# Need help in selection of motors

Hi everyone . Hope all of you will be fine.
I want to select a motor that can run 20kg load on it at 24 volts? So which motor should i select ? Kindly help with governing mathematical equations if applicable. I shall be very thankful to all of you.

Is it lifting the 20kg or just sliding it along a track? Those are totally different questions with different mathematics.

Can you draw a picture? Even a pencil sketch photographed with a phone is better than 1000 words. (Look elsewhere on this forum for the picture posting guidelines.)

We have to rotate the load continuously e.g that load is connected to my servo motor and i have to rotate that motor at an angle of 270 deg so which motor should i select? Please answer now

NOW

I understand if English isn't your first language. Asking for help "now" is considered rude.

This forum has thousands of people that need help and a smaller number of people who give help. If you want to be helped, you must provide more information than you type into Google. If all you have is a Google query then go there instead.

so... Is this axis of rotation horizontal or vertical? How fast do you want the weight to accelerate or stop? You've specified an angle but is this angle intended to be measured accurately? Do you intend to stop at intermediate points?

Can you give the distance between the weight and the center of rotation or the second moment of inertia for the whole moving mass?

Morgan s yes you are right english is not my first language if my words disturbed you then i am sorry.
The speed of the motor is 60 degrees per second and no other data has been specified to me as asked by you. But you may assume anything . Please help me with it just assume the missing data please

Post a diagram of the machine you want to make as you don't seem to be responding to some important questions.

...R

There's no way to know anything without the details - the load will have moment of inertia depending
on its size and shape, and could be eccentric from the shaft axis which could mean torque from
gravity unless the shaft is vertical.

torque to accelerate a load with a MoI of L is given by

T = L dw/dt (ie MoI times angular acceleration)

MoI is calculated by summing m r^2 over all mass elements m, r being the distance from the axis of
rotation.

Torque due to off-axis centre of mass with a horizontal axis is
T = m g r where m is total mass, g is acceleration due to gravity and r is the off-axis distance of the
centre of mass

And power a motor needs to work against a torque T is

P = w T

w = angular velocity in radians/sec
T is torque in Nm
m is mass in kg
MoI is measured in kg m^2
P is power in watts

You need to provide more details of the mechanical setup.

The critical question is "Is it lifting the weight against gravity?" Then you use the T=mgr formula. If not, then you only need to consider inertia.

If it's sometimes lifting and sometimes pushing down like it rolls along like the wheel of a car, then you've got even more calculations as there may even be braking required while the weight is moving down.

Lets consider that i want to select a motor for a radar system you might be well known with radar system and load on my motor is 20kg and voltage of motor is 28 volts speed is 60 degrees per second and i have to rotate the motor at 270 degrees. May be the problem is more clear now ad i tried

Radar, like at the air-traffic control tower? It seems odd to only need 270 degrees but anyway, that's not important.

So that's a purely vertical axis and it's never lifting the weight against gravity. That simplifies a lot of things.

If it is really radar then it's really important to measure exactly where the beam is pointing at all times. That way radar reflections can be plotted on the screen in the correct position. An absolute encoder is a necessity. I really like the MA3 encoder from US Digital but you may have difficulty getting your hands on their products outside the USA.

So the critical question becomes "How quickly must it accelerate to speed when it starts from each end of the 270 degree arc?" If you must have it up to speed within 5 degrees of starting then that's a pretty sharp acceleration. You would need feedback from the encoder or another speed sensor to be able to drive the motor at full power to accelerate and then low power to keep it turning smoothly in the middle.

Once again, if it really is radar, then you probably don't need the speed to be all that accurate - the encoder takes care of the positioning and alignment. The specification then becomes something like "Must complete one 270 sweep in 4.5 seconds." The power calculations for this will look more like calculating how much energy is required to accelerate the weight and then decellerate it again at the other end.

You said it's "continuous", meaning that it constantly sweeps back and forth 270 degrees to scan the area every few seconds until it's turned off? Then I would not try to have a motor start and stop the rotation. Make a linkage like your car windscreen wipers where a motor drives continuously in one direction and the linkage converts this into an oscillating motion. Then the friction in the linkage will be much higher than what you get from the weight being moved and that's a lot more difficult to calculate. But doing the experiment is easy.

Is this "radar" on a moving vehicle? Then you need to have extra power to allow for the fact that the rotation won't be vertical and it does have to fight gravity and vehicle accelerations.