DC motor selection

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Hi all,

I have some questions regarding the selection of DC motors. (hope this is the right place to post such questions)

My motor driver circuit design can supply a maximum 2A and has over current protection (via utilizing a sense resistor)

So if my max. current is 2A do I select a motor with a maximum running current of 2A?

I'm unsure about stall current, as I understand this is the current drawn by the motor when its trying to turn but can't ie not enough torque to move the load.

Am I right in thinking that my over current protection circuit should kick in if the motor stalls and I should choose a motor with a maximum running current of around 2A to best match my driver circuit?
Ie: a motor with a max. running current (or Max. continuous current) = 1.9A & Stall current = 6A would be well suited?

Also if I use a gearbox attached to the motor this will not affect the electrical properties of the motor, just the rpm/torque trade off right?

Thanks for your help
Andy

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In answer to your second question, you are correct. The gearbox will only impact the RPM and Torque, not the electrical properties of the motor.

Your first question is harder to answer. Perhaps others with more experience can share their views.

I would prefer to pick the motor and gearbox, etc., to meet the physical requirements of the project, THEN chose a driver to meet the motor's needs. You appear to be doing this in the opposite order.

The startup current for the motor always exceeds its running current. If the driver can not meet the peak current needed for starting the motor, and is always going into its protected current limited operation, every time you start the motor, it is not clear that it will start properly, or reliably. I believe you need some overhead current drive capability.

Robot, or something else?

JC

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yep, planning for a robot.
The driver can supply 2A DC (constant) and peak 2.5A for 10ms, so I guess I could have a startup current up to 2.5A but might need a delay on my overcurrent protection

~Andy

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Seems like a 12V motor that draws about 1A with moderate load would still bump the 2A current limit when starting or going up hill. Stick the ammeter on it and hold the armature with a rag and see how much it draws under load.

Imagecraft compiler user

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problem is I don't have the motor yet :)
I am trying to figure out which motor would be suitable.
But you make a good point bob, once I get the motor I will also test it

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ant452 wrote:
So if my max. current is 2A do I select a motor with a maximum running current of 2A?

You should select a motor with a continuous current of no more than 2A and a stall current of no more than 2.5A.

ant452 wrote:
I'm unsure about stall current, as I understand this is the current drawn by the motor when its trying to turn but can't ie not enough torque to move the load.

Correct, but also the current drawn when you first start the motor. If there's no resistance, it will quickly drop from this value to it's current value for the resistance provided. The more resistance, the closer to it's stall current the motor will draw until it's so much resistance it can't move and it draws the stall current. The stall current shouldn't be more than the max current allowed by your driver or it'll blow the driver when you first start it.

ant452 wrote:
Am I right in thinking that my over current protection circuit should kick in if the motor stalls and I should choose a motor with a maximum running current of around 2A to best match my driver circuit?
Ie: a motor with a max. running current (or Max. continuous current) = 1.9A & Stall current = 6A would be well suited?

Your over-current protection should kick in if it's stalled for too long as that means you're doing nothing and wasting power. However, you shouldn't kick in right away as you will always draw up to the stall current at the moment you apply power.

The 6A stall current would definitely not be ok if your driver only allows 2.5A max.

ant452 wrote:
Also if I use a gearbox attached to the motor this will not affect the electrical properties of the motor, just the rpm/torque trade off right?

This won't directly affect the electrical characteristics per se, but will have an effect on them if there's much resistance. For example, if the robot is so heavy that the motor itself can barely move it, then you'll always be drawing close to stall current, as it won't be moving much. However, if the gearbox provides enough torque to allow the motor to freely spin, then you'll draw closer to the free-running current. You'll of course be running slower than without the gearbox, though.

Clancy _________________ Step 1: RTFM Step 2: RTFF (Forums) Step 3: RTFG (Google) Step 4: Post

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Thank you for your replies.
clpalmer you pretty much covered everything, I greatly appreciate you taking the time to provide such an answer.

I am unsure whether to continue to use the Driver IC I was going to or make my own custom one based on a couple of N & P mosfets, either way your answers still apply and I'm very greatfull, thanks again

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Driver IC is easy, but will cost more for high current. A simple H-Bridge might be a touch more design/assembly, but isn't difficult by any means and for a couple $$'s, using larger FETs like DPAK/D2PAK/etc, you can get 20A+ without a problem.

Clancy _________________ Step 1: RTFM Step 2: RTFF (Forums) Step 3: RTFG (Google) Step 4: Post

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I am curious. This seems like a backwards way to design a robot. Normally, of course, you would choose a motor that will get the job done and THEN design a controller to meet the requirements. Now, I've had to do projects with "parts on hand", is that what is going on here or is there another mysterious reason for this course of action? Just curious is all.

Go electric!
Happy electric car owner / builder

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This is in relation to building my first robot so I guess I was more curious as to the general parameters of importance when interfacing to a DC motor.

The specs I mentioned were for a very common H bridge drive IC which I thought I could maybe use.

Now that I better understand the DC motor parameters I think I will choose a more suitable motor/driver combination, most likely a custom mosfet H bridge. As high current N & P mosfets are pretty cheap and that way I'll have plenty of current for my application.

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Are you driving the motor with higher voltage than the AVR is running at (ie. motor on 12V, AVR on 5V)? If so, you should look at logic-level FETs, which should switch full-on with the 5V signal, even if the source is 12V.

Clancy _________________ Step 1: RTFM Step 2: RTFF (Forums) Step 3: RTFG (Google) Step 4: Post