Hello,

I am currently trying to decide on which gear ratio to choose for the micro metal gearmotors for a robotics project and I have to mathematically justify my decision. So…

I have performed some simple calculations using the following equations:

V = IR + E

(V = voltage applied to motor terminal, I = current into motor, R = resistance of armature, E = back emf),

E = kw

(k = electrical constant, w = angular speed of motor)

and

T = kI

(T = motor torque, k = torque constant (equal to the other k above as well)).

Using the above, I get very different results for “k” depending on how I calculate it.

Method 1: k = (6volts) / (no load speed)

Method 2: k = (stall torque) / (stall current)

So, about the stall torque:

Is that a number that you all measured, or is that a number that you calculated as I did above, and then rounded?

Also, about the no load speeds listed in the table:

Why don’t they match with the gear ratios? If I compare the 5:1 with the 30:1, it’s plain that 2500RPM/(30:1/5:1) = 417RPM and not 440RPM. So, I don’t know which numbers to go with.

As another example of the kind of contradictions I keep coming across:

If I calculate the torque constant using method 1 above for the 210:1 gear ratio (non-HP), i get the following:

k = 6V/60RPM = 6V/(60/60*2*pi rad/sec) = 0.955 (newton-meters per amp).

WIth the second method, I get:

k = (1.3kg-cm *(9.8newtons/kg)*(1meter/100cm)) / ( 360mA ) = 0.354.

Quite different indeed. I mean, I don’t think I can attribute a difference of a factor of 2.7 to rounding errors…

Any advice would be most greatly appreciated.

Thanks.