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Topic: Minimize Transistor Count for Multiple Motors - Same Speed and Direction (Read 226 times) previous topic - next topic

everdrawn

Hi all,

New to these programs and automation, and didn't see this question posted anywhere so I'm reaching out for some help.

I'm working on a project that needs multiple (number is shaky, but think 30+) small DC motors to drive a single shaft. The speed and direction of the motors will vary, but each motor will always be operating at the same speed and direction as every other motor. The motors will draw 12 Volts and .5 Amps.

I've looked into using H-Bridges, but most cannot support more than 2 motors of this size, meaning I'd need 15 to drive 30 motors. I'd like to take that count down a bit. Any ideas?

Paul_KD7HB

You wrote: " The motors will draw 12 Volts and .5 Amps.". But that is the running current. What is the starting/stall current? Will be many times the running current.

Paul

jremington

Please post a link to the product page or data sheet for the motor.

everdrawn

Stall current is 3.9 Amps, I'll try to hunt down the data sheet.

Edit: Here's the data sheet

https://www.mpja.com/download/18298md.pdf

jremington

It is hard to imagine how you can have 30 "small D.C. motors" driving a single shaft, or even why anyone would try.

One appropriately chosen motor is all it takes, with 30x less opportunity for failure.

everdrawn

One larger motor is superior in just about every way. More compact, more efficient, cheaper, etc. However, this particular application demands a certain form factor that is more of a flat plane than a large cylinder. It's certainly less efficient, but should yield interesting results nonetheless.

Paul_KD7HB

One larger motor is superior in just about every way. More compact, more efficient, cheaper, etc. However, this particular application demands a certain form factor that is more of a flat plane than a large cylinder. It's certainly less efficient, but should yield interesting results nonetheless.
Your engineering studies should have pointed out there will be ONE motor that will do the majority of the work because you are not controlling the TORQUE of each motor. Controlling speed and/or current is not good enough.

Paul

everdrawn

Given that these motors are cheap, I don't doubt the work done by each will be perfectly equal. But surely they ought to be close enough to where supplying equal current to each motor should generate close to the same torque? If you suspect that's not the case, then perhaps I'll need a way to control the torque as well.

Paul_KD7HB

Given that these motors are cheap, I don't doubt the work done by each will be perfectly equal. But surely they ought to be close enough to where supplying equal current to each motor should generate close to the same torque? If you suspect that's not the case, then perhaps I'll need a way to control the torque as well.
That would be creating a MONSTER!

Paul

MarkT

You change direction, you need an H-bridge.

You need to drive several motors in parallel, you need a bigger H-bridge - they are available upto extremely
large currents, although it will get more expensive above a few 10's of amps.  Check polulo.com
[ I will NOT respond to personal messages, I WILL delete them, use the forum please ]

everdrawn

Good site, thanks Mark. It does get pricey when you get above 10 amps, but that's a lot of effort saved over hooking up so many smaller ones.

I did some more research yesterday and found that a 'Darlington Pair' of transistors might enable me to construct my own h-bridges that meet the standards for my application more precisely. This route may be cheaper than pre-built heavy load h-bridges as well, but I think I'll need to do a bit more reading to ensure I know what I'm getting into.

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