[mike_Kilroy]

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!

[cnc2]

Thanks for the reply & calcs, Mike !

This time your calculation's results are more coherent/realistic, if the motor when slowed down to half its speed, draws up ~2.5A then, your 3.2A continuous rating is more likely to be correct...far away from that 8A thing.

Well, when measuring the resistance, the results are weird. The meter's battery is running low, I'll have to replace it. It's a digital multimeter so, a low battery might greatly influence the results.

Measuring resistance from the motor's connector (through the wire & brushes) I find different results in one position I find ~19 ohm, in an other I find ~5ohm & I find ~10 ohm in an other position....WEIRD !!! is that a poorly recoiled motor ??? is the multimeter going nuts ???

Will replace the meter's battery & see if I get other results...from the net, I've found that resistance should be measured directly on the commutator bars to have accurate readings & that measuring through the brushes will give higher numbers, this makes my numbers sound more weird...

The motor casing has a hole in it ~1/2" diameter, when I remove the plastic cap I can see the commutator bars, but I can't pass the two leads through that hole to make a measurement...How is it meant to be done ?

Thanks !
cnc2.

[Al_The_Man]

If you read the link again in post #16 it mentions from experience take the resistance reading like so:
"To measure the winding resistance, lock the rotor so it doesn't turn and measure the current with a small voltage applied (so as not to exceed rated current) Don't bother using a multimeter's ohm range- not worth the effort."
Take a few readings and find the highest current (lowest resistance).
Al.

[cnc2]

Thanks for your guidance Al !

I have finally found te time to make the measurements & finally found a way of locking the motor's shaft. At 6V the motor is relatively slow, so trying to lock its shaft with my fingers doesn't burn but it was quite hard & still turned at around 1or2RPM, so I used a metal plate which had a hole in it and forced the tapered shaft into the plate's hole & it stopped completly.

Well, with the battery charger at 6V & the shaft locked the ammeter reads ~2.46A to 2.48A. Which gives a resistance of R=U/I, R=6/2.48, R=2.42 Ohm.

Mike, Why is it so far from your guessed 30 Ohm ? What's wrong ?

Thanks !
cnc2.

[Al_The_Man]

I checked a couple of DC motors I have of comparable size and they average out at 2.6ohms.
Al.

[cnc2]

Thanks Al ! this confirms my measurements are not so weird.

Mike, something is wrong with your numbers

Thanks !
cnc2.

[mike_Kilroy]

2.5 ohms makes much more sense than 30 ohms.....the rpms and volt readings you gave at the higher voltages have to be off then. which do you think is most likely off, the voltage or the rpms? with this R value, we can stick it into the equations and solve for the other variables instead. the equations work but we need right data for them. i believe this 2.5ohm for R for this size motor.

[cnc2]

I know the RPM is most likely to off because I could get half or double the reading just by holding the sensor in a different position, I know this last intensity measurement is accurate since it gave a realistic result for the motor's R.

When the shaft was locked, the voltage was between 5.98V & 6V, but when the shaft was spinning freely, the voltage increased to around 8V & I think it was the same in the previous measurements.

Thanks !
cnc2.

[mike_Kilroy]

If you dont mind my continuing to poke around on this for fun, i;m going to put the 2.48 into the equations and see what motor constants pop out now. then I will go look at some similar size 2.5ohm motors low voltage motors and see how they compare later today.

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

first Kb=(16.5-.22*2.48)/1.03 = 15.5 v/krpm
2nd Kb=(24-.22*2.48)/1.80 = 13.0 v/krpm
3rd Kb=(8.4-.22*2.48)/.535 = 14.7 v/krpm

call it 14.7v/krpm....

ratioing to get Kt, Kt= (3.3/23.7)*14.7= 2.04#-in/amp. if motor is rated .5nm (4.4#-in) then continuous current rating is 2.2amps.....

and your no load friction to turn the motor of .22amp is .5#-ft. we can compare this friction to similar size motors too to see if it makes sense...

[mike_Kilroy]

The second motor is a "MOTRONICS Corporation" PT.NO. 801679-001 REV.G CAT.NO. 31107-14-210-05 INS. CL. H VOLT 24 DATE CODE 0183. It's a PM DC motor.
OK for rating.... let me convert to english: 4.9" long, 3" dia.

call it 14.7v/krpm .... Kt= . 2.04#-in/amp.

Back to this to compare to similar 24vdc rated motors (it is important to compare to 24vdc rated motors rather than 90 or 120vdc)....

NT2146C inland Kollmorgen 2.8"OD(would be 3" with body around it) x 2.5" long (would be around 5-6" long with bearings). 13#-in peak, 27vdc, 10ohm, 5.6#-in/amp, 63v/krpm.... not good.

try NT2146A winding for 2.7ohms: 2.9#-in/amp, 32v/krpm..... not good.

wow..... my torque constant conversion of Kt=10v/krpm*(3.3/23.7) was wrong - not sure why this shows up wrong on a brushless motor but per double check of these parameters on NT2146 and googling Kt=Kb*xxxxx I see my mistake:

Calculating Motor Performance
Use these handy equations to calculate steady state motor performance. A spread sheet will help in visually graphing motor parameters. If the Torque constant is not supplied by the motor manufacturer, you can measure the motors no-load RPM/Volt and use the following equations to calculate the torque constant.
Torque constant: Kt=Kb x 1.345
Current draw of motor: I = [V-(Kb x kRPM)]/Rm
Torque output of motor: J = (Kt x I) - (Kt x Inl)
RPM of motor: kRPM = (V - RmI) / Kb
Power output of motor: Po = (J x RPM)/1345
Power input: Pi = V x I
Motor efficiency: Eff = (Po/Pi) x 100
Current at peak motor efficiency: Ie max = Sqrt [(V x Inl)/Rm]

Symbol Definitions:
Eff = Efficiency
I = Current
Iemax=Most efficient current
Inl = No load current
J = Torque (oz-in/A)
Kb = Voltage constant (Volt/1000 RPM)
Kt = Torque constant (oz-In/A)
Pi = Power input (Watts)
Po = Mechanical power output (Watts)
Rm = Terminal resistance
RPM = Revolutions/minute
V = Voltage

so real Kt for your motor would be your 14.7*1.345=19.8oz-in/amp /16=1.2#-in/amp so X 4.4#-n cont = 5.4amps continuous....

trying NT2171 to get similar size motor with similar Kb=13v/krpm, it is 1.1ohm, has Kt=1.1#-in/amp, rated 7.5vdc.... but its B winding is 21v/krpm, 2.7ohm, 1.8#-in/amp, rated 11vdc.

I am guessing your motor does not have rate earth magnets so has lower Kb/Kt for the same resistance, so if we assume this then it probably still jives to call it:

- Kb=13.7v/krpm so 1750rpm max speed
- Kt=1.2#-in/amp so 5.4amp continuous for 4.4#-ft continuous rating

[mike_Kilroy]

and one last check of power....

5.4a*24v=130w *.75 (typical conversion on ceramic magnet motor)=97watts

97w/746w/hp=.13hp

.13hp=1750rpm*T/5252; hence T=.39#-ft*12=4.7#-in!!!!!!!!!

IT FINALLY JIVES!!!!!!!!!! (maybe

[cnc2]

Jives ???

[mike_Kilroy]

it doesn't?

Jives; matches; makes sense?

[cnc2]

Ah thanks for the explanation

I don't know if it matches because, I have no idea of what a 4.7oz-in motor is like & in my opinion it should be easy to stop by hand.

#-in <=> oz-in, right ?

Thanks !
cnc2.

[cnc2]

Ah, but stall torque can be ten times the continuous torque, so I am trying to stop 47oz-in by hand, instead of 4.7oz-in.....might make sense

Thanks !
cnc2.

[mike_Kilroy]

I made lots of typos in my replies I see - sorry. i usually write those later at night (after some beers too).... Not sure where you are getting 4.7in-oz torque from, but must be one of my typos? copying from post 30:

so real Kt for your motor would be your 14.7*1.345=19.8oz-in/amp /16=1.2#-in/amp so X 4.4#-n cont = 5.4amps continuous....

trying NT2171 to get similar size motor with similar Kb=13v/krpm, it is 1.1ohm, has Kt=1.1#-in/amp, rated 7.5vdc.... but its B winding is 21v/krpm, 2.7ohm, 1.8#-in/amp, rated 11vdc.

I am guessing your motor does not have rate earth magnets so has lower Kb/Kt for the same resistance, so if we assume this then it probably still jives to call it:

- Kb=13.7v/krpm so 1750rpm max speed
- Kt=1.2#-in/amp so 5.4amp continuous for 4.4#-ft [*** typo - should be of 4.4 #-in] continuous rating

You measured your resistance at 2.48ohms, so with a 24v power supply source (motor voltage rating) you can push 24/2.48=9.7amp max into the motor at stall, so it should produce 1.2#-in/amp * 9.7a= 11.6#-in or say 1 #-ft of torque max.

[cnc2]

Thanks for the reply Mike !

I made lots of typos in my replies I see - sorry. i usually write those later at night (after some beers too)....

Drinking any kind of biofuel can be hazardous to your health :nono:

Not sure where you are getting 4.7in-oz torque from, but must be one of my typos?

I got the 4.7#-in (4.7oz-in) torque from post #31

copying from post 30:

so real Kt for your motor would be your 14.7*1.345=19.8oz-in/amp /16=1.2#-in/amp so X 4.4#-n cont = 5.4amps continuous....

trying NT2171 to get similar size motor with similar Kb=13v/krpm, it is 1.1ohm, has Kt=1.1#-in/amp, rated 7.5vdc.... but its B winding is 21v/krpm, 2.7ohm, 1.8#-in/amp, rated 11vdc.



You measured your resistance at 2.48ohms, so with a 24v power supply source (motor voltage rating) you can push 24/2.48=9.7amp max into the motor at stall, so it should produce 1.2#-in/amp * 9.7a= 11.6#-in or say 1 #-ft of torque max.

Ah, it's more realistic & closer to your early 8.5A rating
If 11.6#-in <=> 11.6 oz-in then 11.6oz-in <=> 0.08Nm ??? sounds a bit low...or is it just me ?

Ahhh, if that Motronics guy could tell us on what kind of machinery this motor was used, it'd give me a realistic idea of the possible applications for this motor.

Maybe you can give me the possible applications of comparable size motors ?

Thanks !
cnc2.

[mike_Kilroy]

Drinking any kind of biofuel can be hazardous to your health :nono:

heehee! Ya, but it sure tastes good!

I got the 4.7#-in (4.7oz-in) torque from post #31

Ah I see. sorry. it is easy to convert from one set of units to another - just multiply or divide of the difference - like above should have said 4.7#-in * (16oz/#) = 75.2 oz-in torque...

If 11.6#-in <=> 11.6 oz-in then 11.6oz-in <=> 0.08Nm ??? sounds a bit low...or is it just me ?

Here is good site to convert units: Online Conversion - Convert just about anything to anything else

Maybe you can give me the possible applications of comparable size motors ?

Anything needing that size and torque and speed to go. no one most common application. We built one of this similar size into a housing recently to rotate a prism for the air force; they shot an infrared light beam thru hole in the motor shaft to the rotating prism and it then bent the light 90 degrees and around the perimeter were pickups to read the light; they said it increased the infrared camera's resolution by about 1000. We build similar size motors into small rotary tables for coordinate measurement machines. We sell this size motor for use in glue dispenser guns to push the glue out. Just lots of applications.

[cnc2]

Thanks for the reply Mike !

I used this convert utility Convert.exe, but I didn't know the name of this #-in unit & I assumed it was just an abbreviation of oz-in...I was wrong. 16oz/#... Thanks for the tip....after testing several units on the conversion website you linked to, I've found that #-in is called pound inch...Sorry, I'm a Metric guy

So, 11.6#-in <=> 185.6oz-in <=> 1.3 Nm it's realistic compared to the torque I felt when I did try to stop the shaft.

Man, that camera example is just wow !

Thanks !
cnc2.

[Al_The_Man]

Maybe you can give me the possible applications of comparable size motors ?

Thanks !
cnc2.

When designing an application it is handy to run it through a Motor size checker, this will give you an idea as to whether the motor load falls into the accepted limit of motor motor to load inertia ratio of 10:1 max.
You can still get the free Kollmorgan program from Electromate Industrial Sales
Al.