View Full Version : how to choose dc motor ratings

08-08-2008, 11:57 AM
i am stuck in some mechanical calculations..... whatever i could remember of my school days is here... do correct it if wrong....

say i need a linear speed of 150cm/s..... if i use 15cm circumference(2.4cm radius) wheels then i need 10rps or 600 rpm....
now if the weight of my assembly (including everything) is 700gm then i need a torque of
=.7 * ??????? * 2.4
1) what about acceleration???
2) mostly i found on net that motor is rated as kgcm.... from above specification my kgcm rating is .7 * 2.4 =1.68kgcm..... does this means , the motor should be rated for 1.68kgcm at 600 rpm????
3) i find out on various online shops motors rated 12v 300mA 'X' rpm 'Y' kgcm .... where X*Y= constant=120...... for me this requirement is nearly 1000 (1.68*600)..... so i dont think they will do no matter whatever gearing ratio i use.... am i right????
4) i found on one site motors with very high rating... but their cost was around 8000/- ..... simply unaffordable..... do i have any alternative???? something below 1000bucks....
5) in all above calculation i neglected friction.... what is generally the efficiency out of ideal value after taking friction???
6) i read somewhere that the torque neede is high when we start and low thereafter.... will this effect the torque requirements????
7) is there a formula to find out torque (kgcm) rating of motor if we know its voltage , current and rpm ratings?????

this mechanics is making my head heavy...... please answer as much as you can.... some links to knowlege are most welcome.....

08-08-2008, 12:32 PM
acceleration means that of u'r roover
could u explain u project to me

08-08-2008, 01:05 PM
power= voltage *current (watts)
N speed

08-08-2008, 02:40 PM
initially i am just building the movement platform........ simple 3 tyre construction using differential drive........ i want it to have enough torque so that i can use it for different robots...... currently its a line follower, later it will be a speech controlled bot or some image follower type.....

08-08-2008, 08:56 PM
refer my earlier post


08-09-2008, 09:18 PM
Another good resource for helping with motor selection is here:


This is the main site: http://www.np.edu.sg/alpha/nbk/index.html

08-10-2008, 01:17 PM
tanks everyone......