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I have a particular set of requirements that call for a relatively high stall torque for the space available (300 ozf-in for < 8cm length of motor). The motor would be operating at a relatively low speeds (max speed of motor can be between 10-100 rpms) at a torque of about 200 ozf-in. The diameterical space available is less than 3“.

Initially I looked at some brushed DC motors and while there are some that meet these particular set of criteria, there is not much choice in terms of that package size.

So I decided to look at some BLDC motors usually used by hobbyists but the stall torque, rated torque, or speed-torque curves are not published anywhere. The motors quote only speed characteristics and I have yet to find one that publishes speed-torque curves.

I was wondering what kind of torques I could expect from these motors and if it would be sufficient for my needs. Are there any alternatives to the hobbyist motors that also come in a small package size?

I was also wondering if these motors could be operated at the low speeds I need (100 rpm), due to the commutation of the coils in the BLDC motor. If so, what kind of adverse effects could be expected by running it at such low speeds (maybe even below 100 rpm)?

Thanks for your help.

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  • \$\begingroup\$ Information is missing \$\endgroup\$ – Andy aka Oct 21 '14 at 7:59
  • \$\begingroup\$ If you have the speed-voltage constant for a motor, you also have its current-torque constant since there is a very simple relationship between them. Does that supply the missing information? Incidentally reading your requirements, I'm thinking geared motor. \$\endgroup\$ – Brian Drummond Oct 21 '14 at 9:23
  • \$\begingroup\$ What max diameter can it be? | kg.m = oz.in/1385. Power = RPM x kg.m = 100 x 200/1385 = 14 watts . | That's inside the range of an electric drill motor but the 8mm length is "very difficult" depending on what diameter you can tolerate. IF diameter is "free" you cn do it by making a large dia wheel. A Fisher & PAykel smart drive motor probably meets your reuirement. They use Ferrite magnets. Dia will decrease substantially with rare earth magnets. |Drive Calulator program old excel and new standalone pgm. \$\endgroup\$ – Russell McMahon Oct 21 '14 at 11:03
  • \$\begingroup\$ To some extent, BLDC and stepper motors are the same idea optimized for different goals, and you might do better on the stepper side of the divide. A more "pancake" type of design will product more torque than a "can" type. But it's not clear that your application should rule out gear or belt reduction; also you might be able to use a long, thin PMDC motor mounted perpendicular to the intended output direction, with right angle gearing. \$\endgroup\$ – Chris Stratton Oct 21 '14 at 17:32
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What max diameter can it be?

kg.m = oz.in/1385.

Power ~= RPM x kg.m
= 100 x 200/1385 = 14 watts.

That's inside the range of battery electric drill motor but the 8mm length is "very difficult" depending on what diameter you can tolerate.
IF diameter is "free" you can do it by making a large dia wheel. A Fisher & Paykel smart drive motor probably meets your requirement. They use Ferrite magnets. Dia will decrease substantially with rare earth magnets.

Drive Calculator program old excel and new standalone pgm.


Excellent - relatively simple Simple analysis for Brushless DC motors

Drive Calculator V3.4 free

Old Excel version and link to new

Manual for V3.4

25 e-flight calculators - some relevant

Motocalc download - 30 day free trial then $40

Useful related

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  • \$\begingroup\$ Thanks for the useful tools. Let's say I rephrased the problem to a required torque (stated in the original question) while speed can be anywhere between 10 rpm - 100 rpm. Does this substantially increase the options available to me? The diameter can't be more than a couple of inches across. \$\endgroup\$ – Shardul Upadhyay Oct 21 '14 at 17:04

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