# Engineering approach to choosing a motor, voltage and gears

I am working on a project to automate the opening and closing of my venetian blinds at home, by attaching a motor with an H-bridge to them. I started with a practical trial and error approach to it. For the motor I just got a small one I had spare, the kind you would use for an RC car, and for voltage I was connecting straight to different batteries and see what was the voltage needed to give enough power to the motor to open the blind. I didn't do any gearing to increment the torque.

After finding what I thought worked, I burnt a couple of motors, because (I think) I'm applying too much current to them.

I would like to use this opportunity to approach this project from an electrical engineering approach, the kind of thing you would do at university, where is more like a theory exercise to calculate your values in your design, and then apply it to the real world and fine tune the values to what's different from the ideal world of the theory.

I have a background in computer science and software engineering and it had a strong foundation on electronics and circuits, but nothing with motors.

I would love to know what kind of material, be it books/blogs/... would I need to study from the academic (maybe) electrical engineering world to not approach this problem blindly.

• To specify a motor you need a torque requirement, so this project begins as a a mechanical problem not an electrical one. Have you looked into ways of measuring the required torque, and have you checked this previous question on the same topic? electronics.stackexchange.com/questions/36481/… – user133493 Aug 13 '18 at 9:12
• Try to find a work-gear motor. They have less mechanical feedback. – Oldfart Aug 13 '18 at 9:28
• Hi Replete. Yes, precisely, I'm trying to start solving the problem by calculating the torque needed from the weight I need to lift, etc... rather than blindly trying or based on what worked for someone else. – palako Aug 13 '18 at 9:38
• Q: How do you make a Venetian blind? A: Throw sand in his eyes. But seriously folks ... Can you attach a lever of known length and some means of attaching mass to apply some torque and make some crude measurements? e.g., Lever, thread, paper cup | fill the cup until enough torque is generated to rotate the mechanism | weigh the water (kg) | x 9.81 gives force (N) | x lever radius (m) gives torque (Nm). – Transistor Aug 13 '18 at 9:51
• @Transistor - you have just measured the frictional components but not the inertia of the system. But a required starting point. (easier to use solid weights - say washers on a paperclip, than water in a cup) – D Duck Aug 13 '18 at 12:50

Selecting a motor for a given application requires determining the torque vs. speed characteristic curve for the driven load. How much torque is required to overcome static friction and get the load moving very slowly? How much torque is required for steady operation at any given speed up to the maximum operating speed? Those figures are determine by the friction of the driven equipment and the work done in the process. Then the inertia needs to be determined. The torque required to accelerate the load is the friction and work torque plus the torque of mass X acceleration. That is all mechanical engineering.

To select a motor, you need to determine the toque vs. speed capabilities of available motors and find one that meets or exceeds the torque vs. speed requirement of the load. You need to know how to interpret motor specification sheets and convert the data to speed vs. torque curves.

Many applications require only a means of starting and stopping the motor based on the requirements of the driven equipment. Small motors can simply be turned on and off. They will accelerate based on their capability and the requirements of the load. The load will coast to a stop when power is removed from the motor. For some applications, simply turning the motor on results in acceleration that is too stressful. Acceleration can be controlled with an electronic speed control (ESC). Some applications require speed control for process or energy saving reasons. Brushless DC motors require ESC because of their design. Brushed DC motors larger than a few hundred watts require ESC to control the starting current. AC motors may require a soft starting system or ESC depending on the capability of the source to provide starting current.

The above is a summary of the basic considerations. There are a lot of approaches to learning the details. I recommend looking to see what is available and determining what is easiest to understand.

• Thanks for your answer Charles. Can I find out those (torque to overcome static friction, torque for steady operation) via equations from the dimensions and weight of the blinds? Also, once I find out the torque, what kind of considerations would I take to get a motor that has enough torque to be connected directly, or instead use some gears between the motor axel and the blind axel? – palako Aug 14 '18 at 10:46
• You can calculate the required torque, but you need to know or be able to estimate the coefficient of friction for all of the relevant pieces. That is a mechanical engineering task. I expect there are references that would help, but that is not my area and not the area of this forum. I think a stepper motor may be able to do the job without gears, but gear-motor units are also available and may be better depending on size, price etc. If you use a gear make sure to consider friction in the gear. – Charles Cowie Aug 14 '18 at 11:18

Your motor needs to be sized such that the blinds open/close in a specified time.

The torque of the motor needs to overcome the friction in your system and the inertia. The motor needs to produce a force to accelerate all the mechanical bits and then you need to decelerate them to a stop. (remember $F=ma$) Torque is a rotary version of force, moment of inertia is the rotary version of mass.

The more force you can apply the faster it happens.

If you can wait forever, you can size the motor just bigger than the friction.

You need a mechanical equivalent of a schematic to work out the torque and moment of inertia for your system.

You need some method for your system to know when to stop accelerating and start decelerating - perhaps a pot on your blinds for feedback.

You can change the torque you apply with a set of gears etc, but this comes at a cost of speed and often makes feedback difficult as there is now backlash in your mechanical system.

Search for sizing a small servo motor

Now that you have this you need to design your control electronics and size the driving H-bridge.

• Thanks D. Duck. Would you please have a look at my comment on Charles Cowie answer? – palako Aug 14 '18 at 10:48
• @palako It's a big field - you can usually want to make friction a small part of the problem and oversize your motor. I'd would say your question is an electrical problem as I was taught this in EE school in the 1980s - in two separate courses. And you can convert the mechanical models to an electrical equivalent to solve. – D Duck Aug 15 '18 at 15:10
• I'm happy to pick up a big fat book if needed. I started this thread more to learn about the topic and the approach to the problem than rather just find the value of the motor I need. Any recommendations are super welcome. – palako Aug 16 '18 at 9:42