What size motor do i need?

[Mtronics]

Hi, i have to spec a motor, what i need to know is how do i figure out there torque required? it is a vertical shaft, bearing a load of 4kg, load diamter is 100mm. motor run time per cycle is 30 sec, so it needs to start quick, and stop quick. Im limted to a 750W input supply. length of shaft approx 100mm. any help would be great

 

[electricpete]

Others can answer better I'm sure, but some clarifications are in order.

The gravity weight of 4kg acts at a torque radius of 100mm, correct?
During this 30 seconds, do you expect roughly constant speed or roughly constant acceleration or something else? If constant speed, how long to reach the target speed? How long the total distance travelled in 30 seconds?


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(2B)+(2B)' ?

 

[Mtronics]

yeah, say an rpm of aprrox 60rpm. as it will only have about 30 seconds for the cycle it needs to be up to spped relatively quickly. i'll be using a bevel gearbox as to allow vertical rotation and support to load. imwondering can i figure out the torque required just knowing the load mass(4kg) and radius of the load(50mm)? as in T=FxD (force by distance or is it diameter?)

 

[desertfox]

Yes its force X radius however your torque will vary from zero when the load is at 6 or 12'oclock to a maximum at 3 and 9'oclock as it rotates.
You need to consider acceleration time for the motor to drive load at required velocity.

 

[electricpete]

yeah, say an rpm of aprrox 60rpm

Are you planning on using gears (maybe a motor with built-in gears)?

How often does the cycle repeat?



=====================================
(2B)+(2B)' ?

 

[electricpete]

I'm sure you could do this, but a simplistic calculation is
Mass = 4kg
Weight = Mass*g = 39.2N
R = 0.1M
Torque = Weight*R = 3.92 N-m
Power = 2*Pi*Torque*Speed = 24.6 watts (using speed = 60rpm).

That is the just the power required to keep the weight moving at a constant speed (the easy part). Need to think about how much additional torque you need available to get proper acceleration as mentioned and consider duty cycle and other factors to come up with good solution. I'm not the right guy to select this motor.


=====================================
(2B)+(2B)' ?

 

[Mtronics]

30 second cycles continuously, ideally upto speed in a second or so,60 rpm,built in gears will be the preffered option, the load force will be a constant, the part being placed is negligible. yeah its ensuring i have enough torque for start up i need to know. also,what about stopping?