View Full Version : DC Motor - Power Problem

11-22-2007, 12:48 AM
I just made a bump-and-go robot (hits an obstacle turns around and speeds off in a diff direction)...it worked fine on breadboard but now when i place wheels on ground it doesnt move, but works when held off the ground.

Motors - 5V, 100 rpm
Battery - Duracell 9V

What should i do to fix this problem? =/

Vyder =)

11-22-2007, 01:19 AM
The Torque of your motor is very low.

1) Try to minimize the weight of your robot
2) Go for a Higher Torque motors


Best regards,


11-22-2007, 08:50 AM
@Vyder: Pratik is right, you probably need a higher torque motor, or, it is possible that your battery is dying. Try again with a fresh battery. If your robot, still does not move, you need to replace your motors.

11-22-2007, 03:03 PM
Are the wheels mounted directly on the motor??
if so, must use a geared motor.

or, use the shaft of the motor touching the ground as the wheel ......

11-22-2007, 06:54 PM
Is a higher torque motor the only way? i was hoping to keep this project cheap...higher torque will mean more expensive (and ill have to wait till saturday to visit lamington rd again...tests at skool... =(

@pratik: any lighter and this robot will fly...ive used the lightest plastic box I cud find at crawford market...

@debu: new batteries dont fix the problem...but connecting it to battery eliminator (5A current) made the robot zip around the room...but not a practical solution =P)

@ docel: yes the wheels are damn light (plastic hub thing with black foam around it...fell in love with them at first sight...)...but wont the shaft slip?...tubing or sumthing for grip?

Wont a L293D amplify the current or sumthing?


11-22-2007, 08:33 PM
but connecting it to battery eliminator (5A current) made the robot zip around the room.

If it did, use normal AA battery!!! Novino, Nippo.... :D
series connect it to get your required voltage!!

It should work - never underestimate these old friends - even though they wont last much long.

You can use rechargeable batteries - Nicd or Nimh. economical in the long run -

9V batteries are useless to drive motors. NEVER use them to drive motors.

just curious - the motor is 100 rpm?? so it have to be geared already!! or is it??

11-22-2007, 10:49 PM
9V batteries are useless to drive motors. NEVER use them to drive motors.


@ Allbits: Thanx!...ive got some rechargeable batteries lying around...going to go try it right away...and how is a 1.5 V battery better than a 9V??...
I'm not entirely sur what "geared" means...i hvnt added gears if thats what you are asking...

Thanx again,


11-23-2007, 12:33 AM
If it draws 5A, if it "zips all over" and if it doesnt start off on low current, then it MUST be a wheels attached to the Motor direct, and not a geared motor!!!

You said 5v 100rpm, so it is certainly a geared motor. In that case, it should move a weight of around 1kg easily. If you bought it for Rs. 175/- at L road, then the current should not exceed 300ma @ 12v.

1. Allbits meant 1.5V cells in series ( a "battery" of 6 cells in series) to give 9v.
The 1.5V cells are ~1AH, where as the 9V bat is ~ 400mAH. Besides, these batteries have hi internal resistance and limit current.
2. The Nicds and Nimhs have very low internal resistance. This allows large current through the load.
Simply put, the 1,2v nicd will give better motor performance than the 1.5v cell.

3. I asked you the question: have you attached the wheels to the motor shaft?? If you cant count the no of rotations of the wheels, then it is too fast. If it runs off the ground and not on the ground then the torque ( turning power) is low. You cannot attach wheels to a dc motor without a gear box.

4. A gearbox is a set of gearwheels that are attached to the Motor shaft. The output shaft is attached to one of the geared or 'toothed' wheel. The geared combo will reduce the Motor speed by the teeth ratio and increase the torque correspondingly in the same ratio. Torque is the

For eg: a DC motor will have an average RPM ( Revolutions Per Minute) of some 3000 to 20000. The torque will be some 20gms/cms. This means the motor shaft can move or displace 20gms weight for 1cms per revolution. If you tie a weight to a long thread one end of which is fixed to the motor shaft, the thread wind over the shaft when the motor turns. This will lift the weight at some rate ( height per unit time.) This is a crude way of measuring motor torque and is NOT a scientific definition of Torque.

You can also test the torque by hand. Apply emf to the motor and try to stop the shaft by pressing with your 2 fingers. If the motor stops , it is low torque. If your finger gets burnt then it is hi-torque :lol:

5. Most cases, the shaft moves the body. The tubing will help increase the friction, as well as the torque (more or less diameter).

You have a 12v motor i think, and not of 5V. If it is rated at 12V, it will not work on load @ 5V.

11-23-2007, 10:11 AM
Just increase the supply to motor, use hbridge at high potential than your circuitry if your circuit is having any 5volt specific component .

11-24-2007, 09:39 PM
One more important factor is wheel size
Simply divide the torque by the radius of the wheel (1/2 of the diameter).
For example if using a 8 inch Diameter wheels with 1 Kg cm Torque motor then

(1 Kg cm of torque / 4 inch radius = 0.25 Kg cm of torque)

Bigger wheels = less torque

Best regards,


11-24-2007, 09:45 PM
That post was overwhelming...need time to chew over that...will post back with more questions (im sure im going to have some) =)

Thanx a lot guys for all your help!!!

Vyder =)

P.S. Ill be back!

01-22-2008, 07:36 PM
Worm gearbox is help to u to reduce speed and increase torque

aluminum worm gearbox is less in weight
and also it is with encoder.

gearbox ration is 20:1 so it can make higher torque which you are require

and size is also small

i used big gearbox for same function in SVNIT competion and NIRMA competition.

now i am going to use this gearbox in Micromouse 2008 , in techfest IIT bombay.


01-23-2008, 06:44 AM
I think this is not a motor or greabox issue.
it is a battery issue.
9V battery cant supply enough current. (usually ~300china-400mA)
(point made by Docel and Allbits.)
Thats the reason that AA cell powered car/truck toys are most common, and not 9V.

Just change the battery to -
either a 6V lead acid.
or multiple nicd cells in series to create 9v.
if your motor is 100rpm, it will take this load, and you also get benefit of recharge.
(or even multiple AA cells.)

Good Luck!

01-23-2008, 11:28 AM
1 Kg cm of torque / 4 inch radius = 0.25 Kg cm of torque


be careful with the units!!! it is a cm/inch division..
It is Kg on the right hand side, not torque.
Which simply means, the bigger the wheel radius, the less weight it can handle.

Choose the smallest diameter possible for your required speed.

01-23-2008, 01:25 PM
Yes u are right allbits all units must be in inches

Some good information about building a robot


Best regards,


01-23-2008, 02:29 PM
I didn't say all units has to be in inches.. I just meant be careful when you post equations or calculations, all units has to be the same. The dimensions should match.