expected to stop at low speed. An optimal speed controller has a negative output resistance, compensating the armature resistance to achieve almost load torque independent speed. Or even better, use a speed sensor to control the speed in a closed control loop.
 

Post your schematic please.

What you are experiencing is the classic problem of a motor needing more 'push' to get it started than when it is running and also needing a certain amount of power supplied to it to turn at all. Additionally, the motor power isn't directly related to the current flowing through it and the transistor/control do not control the current linearly.

The only real way to get absolute speed control is to have a feedback mechanism that monitors the speed and controls the motor voltage dynamically to ensure it tracks the setting you selected - it can get quite complicated!

Brian.
 

Hi,

bzblues said:
I've build a DC Motor speed control with a NPN transistor.
Click to expand...
This can be
* linear constant voltage
* linear constant current
* linear soemthing inbetween
* PWMd slow decay
* PWMd fast deacy
* ...

All depends on your circuit. Thus: Show your schematic and give informations.

Klaus
 

I presume your motor is a DC series type universal motor, the kind often used in the electric drills, mixers and the like. Their speed control is most commonly a linear voltage but this is the cheapest and the worst solution.

As others have rightly pointed out, to have a meaningful speed control, you need a feedback mechanisms. You need to monitor both the current and voltage. The process can become complex if the load is also variable.
 

Hello everyone! I forgot to tell that I'm really new in electronics, veeery new! so I still don't know how to read schemantics propperly.

I just watched some youtube tutorials and managed to get one done.

I'll post a wiring drawing so you can see what its about. I saw on the tutorial video that there is a ceramic disc capacitor of 104 (gueesing thats pF) soldered beetwin the motor lugs, but I don't see that makes any difference. It's all powered by a 9V battery.

Thank you!
 

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The circuit corresponds to the expectations in post #2, (almost) constant current control. Could be changed to almost constant voltage by using this circuit, possible lower pot resistance required (low kohm range).



No doubt that it's a PMDC motor (constant excitation, speed ~ applied voltage).
 
Hi! thank you! So, I'll try it, you say that this wiring would be better to manage the speed of the motor?
 

Awesome! I'll try it then and let you know how it came out! thank you!
 

What you would like, is a closed loop controller
which adds I*R from drag and load torque, to
the V=k*speed term. Not all of it (which would
make speed increase with load) but maybe 95%
of the I*R motor-voltage term.
 

dick_freebird said:
What you would like, is a closed loop controller
which adds I*R from drag and load torque, to
the V=k*speed term. Not all of it (which would
make speed increase with load) but maybe 95%
of the I*R motor-voltage term.
Click to expand...
Yes, see optimal controller sugegstions in post #2. Howewer, switching from almost constant current supply to constant voltage supply will considerably extent the controllable speed range. The OP didn't yet specify speed constancy requirements.
--- Updated ---

I also guess that the design of constant speed controller might exceed the scope of this project.

Here's a controller with armature resistance compensation for a DC-micromotor with about 100 ohm terminal resistance. Could be scaled to larger motors.

 
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