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I have two same sized DC brushed motors found in different variants of an appliance of the same brand. They are possibly equivalent, but they have different labels.

One is labelled with:

(Johnson Electric logo)
31348
3K3861

The other is labelled with:

3501-0011-01  REV C
CIM-2450-1
24 VDC  0.9 A
50 RPM /15 LB-IN  9-95

The diameter of the motors are 27.5mm, and length of the motor bodies is 32.5mm.

What does the last line, beginning with "50 RPM" mean? Surely it doesn't simply mean the nominal speed, since 50 RPM is way too slow. Does it have something to do with the concept of "motor regulation"? This online catalog meantions motor regulation in units that I am not sure about, e.g. on page 50, for motor model HC313G-001, it says Motor Regulations: 304.857 Rpm/m-Nm . Huh, is that rpm per millinewton-metre?

Johnson Electric HC313G-001

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  • \$\begingroup\$ Label on second motor suggests it has a gearbox attached, then 50rpm could make sense. Motor regulation suggests it slows down by 305rpm for every mN-m of load (from its no-noad speed of 6900 rpm) \$\endgroup\$ Mar 30 '15 at 11:41
  • \$\begingroup\$ Thank you Brian and Brad. It does make sense to me now. The second label was stuck on the motor can but actually refers to the motor+gearbox. BTW, does 1 mN-m mean 0.001 newton-metre? \$\endgroup\$
    – Doochz
    Mar 31 '15 at 3:16
  • \$\begingroup\$ Yes. 1 mN-m means 1 milli-Newton-meter, or 0.001 Newton-meter. Usually if the gearmotor is sold from the manufacturer and not as separate parts, the ratings refer to the gearmotor and not the motor by itself. \$\endgroup\$
    – Eric
    Mar 31 '15 at 4:18
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50 RPM suggests that the motor is actually a gear motor. So the rated speed of the gearmotor is 50 RPM, the rated torque is 15 in-lbs and the rated current is 0.9 A. As a sanity check, 50 RPM at 15 in-lbs gives you an output power of ~8.9 W. The input power is 24 V * 0.9 A = 21.6 W. That gives an efficiency of 41.2% at the rated load, which is not unreasonable for a gearmotor of that size.

Motor regulation is a figure of merit for DC motors. It can be thought of in a number of different ways. Some people like to think of it as the slope of the speed torque curve. What it tells you is the motor's ability to turn electrical power into mechanical power. It can be calculated by \$\frac{R}{k_t^2}\$, where \$R\$ is the terminal resistance of the motor and \$k_t\$ is the torque constant of the motor. A lower value indicates better heat conversion.

Some people also use a similar figure of merit called the motor constant (\$K_m\$). \$K_m = \frac{k_t}{\sqrt{R}}\$. As you can see, this is just the reciprocal of the square root of the motor regulation that I talked about in the previous paragraph.

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