View Full Version : need help regarding DC motor

06-02-2007, 04:34 PM
I'm having trouble with my DC motor when the motor is under heavy load. The motor wont stop eventhough the LOW signal is sent to the transistor to shut the relay controlling the DC motor.

The DC motor is 12V planetary gearhead with 500:1 reduction and the relay is rated at 12V 10A.

Can somebody explain to me what on earth the motor doesnt want to stop? And why the relay contacts still intact?Draw to0 much current?

I noticed that many of my teamates having different speed at both of the DC motors driving the robot's wheels. Something to do with bad power distribution? Is there any solution to this? Thanks....

p/s: i'm not from E&E major....

06-03-2007, 01:17 AM
C. D. 8!!!
You have the following problems in hand::
1. Your neglect in posting the actual circuit.
2. DC motors and any other inductive loads have a specific characteristic of producing back emf.,
3. This back emf is the result of varying magnetic field cutting across a conductor( primarily the inductor itself!!!) and developing a emf with opposing polarity to the source voltage.
4. This 'back emf' will travel all around the circuit ( and its individual elements) via the power line, ground etc,. whatever is common in the specified circuit. This leads the transistor to switch intermittantly, depending upon the component layout, lead lengths, Battery condition etc.,
5. All motors have inertia, like any other object.( it will take some time for the motor to start oir stop, even after switching ON or OFF the DC supply)
6. During the 'inertial period' , the Motor will become a producer of numerous signals ( back-emf); this will be interpreted by the transistor as a "relevant" ON signal, resulting in the motor being ON even after the input to the transistor is shut OFF!!! The result is that the transistor will be ON continuously.
7. This phenomrena can be eliminated by suitable By-pass capacitors sufficient to "filter out" these signals.

It wiill be easi6y to debug your circuit and any shortfalls if you can post the Circuit......

06-03-2007, 07:58 AM
Thanks Docel for detail explaination, for the moment I cannot obtain the circuit diagram and post it in here. I will do once I got my laptop back.

I noticed that the motor's by-pass caps ( don't know value ). I'm checking out later. By the way you explain the circumstances of having back emf truly freaking me out because I only aware that relay produces back emf (I'm sucked at electronics) and totally ignored other factors. Surely I need more reading on electronic basics.

And I'm a bit curious, you all familiar with power window DC motor? Here it has good reputations for being dirt cheap, high torque, durable and low inertia ( for me i dont see any inertia at all, like motor braking)

nway, I'm from Malaysia and I'm majoring in Manufacturing Engineering.Nice to meet you all.....

06-03-2007, 02:56 PM
OK, welcome to the forum!
1. Power window or ANY motor has inertia! It is not visible to you, unless you connect a CRO and monitor the 'ringing' ( damped oscillations). it is more with geared motors: a 1/5th turn of your present motor shaft and wheel represents 1/5 X 500 RPM at the motor end!!!
2. Anything that has a substantial 'turns' of wire as a coil will produce a back emf. Also, the back emf can be several times the source voltage :!: This dpeds on the switch-off time.
3. A FreeWheeling diode should be connected, in parallel with the Relay, and in reverse bias. to eliminate back emf.
4. It is not clear how you are driving the Reley and other components used....
5. NO two motors run with the same speed :!: . You have to be Very lucky if they do.
Connect a low value resisto made from solder or nichrome wire in series with the fast motor and adjust the value by trail for equal speed.
Good luck :D

08-12-2007, 08:38 AM
Or just use a potentiometer on each motor to adjust the speeds exactly.

08-14-2007, 01:11 AM
Or just use a potentiometer on each motor to adjust the speeds exactly.

:evil: :?: :?: :?: