Gearing motors together

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EpicentrE
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Gearing motors together

Post by EpicentrE »

Hi all,

I've Googled around a bit, but can't find a definitive answer to this, so I thought I'd ask here. I'm looking at a design that will have two brushless motors connected together through gears powering something quite, quite nasty. Now I've read that, at least with standard brushed motors and IC engines, you can do this without too much of an issue; you obviously won't get double the power, but it won't be that far off. If the speeds of the motors are mismatched, then it will simply run at the speed of the slowest one, which again will cause some losses overall, but assuming the difference isn't too great it shouldn't be a huge issue.

I'm interested as to whether the same is true of brushless systems. Because they are controlled using strictly timed pulses from the ESC, would one motor being slowed down by another motor that was slightly slower cause issues? Would the faster motor pull a lot more current due to constantly "pushing against" the slower one?

I've also researched running both motors from one ESC, but as far as I can see this is generally considered a bad idea. However if there are any smartypants ways to make two brushless motors be matched in speed I'd be interested to hear them.

Thanks in advance!
Scott Fyfe-Jamieson, Captain of Epic Robotics. Champion of AWS38/41/42.
http://www.epicrobotics.co.uk
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BeligerAnt
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Re: Gearing motors together

Post by BeligerAnt »

What you describe should work as well with brushless motors as with brushed.
Whilst brushless ESCs do generate correctly-timed pulses to drive the motors, the timing is actually controlled (or at least moderated) by feedback from the motor windings. This is the only way a motor can cope with a varying load - for a given input, speed will drop as load increases.
You will need two separate ESCs. I couldn't ever see two motors running off one ESC unless they were very well matched and seeing exactly the same load. Given the variability of cheap RC brushless motors, trying to use a single ESC could end very badly!

Sounds scary! :o
Gary, Team BeligerAnt
Ellis
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Re: Gearing motors together

Post by Ellis »

Could you run through a custom or model differential? I had a Lego one (might have been K-nex) years ago that would have been shredded by brushless power but something like it could be built for purpose. On this scale delrin bevel gears and such are cheap. I want to look into this at beetle scale for a FBS.
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bitternboy
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Re: Gearing motors together

Post by bitternboy »

The concept of a dual brushless set-up using a differential makes my mouth water. What's the reasoning behind using two motors instead of just upping the size of the current one though?
Jonathan Atkinson
Before you criticize another person, first walk a mile in their shoes. Then, when you critisize them, you'll be a mile away and have their shoes.
EpicentrE
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Re: Gearing motors together

Post by EpicentrE »

Gary: Thanks; I thought as much, but wanted an expert opinion.

Ellis: Theoretically yes, but it wouldn't work in this design and it would add a lot of weight also. If what Gary's said is right it shouldn't be required

Jonathan: In this instance, it's because it's for powering two separate disks, but they need to be synchronised. I've looked at connecting them to a single motor, but in this particular design it wouldn't be practical for a variety of reasons.
Scott Fyfe-Jamieson, Captain of Epic Robotics. Champion of AWS38/41/42.
http://www.epicrobotics.co.uk
Ellis
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Re: Gearing motors together

Post by Ellis »

Ah, you want the two to run in sync. Guessed you might have just wanted two motors behind a massive weapon, hence a differential would work.

Also yeah the thought of running a motor with more reduction/lower KV into one side of a differential and a higher speed input on the other sounds fun. Could make for ridiculous spin up times in the bigger classes, with the right programming it could be like an automatic low-high gear shifter.
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