# Motor Design Assistance

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#### Benjamin

MODERATORS NOTE: We usually do not post homework or project type questions. This poster seems to have a simple question and does not expect the readers of the forum to do all his work. So I'm posting it.

Hi! I'm currently a senior designing a lunar rover for a project. I need help in determining what motor I should use for my project. Here's my information.

My lander is about 40,000 kg and will have 6 separate individually powered tracks. The tracks will be how the lander moves around the surface (obviously).

How do I determine what size and type of motor I should use in my design?

Thank you for your help, it is greatly appreciated.

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#### William Sturm

In this order (mostly):

Calculate peak torque requirements.

Calculate continuous (average) torque requirements and ensure proper cooling.

Calculate motor inertia to load inertia within reason (within 10:1 ratio)

Calculate maximum speed required.  (Hint: gearing affects all of the above)

Consider available supply voltage and amperage.

Consider environmental concerns (protection from atmosphere)
Possibly consider efficiency %.

Consider short term and long term cost.

Find a motor that meets these requirements and hope that it will fit in the space available.

Have fun learning...
Bill Sturm
Abbeytronics LLC

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#### M Griffin

Cooling will be a big problem. There is no atmosphere on the moon, so there is no ambient air to cool the stator or rotor. Also, lubrication will be a problem as standard oil or grease won't work in that environment. If this rover is actually supposed to be able to work on the moon you will need a custom motor design.

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#### Ben

Actually this is just a design idea. I don't need a ton of detail, just how to do the basic calculations to determine how much torque and everything the motors must deliver. If anyone has any type of motor that's currently out there that could possibly be used for this, that would be excellent. If anyone knows how to do the basic calculations for torque, as well as gear ratio stuff, that would be awesome.

Thanks so much for your help, I'm greatly appreciative.

W

#### William Sturm

Brushless DC (BLDC) servo motors are commonly used in industry, they MIGHT be a good chioce.  Danaher Motion sells a wide variety with good data sheets.

As far as the calculations, have you had some Physics classes?  If so, you should be able to work it out.  A copy of the "Machinery's Handbook" would give you some basic ideas and formulas.

Bill Sturm
Abbeytronics LLC

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#### M Griffin

The torque and gear ratio calculations are basic physics. You need to look this up in a physics book. These would be *inputs* to motor calculations, not outputs. You have to work all this out before worrying about the motor.

If you are just at the concept stage, then just assume the motor can be run at variable speeds between 0 to 3000 RPM. The weight and required performance characteristics of your rover (acceleration, hill climbing, etc.) would determine what torque and gear ratios you need. Once you've done that, *then* you try to find a motor that fits the requirement.

If you were building a real lunar rover, you would end up going back and forth through this process several times to get the best overall compromise between weight, cost, power consumption, and whatever other characteristics are important.

But for what you are asking, that is basic high school level physics. I bet you thought you would never find a use for those classes, didn't you?