Homework Help: Torque calculation to determine motor size

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1. May 14, 2015

Matthew Titus

< Mentor Note -- thread moved to HH from the technical engineering forums, so no HH Template is shown >

Hi guy's this is my first post so forgive me if i leave anything out.

I'm designing a solar tracking device that needs to be mounted on a roof and i need help calculating the torque required to rotate the assembly.

i know i should use the formula T=Iα. I could get the inertia values from my inventor CAD drawings but i have no idea which alpha to use. also i have a wind loading of 1.4 MPa how would include this in my torque calculations.

any help would be appreciated thanks

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2. May 14, 2015

Staff: Mentor

α will be tiny, the device just has to do one rotation per day. Wind load will probably determine the maximal torque, with gravity as additional influence to take into account.

1.4 MPa does not look reasonable, that is 14 times the atmospheric pressure.

3. May 14, 2015

Hesch

Tracking the sun, you must use a very low gearing ratio, so that the torque needed is due to friction in the gear. You don't want your solar panel to be whirling around by 1500 rpm?

Some large telescopes uses 800W motors when tracking the stars ( using maybe 100W due to low speed ).

4. May 14, 2015

Matthew Titus

i used the formula P = 0.5xpxv^2 which gave me an answer of 470kPa and then multiplied it by 3 as per the standards one of my lectures gave me. there are so many factors that they consider in the standards so i just took the basic one as it is not really needed for my project. so would i be over designing if i use such a large safety factor?

5. May 14, 2015

George Zucas

You need to determine two types of power requirement, static and dynamic powers. For the static power, you need to include friction, gravitational forces, and in your case the wind resistance for example. Dynamic power is required for acceleration. Assuming you don't need much acceleration (maybe you do), the static power requirement will dominate as mfb said.

I have a few formulae that pretty accurately approximate the power requirement in this case but before that maybe more experienced engineers and science people will teach us new things so I won't give them to you :)

6. May 14, 2015

Matthew Titus

haha nope certainly not. i haven't done any gear ratio calculations as of yet because i need the power input from my motor in order to decide on a gear material etc. so u would suggest using something close to 100W would be sufficient enough?

sorry for being such a noob i have no idea whats going on in motors

7. May 14, 2015

Matthew Titus

haha anything would do for now. i thought i just needed to calculate the inertia torque and torque due to wind and gravitation and then from there calculate the power if i have the rotational speed.

8. May 14, 2015

Hesch

Yes, for sure it will. But my point is, that the friction in the gear must be regarded as for ( 95% of ) the power needed.

And incidentally I think that a large telescope is bigger, heavier, more sensitive to wind, and so on, than your solar panel.

9. May 14, 2015

SteamKing

Staff Emeritus
The stagnation pressure formula is P = (1/2) ρ V2, where:
P is the pressure in pascals
ρ is the density of air in kg / m3, and
V is the wind velocity in m/s

At sea level and a temperature of 15 °C, ρ = 1.225 kg / m3 {That is a little over 1 kg / m3}

Working back from P = 470 kPa, the wind velocity would have to be approximately V = 875 m/s!

The speed of sound is only about 340 m/s.

You're trying to design the world's first solar tracking device capable of operating in winds of Mach 2.5!

Are you sure you didn't use the density of water instead of the density of air in calculating your wind pressure?

10. May 14, 2015

Staff: Mentor

A solar tracker for supersonic airplanes!

Also, you have to convert that pressure to a torque.

11. May 15, 2015

Matthew Titus

haha yeah sorry i used 1200 instead of 1.2

12. May 16, 2015

Matthew Titus

can u please help me out with converting the pressure into torque.will the pressure act as a UDL on my panel?
i have assumed that it does and that it will give me a point load in the centre of the panel.however this is where i get confused because T=Fr and if the force is in the centre then there is no distance to multiply with.

im only doing applied strenghts of materials next semester so my knowledge is not all that great as i am doing very basic beams at the moment

13. May 16, 2015

Staff: Mentor

A constant force over a symmetric shape won't give a torque, right. The worst case would be full wind load on one side and no load on the opposite side. That is highly unrealistic, but it can serve as upper bound. I guess a better estimate would need some model how wind flows around your structure.

14. May 17, 2015

CWatters

Pretty difficult if you have several rows of panels. The flow over the front row might mean it's turbulent when it hits the back row?

I think I would cheat and look at what other tracking systems use, build one and test it?

15. May 17, 2015

Staff: Mentor

Well, you can beat everything with safety factors. On the other hand, tracking the sun is nothing completely new, there should be tons of examples around.