# Selecting a DC motor to run at 25% of stall current

Hi,

I am new to electronics / motors, though I have done a lot of reading recently!

I have a project that requires a motor to turn at 60 rpm. I have calculated the required torque to be 13.7 kg cm 190 oz in.

The DC gearmotor pages state, “…a general recommendation for brushed DC motor operation is 25% or less of the stall current”.

Does that mean I should multiply my torque, RPM or both values by 4 to account for the 25%?

Any advice would be greatly appreciated. Thanks!

Hello.

Generally, a gearmotor drawing 25% of the its stall current will correspond to it applying 25% of its stall torque at 75% of its free run speed. (There is an inverse relationship between torque and speed.) However, if available, the ideal resource for assessing this is the motor performance curve which shows the relationships between a motor’s torque, speed, current, output power, and efficiency.

We have performance curves for most of our metal gearmotors, including all of our micro metal gearmotors, 20D gearmotors, and 37D gearmotors, in motor datasheets that are available on each gearmotor’s product page under the “Resources” tab.

Unfortunately, we have not a datasheet for our 25D gearmotors yet, but do not worry, those are coming! In the meantime, if you need performance curves for one of our 25D gearmotors, please email us at support@pololu.com, and we might be able to get you preliminary data for a particular gearmotor.

- Patrick

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Hi,

Thank you for your reply, which in combination with the performance curves has [ I think ] given me a better understanding. I cannot find something that matches my original spec. However, if I adjust my mechanism, I can adjust the RPM and torque requirements as follows:

RPM = 120
Torque = 70 kg mm

Note: The device would accelerate to speed, then run continuously (essentially a turntable with uniform weight)

I have selected the following motor #4743 and drawn over the graph (attached)
This provides me with the required RPM and a little extra torque. However, I would be running at approximately 41% of the stall current.

Questions.

1. Is that 41% of stall current a disaster?
2. The graphs have solid and dotted lines. Is there some significance? (there must be !)
3. It seems logical that the load on the motor would reduce once I have accelerated to speed. I’m not sure how that is relevant, but feel that it is.