Torque on a DC motor spinning a wheel horizontally

[JekyllandHyde]

I need a 12v DC with appropriate torque. In my design, the axle of the motor would be pointed straight in the air and I want to affix a wheel to it and have the wheel spin flat, sort of like a pottery wheel sort of design. I know what the wheel is made of, its dimensions and approximate weight. How do I calculate the amount of Torque the DC motor needs to have to turn the wheel and spin at or near its peak listed RPM? DC motors are listed by torque numbers, so I wanted to get one strong enough

 

[metastable]

You could use an open source VESC-based controller with your 12v battery, and use perhaps a 192kv 6374 or so brushless motor which is a common rpm/volt for electric skateboards, then set the controller up for speed control mode and the controller will send whatever current is required to maintain constant speed as the mechanical load on the wheel changes. You can use a belt with 2 different size pulleys as a gear reduction to improve efficiency if the max rpms of the motor are too high for your application.

 

[JekyllandHyde]

thats a great idea, but a whole a whole complicated expensive design, I was looking for cheap small 12v brushless motor 100rpm or less that I can use a PWM motor controller with for speed modulation. that's why I asked about how to calculate torque. I know what wheel setup I want to turn, but DC motors are listed by torque rating mostly, so I need to know how to calculate how much torque I need to operate the wheel as intended and spin it as fast as I need

 

[JBA]

If your horizontal disc is fully spin balanced, once the inertia of starting it spinning is overcome, then the motor will only require enough torque to overcome the friction in the bearing(s) supporting the disc. If you use good ball or roller bearing(s) then that friction will be very low. Apart from that, the only torque required will be what is required for turning the disc against any outside drag you place on the disc.

Edit: Without knowing the intended use of the spinning disc it is not possible to describe what you need to determine the required motor torque.

 

[CWatters]

Torque determines how fast the motor can accelerate your wheel, but once up to speed the motor only has to overcome friction, air resistance and any other load applied to the wheel. So the torque requured might be modest?

To answer your question in detail you will need find a way to calculate or measure those.

 

[JekyllandHyde]

If your horizontal disc is fully spin balanced, once the inertia of starting it spinning is overcome, then the motor will only require enough torque to overcome the friction in the bearing(s) supporting the disc. If you use good ball or roller bearing(s) then that friction will be very low. Apart from that, the only torque required will be what is required for turning the disc against any outside drag you place on the disc.

Edit: Without knowing the intended use of the spinning disc it is not possible to describe what you need to determine the required motor torque.

basically I am making a spirographic drawing machine, where spinning disks will rotate and push, pull, and move rods that are connected to a pen on a large drawing surface to pull and push the pen around creating cool designs. So the motors need to spin the disks that have the rods to push the pens. nothing super crazy. I just wanted enough power to get it going quick, not super quick but enough, then to be able to adjust the system because different speeds cause different designs.

for reference, or if interested this what I am trying to make. or something like it.

 

[Tom.G]

Here is at least ball-park information on torque.

The video specifies size 17 stepper motors, and they seem to be approximately cubicle. Some Google research shows size 17 steppers with holding torque from 27oz-in to 113oz-in. The motors that are approximately cubicle have a holding torque between 36oz-in and 62oz-in. The torque rating varies depending on the how the windings are driven and somewhat with manufacturer. Stepping torques will of course be somewhat lower (roughly 10% - 25%, varies with speed)

Cheers,
Tom

 

[metastable]

With such minimal torque requirements (drawing on a spinning sheet of paper), in current control mode a BLDC motor will be spinning very near its peak rpm per volt.

Suppose your target rpm is 100rpm and you choose a 192kv BLDC motor along with a nominal 12v battery (and your mechanical load is a spinning sheet of paper).

12v * 192kv = 2300rpm

to get to your target 100rpm output you could use a 23:1 gear reduction, which sounds impractical.

--------------------------------

Another possibly better option would be to use the controller in duty cycle control, which controls the BLDC effective voltage.

Instead of 23:1 gear reduction, we use BLDC Duty Cycle Control mode:

12v * 192kv = 2300rpm

2300rpm max / 100rpm target rpm = 23 ratio

0.521739v BLDC effective voltage required = 12v / 23 ratio

0.521739v BLDC effective voltage required / 12v battery voltage = 4.347826% duty cycle control

12v * 4.347826% duty cycle control = 0.521739v BLDC effective voltage required

0.521739v BLDC effective voltage * 192kv = 100.17rpm
------------------------

Conclusions:

A 12v battery, a BLDC controller in Duty Cycle control mode at 4.347826% duty cycle (0.521739v BLDC effective), and a 192kv motor gives 100rpm.

 

[Tom.G]

4.347826% duty cycle

Umm...
That motor is going to be coasting during the 95% off-time and may even be back driven by the other motor.

 

[metastable]

With the open source VESC based controllers in duty control mode, during each commutation cycle the controller functions as a buck converter in continuous mode so the current through the inductor (the motor windings) never falls to zero during the commutation cycle. (The inductive windings are shorted to themselves during the duty off time and the current flows inductively. Switching speeds are on the order of 20khz)

https://en.wikipedia.org/wiki/Buck_converter
"Buck converter operates in continuous mode if the current through the inductor (

) never falls to zero during the commutation cycle."

^The motor current in the energized windings during one commutation cycle looks like I-avg

 

[CWatters]

One problem i see is that ordinary DC motors won't be synchronised nor will they run at exactly the required speed. So the resulting pen plots will likely be a random mess that may never return to the starting position.

This was a problem for some spirograph gear combinations. The lowest common multiple of the gear teeth could be huge. Just when you thought you had finished a drawing it would fail to return to the starting position by a tooth and you would either end up with a discontinuity in the pattern or have to keep going for much longer.

The solution is to use stepper motors and think carefully about the relationship between their motions.

 

[anorlunda]

thats a great idea, but a whole a whole complicated expensive design, I was looking for cheap small 12v brushless motor 100rpm or less that I can use a PWM motor controller with for speed modulation.

You may be pessimistic. It is common today for BLDC motors to come with digital controllers bundled into the price. If your object is DIY that's one thing, but if you are interested in the end result at a reasonable price and minimum fuss, purchasing a ready-made solution might be better.

For your spirograph app, you might want to consider stepper motors also. They are cheap, common, flexible, and easy to control.