VEX 393 Motor Question

You are on the right track here. R_M is NOT the motor coil resistance, which you can measure with an ohmmeter (and should be 1.5 Ω). R_M is not a real resistance but rather just a stand-in representation for V_M/I.

Inductance and magnetism do play a part if you want to really analyze the theory and get a fully theoretical P_M. However, you can still do a lot with just Ohm’s Law as long as you are mindful of quantities you can only determine experimentally.

Very good questions!


Well…yes… but you don’t get to control the current. One of the simplest, but hardest, concepts in electronics to explain is that Ohm’s law rules everything. If you hold the voltage constant, the current will be based on the “resistance” that the motor has…but for motors it really isn’t resistance (at least not exactly like a resistor) because we’re dealing with a inductive (a magnetic coil of wire) based system.

Back to the motor: it will draw more and more current as the load increases on the motor, to the theoretical point of “infinite current” when it stalls, but physically that’s not possible.

(trying to keep it on the pre-college/EE level)…


I use a combination of Inkscape and LaTeX. It’s the only solution (only free solution, anyway) I have found that offers the flexibility I need for effectively modeling more complex circuits.

You can get the source SVG and LaTeX files I used from the attached Zip file at the bottom of my first post. In Inkscape, File > Save a Copy > PDF format > Omit text in PDF and create LaTeX file. Then include the exported LaTeX file in a LaTeX document as shown (you can certainly make it nicer, but I was only doing it to take screenshots).

There are free online circuit designers that you can use to still make pretty nice diagrams, but a lot more easily. They will definitely serve your purposes just fine for this experiment :slightly_smiling_face:


No dude, you’re not at all being rude. The more I look into this, the more I see the many ways this experiment could fail. I realize that what I’m trying to do is too advanced for me and I should aim for something a little simpler.

I’m submitting my proposal by midnight. I really like your idea. I have a homework assignment to do, but afterward I will draw up a new procedure with your idea. Would you mind looking it over for me at about 7 pm?

Yeah, sure. 7 PM what time zone?

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For those interested in more details on the 393 and (now discontinued) 269 motors, we dug into this in depth several years ago. The information is spread over several topics and a bit disjointed due to the nature of the forum, but a good place to start might be here which includes links to some of the earlier topics.


Oh, sorry, didn’t see this. 7 PM ET, so 15 minutes from now.

Ok, so this is what I’m thinking:

  1. Using a breadboard and jumper wires to make electrical connections, connect a winch that is powered by a single DC motor in series with a voltage source and an ammeter.
  2. With no mass attached to the end of the winch, turn on the voltage source and record both the current displayed by the ammeter and how long it takes for the winch to roll up a designated length of string.
  3. Reverse the polarity of the motor on the breadboard to unwind the winch and bring it back to starting position.
  4. Perform steps 2-3 19 more times (a total of 20) to reduce error.
  5. Tie an x gram weight onto the end of the string, and repeat steps 2-4 so it has been tested 20 times.
  6. Repeat steps 2-5 so no mass, an x gram mass, y gram mass, and z gram mass have all been tested 20 times each.

Also, quick question: So you’re saying that increasing the mass the winch has to lift will change the current travelling through the motor?

Breadboard is probably fine, but technically it’s not really advised to use a breadboard with anything high current like this. If possible, just use alligator clips or something to connect stuff.

Personally I’d say go with more data points and fewer runs at each point, doing it 20 times is just excessive, and you’ll get a clearer idea of the curve with more data points for different weights. 3 runs is probably fine, I’m lazy and would probably just do one.

Make sure your weight range represents a good range of value from no load to nearing the stall torque (1.67 Nm with the default gear ratio) of the motor to get the most comprehensive data, Not sure if you know the physics regarding torque or not.

And yes, as you increase the amount of power the motor has to use (in this case, increasing the weight it’s lifting), current increases assuming you keep voltage constant. This continues to go up until you reach stall torque, at which point the motors stop being able to move, and the current is at its highest.

To use a very imperfect metaphor, you try to push a light box it doesn’t take much effort (current). The box gets heavier, you’re working harder. Eventually it’s so heavy you’re pushing with all the strength your muscles have and the box still isn’t moving. The box could be 500 pounds or 5 million, either way at this point you’re using the same amount of effort and getting equally nowhere. Don’t read too much into this metaphor in trying to understand the relationship between current resistance and voltage, it’s not great.


No, it’s cool man, I get the picture lol.

Also, I apologize for asking you so many questions. I just have one more and I should be good.

Unfortunately no, I’m not too well versed in torque. I’m currently taking AP Physics 1, and we’re on Unit 3 which is Circular Motion and Gravitation. Torque and Rotational Motion doesn’t come up until Unit 7.

I looked up another post on vex forum regarding torque. To quote someone else,

"Torque is measured in inch-pounds. 1 inch-pound is the amount of torque required to lift a 1 pound weight that is 1 inch from the axis of rotation of the motor output. So if you had a motor that supplied a torque of 1 inch-pound, and you put a wheel on it with a radius of 1 inch, the motor would be “pushing” through the wheel with a force of 1 pound.

The maximum torque of a 269 is 8.6 inch-pounds and the maximum torque of a 393 is 13.5 inch-pounds."

If my winch is lifting something a foot or so away, I’m not sure how this would affect the torque.

I found a hanging mass set on Amazon. I can add up to 10 g masses onto it, meaning I can have 10 data points (10 g to 100 g).


Do you think these are still a bit too close together to see any noticeable difference? If so, do you think I should go for something like 100 g - 1 kg? I’m not sure what the limit is here until I reach stall torque.

OK, basic physics lesson. If you’re already in physics, some of this should be review.

F=MA. F is force, M is mass of the thing you’re lifting in kg, A is acceleration due to gravity. We’re gonna say acceleration is 10 m/s^2 because it’s actually 9.8 and 10 is easier.

So if you have a 100g mass, 0.1kg * 10 m/s^2 gives you a force of 1N.

So you a motor and let’s say the stall torque is 1 Nm. The units there are Newton meters. What that basically means is that 1 = Newtons of force * meters from the center of rotation (motor shaft). I.e you have a pulley 1 meter in radius on a motor. So, if you try to pull up your 100 gram weight on a motor with 1 Nm of torque with a pulley with a wheel a meter in radius or greater, it will stall. Any less than that and it’ll be able to pull it up.

Alternately, if you have a a 200g weight, now your radius needs to less than 0.5 meters, because you’ve doubled the number of Newtons of force you need, so you have to halve the number of meters.

Now, if your pulley has a 10ft rope or a 5 inch rope hanging down, doesn’t matter (we’re ignoring the weight of the rope), all that matters is the radius of the pulley wheels, and the weight of the thing being lifted.

Did that make sense?


Also, inch pounds and Newton meters are different units that measure the same thing, torque. Except inch pounds uses imperial which is awful never ever use imperial. (Newtons and pounds are both measures of force, inches and meters are both measures of distance)


Oh ok, so if a 393 motor has a stall torque of 1.67 Nm, then I should use 10g - 100 kg masses, as the greatest mass I’m lifting, 100 g, will take 1 Newton to lift 1 meter, which is less than the stall torque of 1.67 Nm. But if I were to use the 100 g - 1 kg masses, then the winch wouldn’t be able to lift anything greater than 1.67 Nm. The 200 g mass would take 2 N to lift 1 m, the 300 g mass 3 N to lift 1 m, and so on.

Thank you so much, man. You quite literally saved my science fair project from being a disaster. I think it’s really starting to come together now.

I think you might still be misunderstanding torque. It’s not a matter of how high it can lift it, it’s a matter of the radius of the pulley (so, wheel or whatever) attached to the motor.

Imagine in this picture that instead of two ropes, there’s just the one going down, and the wheel is hooked to the 393 motor. The distance you’re measuring is the distance between that center dot and the outside of the circle (technically a little bit inside, it’s the distance between the center of the circle and where the rope is coiled around)



I have prepared a high quality diagram.

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I already built the winch I’m using for a project for my engineering class a couple of months back. This is what the pulley looks like:


The pulley itself is just a shaft. Let’s say, for convenience sake, the radius is 0.1 meters. So, using Newtons acting on a 100 g hanging mass due to gravitational force times the radius of the pulley, I get 0.1 Nm.


So I should use greater masses, depending on the width of the shaft, to maximize the spread of data points and for me to see a visual difference depending on the weight of the mass I’m using.

Am I on the right track here, or is there something else I’m missing?


Do you have any way of making an actual pulley? The radius of that pulley is not 0.1 meters, it seems to be more like 0.003m. So you’re going to have a hard time nearing stall torque that way.

EDIT: not sure I already said this, the stall torque on these motors is 1.67 Nm, you’re gonna want like… 1.3 ish to get a good range while still being sure you’re not going to stall it.


Good point. Something to consider. I know VEX makes pulleys but they’re still way too small for my liking. I could look into constructing one on my own. I could make the radius large enough so I wouldn’t have to worry about getting too many weights.

You also bring up a good point on stall torque. I’ll try my best to vary the data points as far as possible.

Just to throw one more thing into your experiment: you already know that power (watts) is Volts x Amps. But with your pulley setup and a stopwatch, you could also measure power by how fast your motor is able to lift the weight over a given distance. You could then compare your electically measured power by your mechanically calculated power. I’ll leave the formula to you…it should be in your physics book, you’ll end up with a very small fraction of a horsepower.


You can just gear it down to decrease how much input force is needed to stall it. An 84t driving a 12t gear would decrease the torque required to stall by a factor of 7

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