DC motors: what voltage and torque?

[matta757]

Hi everyone,

I am hoping someone can guide me in the right direction on which kind of DC motors to buy. I am purchasing them to power my 2 throttles and 1 speed brake. I have already gotten the Opencockpits DC motor control card.

Now I need to know which voltage motor I should get; 12V or 24V? I read in the manual for the Opencockpits card that it can handle motors up to 36V, but I do not really know what to look for in terms of the best solution. What kind of voltage should I be looking for? What role does voltage play in this equation?

I also am curious about torque. I am assuming that I want motors with reasonably high torque, since the levers and the tension they have are fairly heavy. Is there anything I should do in particular to decide how much torque the motors need? Is there some sort of balance between torque and speed? Is it possible to get a motor that is too fast? I can get them from say 1 RMP to over 100 RPM, but the faster ones have less torque. The speedbrake lever is quite a bit lighter, so would you suggest going with a faster, lower torque motor for that?

Thanks for any help! Regards,

Matt

[fordgt40]

Matt

I would look at this from a different angle.  Firstly, establish the time it should take for the throttle levers to move their full travel - possibly around 4 secs? This info should be available on the net, probably by timing a commercial product demo. This will then give you a relationship between motor rotation speed and throttle travel timing to arrive at the necessary gearing ratio. You could then try using a fishermans spring balance to determine the force in inch/oz to pull the levers. With the gearing, I doubt that you will need a powerful motor, certainly no more than 12volt - remember that the Symulatory throttle uses servos running on 5 volts. Unless you have a degree in maths, it might be better to buy one motor and "suck it and see"

David

[matta757]

So a motor with a higher voltage is more powerful? I am clearly a novice to these matters. The ones I am looking at are basically the same price, so would it make sense then to just go for the 24V one?

[fordgt40]

Matta

Basically yes, but you should not require much power here, particularly as you presumably will need gearing. Focus on getting the correct gearing first and then see what motor options are available. If it helps, I will not be thinking about any motor greater than 12volts when I come to changing my throttle assembly. Also remember that the greater the voltage the more costly  other items become eg power supply.  Also bear in mind that the Opencockpits card has only one external power jumper, ie the same voltage for all six motors and I have doubts that it can cope with much total amperage at 24volts.

David

[matta757]

Hey David,

So question then about gearing... I am using toothed gears to turn my pots with my throttles. So when I am trying to decide what size gears to attach to each motor, should I be using gears that are larger, smaller, or the same size as the ones connected to the throttle levers? I am assuming using a smaller gear on the motor than on the lever will result in more stress on the motor?


Matt

[fordgt40]

Matt

For a mechanical solution as an example

You have a motor that turns at 60 revolutions a minute. Your arc of travel of the throttle levers is say 90deg (lets keep it simple)

You want the throttle levers to move across that arc in 5 secs - that is a quarter of a rev (90 degs) in 5 secs or a rotational speed of one rev per 20 seconds or 3 revs per minute

Therefore to match the rotational speed of the motor (60 revs a minute) and achieve full movement of the throttles in 5 secs (3 revs per minute) you need a gearing ratio of 20 to 1

So you need a gear on the throttle lever shaft  that has 20 times as many teeth as the gear on the motor shaft. This may be impractical as you would need a throttle shaft gear of circa 150-200 teeth.

So, if you get a motor with a speed of say 12 revs a minute you will need a gearing ratio of 4 to 1 which is quite practical, though it would still be preferable to get the largest gears (number of teeth) within this ratio to avoid  cogging and jerky movement. The gearing ration can be approximate and the speed of the motor adjusted by electronic means.

Hope this helps explain the principles - this is not for the faint hearted E&OE!!

David

[matta757]

I'm sort of following... I am confused though about where you got the 20 seconds/3 revolutions... shouldn't it be 15 seconds/4 revolutions since you're taking about moving 90 degrees/a quarter turn? Wouldn't that then give a ratio of 15 to 1?

I really appreciate your help, sorry that I am so uninformed on these things!


Matt

[matta757]

Actually, I thought of one more thing regarding gears...

I am currently using 48 pitch gears. I know you can also get 32 pitch. Is there a preferably pitch for the motors?

Matt

[fordgt40]

Matt

If the throttle lever travel is through an arc of 90deg that is a quarter of a full rotation which is 360deg. Hence, if it takes 5 secs to travel one quarter of a rev then  one full revolution will be  4 x 5 sec = 20 secs which is equal to three revolutions per minute.

I do not recognise your "pitch" reference - use a gear size that looks right for the use. Too lartge and it will cog, too small and they may strip (particularly if poor plastic) Best answer is to experiment

David

[fordgt40]

Matt

A further thought. As the maths is straightforward, perhaps it is the terminology. When looking at the throttle levers from the side of the cockpit they move in a vertical plane forward and backwards across a arc of say 90deg ie a quarter of a full circle. As they travel one quarter of a full revolution in 5 sec - this equates to one full rev in 20 secs which is 3 revs a minute speed. Still confused?

David

[matta757]

David,

Ah yes, now I follow you. Thanks for the clarification. So using the 12 RPM motor, does that still achieve the same movement in the 5 seconds?


Matt

[fordgt40]

Matt

I suggest you decide yourself what lever speed you want, what is the travel, available gears and motor speeds and then work it out using the maths model I gave above.

If you really get stuck, then come back

Regards

David