Out of complete curiosity, I was wondering how much weight can a low speed motor Iift on a 1:1 ratio? What about high and turbo?
Per VEX specs:
Stock HS TS
14.78 9.2 6.2
Of course we’d have to consider leverage, weight etc…
I’d under-rate these to 60% to account for friction & minimize chance of overheating. Also read limiting motor] to 90 is best bang-for-buck speed/power-wise. We use 100 because it’s more intuitive
I have no idea what you just said.
Well you were the one who asked, @jrobitai just gave the answer. The problem is there is no way to measure in just pounds, since it depends on distance from the axis of rotation and friction and stuff. That’s why the answer is in torque, not pounds. Then the second part of the response basically says that friction makes the motors have less “lifting” power than they would in a perfect world.
I’m not sure I know what you mean? What are the downsides to going over 90?
Our scissor lift uses four high-speed motors so I multiplied 6.2 by 0.6 then multiplied the product by 4 to get 14.88. Would that mean with four motors I would just need to multiply the original amount by 2.4 to get the weight it can lift on 60%?
Make sure you are thinking of Vex motors in the correct way:
Regardless of gearing, electric motors are not engines. They do not perform in the same way either. The torque of an electric motor is directly proportional to the load/resistance applied to the the motor; torque does not increase as speed drops. Torque increases as resistance increases. Setting your motors to full power will simply give your motors full voltage to run on. Yes, as resistance increases, the speed will drop, obviously, and that causes the torque and current to increase. Running your motors at a lower voltage/power value in the program will not increase your torque. While running at a lower power will most likely help the PTC or breaker to not blow, it will not increase your available torque.
@SkinnyPanda Robotics In essence, yes. To simplify the statement:
The only way to utilize the motors at a higher torque without exceeding the PTC current limit is to reduce the RPM that they operate at, allowing higher toque figures to be usable without blowing the PTC. So, in essence, the simple way to look at it is you “increase” torque by reducing speed.
Thank you for the technical clarification though, its important to remember what it actually happening with the motor.
I just didn’t want someone new to see your post and go put their mogo motors at 20 or something because they “get more torque” the sower they go. Didn’t mean to single you out.
Excellent replies all! Thanks!
Sorry my reply was short & not detailed but happy you got some quality replies.
Torque - in/lbs = the weight the motor can lift 1" from axis (divide by 12 for ft/lbs)
Stock = stock motor - HS = HighSpeed Gears - TS - Turbo Gears
(notice decrease in torque as speed increases)
So if you have a stock motor you can THEORETICALLY lift 14 lbs 1" from axle, 7 lbs 2" from the axle etc…
Under-rate to 60%
When ppl design using factory specs they ‘assume’ the specs are optimistic and assume the motor has less power. When I factor-in how prone the Cortex/Motors are to overheating (and stopping mid-match) as well as other losses (friction) my seat-of’pants tells me to count on ~60% . So I suggest multiply motor torque by 0.6 to get a ‘safe’ number for the work
As the excellent posts above show - if we ‘push’ our motors at full voltage we are losing (risking) more than we gain. The motors use Pulse Width Modulation to determine the speed. Easiest way to think of PWM is how long the motor is ‘turned on’ 0 = off, 127 = always on (-127 always one but reversed polarity) and 63.5 on half the time. So motor means the motor is on 70% of the time.
Which BTW you see a lot in real world - like when you turn radio volume >70% it usually starts to distort.
An important note: 85RPM is obviously 85% of 100 (on a stock motor gearing.) However, 85RPM is NOT the same as 85 of 127 motor power. 85RPM equates to about 108 motor power, which is especially important if using true speed. Without true speed implemented, motor controllers pretty much level off after 85 power, just becoming garbled and jumping up and down. With true speed however (because of the “linearized” power to RPM), as I mentioned, the appropriate “best number” is 108 POWER of 127, which should produce 85RPM.
How do you find the true speed of a motor? I know it’s an array, however, I don’t know what value the index represents (I think it’s RPM?), but I don’t know how to find the individual values of the array. Is it the same for all motors or is there a specific one per motor or per mechanical system (i.e. chassis, lift)?
The values in the index map requested input to the actual output of the MC29 in RPM of the motor under load. The index should be created individually for each subsystem in order to ensure the highest accuracy, however the index that 24C posted originally will work better than no implementation.
Good thread…all contributors have good knowledge. I gain a lot of knowledge here.
Correct me if I’m wrong, but aren’t the speed of the vex motors proportional to the voltage inputted? That would mean sending less speed to the motor would tell the cortex to send less voltage to the motor, making it in the end have less power.
@Paul Copioli explained it a little bit a while ago but the true thing we’re modifying with the motor values is not truly the power or speed, but the duty cycle.
@Not12G , no, the motors will not spin the same speed with the same power supplied. What they’re working against matters, too. Consider having a cart on wheels and pushing one way while a friend pushes the other way. What you’re thinking it that the cart’s motion has nothing to do with how hard your friend is pushing back against you, but the reality is that both matter.
Seeing this thread, I realize I should send VEX a message. Those are not units for torque. If you’re not doing any conversions, you’re OK. But if you start doing any significant conversions, especially between imperial and SI, you’ll get really messed up values. The unit should be in•lb (or ft•lb, N•m, etc.), not in/lb.
Sort of. The Cortex does not send less voltage to the motor, instead the motor controller turns it off and on at varying rates to achieve the range of power they have, but this is functionally the same as changing the voltage (see this and scroll down to ‘Motor Controllers’ for more info). However, it is still true that this reduces the power of the motor, so I assume all the charts above are referring to a motor at 100% output.
Is there a mesurement in grams?