RPM of DC motors

Hello professionals, a hobbyist here.

I'm working on a 12V DC motor project and I'm having trouble choosing the right motor.

My question is regarding the RPM, which is mentioned when the motor has no load, I understand it's more of a reference point, but I would like to know more specifically if the motor will be able to handle the load. Can I calculate somehow what will be it's RPM under some load?


These are the specs of some motors I consider for example:

Which motor? What is your load torque and RPM?

That's my problem, I don't know which motor I should get. In the table are a few options.
My question is how should I take the RPM that's written because it's relevant for no load.
In practice the load will be around 7kg, for a few seconds only a few times a day (for roller blinds), and I need and RPM of about 40-60, but with the load.
I hope you can understand where my confusion is.

Hard to say without a diagram of how the blinds are being lifted, some kind of hoist arrangement? If so what will be the drum diameter when blinds are up and hoist drum is full? How much time to lift 7 kg from low to high and the distance?

Thank you for your help.
These are roller blinds that are moved up/down by hand with a chain.
I want the motor to move the chain.
My question is more general though: what can one infer from a motor's free load RPM about it's ** (some) load RPM?** The two specs must be related.


That's difficult to predict, if the power supply can hold the voltage up, the speed won't vary much until the motor stalls from overload. I would wild guess the RPM at full load will be 85 ~ 90% of no load RPM.
One thing you could do: Using a spring scale or various weights, determine how much force required to pull the chain when blinds are at bottom and again at top. That would give you a starting value to work from.

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Great, thanks!
One more question please:

For an example, the no load current is 0.2A, the stall current is 5.5A.
What would the current approximately be at a comparable load (<stall torque) ?
Is it a function between the two values..?

I'm asking because I have a 12V 1A and I want to know if should get one with higher Amps.

Thank you

That's something I won't try to guess, it is one of the specifications of industrial motors, I've never seen it stated for hobby type motors, you're lucky if they state the current. The motor will draw near stall current for a short time every time it starts, if your power supply cannot hold this surge long enough for the motor to get up to running speed, it may not start, i would use a PS rated at least 5A.

I understand.
But generally in DC motors, in a regular loaded (<stall load ) motor movement, is the current closer to the stall current or free load current? I hope there may be any sense behind it.

Depends on the (as yet unknown) load.

So it's a function between the no load current and stall current depending on the load's weight?

Then you need to design your system based on how FAST you want the motor to move the chain, not from the other end of the design. Once you decide on how fast the chain will move, then you have to design the mechanism to connect the chain to the motor. The radius of that connection will determine the RPM necessary for your motor to move the chain. From that you can compute the torque necessary.
Chances are, when you finish the rest of the design, the motor stall current will not matter, because the chain or other parts will break before the motor reaches that point.

Yes indeed. I know the that I want 40-60 RPM, I inferred that from the radius of the part you are talking about.
I indeed don't really care about the stall current, I care about the maximum current it will consume practically when in my project, with reasonable-load that is < stall load. The only thing I know is the current with no load, and at overload (stall).
I tried to infer through the stall current and no load current the reasonable-load current, that will be when my project ideally works.
I hope you can understand.
Thanks for taking the time to help me.

It seems like it's not a straightforward thing. This might help:

I imagine someone smart enough could work that into the formula you need. I'm sadly not that person.

I don't believe the size of a roller blind is a factor in the above equations :wink:

Actually the problem to be solved is: Will the current be more or less than 1A?
If anyone could take an educated guess.


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Well, Amps = Watts / Volts.

In your motor table, the max is 12 watts, with a 12v rating that would be exactly 1A. So the answer to your question is probably "no" :slight_smile:

The problem will be whether you will stall the motor, as that will exceed 1A significantly.

@gibbobaz @Paul_KD7HB

The question I meant to be asking can't be answered with yes or no, but with >1A / <1A.
Assuming the motor will not be stalled and it will only be loaded with less than it's stall load, is it reasonable to assume that it will run well with a 12V 1A adapter?
Just a reminder, I'm talking about a 12V motor and ~0.2A no load current and ~3A stall current.

Sorry. You are asking theoretical questions, so you get theoretical answers. Get some motors and test them by holding the turning shaft with two pieces of wood while watching the ammeter.

If you assume that you will be running the motor up to but not over it's rated power, then you are looking at 1A.

Your motor information doesn't give any details about the relationship between the RPM, load and current, so it's impossible to say directly what amps it will pull. Those equations on wiki probably do tell you, but again I'm not sure you have the data for all the inputs.