Test 1
When measuring no load voltage on the charger set to 6V: it reads 6.99V 7.0V
When measuring voltage when the motor is running from the charger set to 6V: it reads around 8.4V (it increased) and bike tach's RPM reads 540rpm/531rpm.
So ignoring friction (IR drop) motor Kb=8.4/.535=15.7v/krpm

Test 2
No load voltage is 12.35V when the charger is set to 12V.
When measuring voltage when the motor is running from the charger set to 12V: it reads around 16.5V (it increased too) and bike tach's RPM reads 1031rpm
So ignoring friction (IR drop) motor Kb=16.5/1.03=16.0v/krpm

At 24V the tach reads 1800rpm.
So ignoring friction (IR drop) motor Kb=24/1.8=13.3/krpm

THE HIGHER THE VOLTAGE THE LESS PERCENT IR DROP IS SO THE MORE ACCURATE THE Kb CALC IS.

At 6V no load the motor draws up 0.22A 0.23A in one direction and 0.24A 0.25A in the other direction but the rpm reading is the same in both directions. At 24V no load it draws up around 0.8A but after trying to stop it by hand it draws 0.22A at no load.
So taking IR into account too, V=IR+Kb*Krpm we can fiddle with two equations and two unknowns so solve for both....

16.5=.22*R+1.03*Kb and
24.0=.22*R+1.80*Kb and
8.4=.22*R+.535*Kb we'll ignore this one for a bit...

R=(16.5-1.03*Kb)/.22

putting this into the next equation

24=.22*(16.5-1.03*Kb)/.22 +1.8*Kb
24=16.5-1.03*Kb+1.8*Kb
24-16.5=.77*Kb
so Kb=7.5v/.77= 9.74v/krpm

So R=(16.5-1.03*9.74)/.22=6.47/.22=29.4 ohms

*** cnc2 please measure the armature resistance of your motor. measure, rotate a tad, measure again, get reading a a few different positions and see if it is around 30 ohms avg (reading is not valid if armature moving remember).

Let's see if it works in all 3 cases you have:

16.5=.22*R+1.03*Kb =.22*29.4+1.03*9.74= 16.5 ok
24.0=.22*R+1.80*Kb = .22*29.4+1.8*9.74 =24 ok
8.4=.22*R+.535*Kb = 6.5+ .535*9.74 = 11.7 close enough as we get low in voltage so less accurate again....

So using THESE numbers, let's say Kb=10V/krpm and so ratioing Kt/Kb from any other motor with same units, Kt=10v/krpm*(3.3/23.7)=1.4#-in/amp or (1.4#-in/amp)* (.113nm/1#-in)=.157nm/amp

whew!

If your motor is around .5nm continuous rating, then it will require 0.5nm/.157nm/amp or 3.2amps continuous. To go 1800rpm no load (not even friction) will require 10v/krpm * 1.85krpm= 18vdc.

course you have such a high resistance motor armature according to this that it also requires another 7v just to overcome friction in itself for the 24v total.

I'm ready for a beer (or three! BEEN a long day!) Let me know if my 30ish ohms jives with your motor.... also you should feel a tad better than my initial 8amp guess from down in the mud voltage/speed calcs dropped by 2x!

THE SAGA CONTINUES!

[quote]Just to make sure: my ammeter is wired in series after the motor( V+ >>motor+; motor- >>ammeter+ ; ammeter- >> charger-) is it the correct way to use it ?

yep

This bike tach is not accurate at all ! its original magnet is still on the bike's front wheel & i'm using a little magnet I scraped from a CD drive lens mechanism, when the magnet has its wide area facing the sensor it reads something & when it has its side (half the previous area) facing the sensor it reads twice the previous number
HMMMM.... well our calcs are only as good as your data

ITS MILLER TIME!