[mike_Kilroy]

where is my reply from yesterday 4pm? cnc2, I sent another long reply but is not here! took me about 20 minutes to compile (:

bottom line was

1) I'm by no means p'ed! but happy u r digging in on this! -
2) I also showed ur rule of thumb is dependent on rpm so using it I got motor rating closer to 1nm
3) my .5nm was based on looking at similar motor sizes and their cont rating (a rule of thumb is similar size motors have similar continuous torque ratings; torque rating is determined by mass and size. speed, peak torque, amps, volts are all engineered and can be almost anything in a given size motor.
4) this cont torque rating is that: continuous - what the motor can put out forever w/o overheating, NOT max or peak torque. so if you apply correct volts and current to this motor rated .5 or .75 or 1nm, it will put out a peak torque - probably around 5nm! that is why u cannot stop it by hand!
5) DO run the motor on 12v car battery and get your bike tach reading; I calced based on numbers down in the mud at 3v... do 12v and we will have a good speed to work with! remember, this test will let us calculate current rating and max speed rating of the motor, not cont torque rating; that is determined by physical size only. two different 'tests' to get your full motor data.

there; this summary reply took only 5 minutes! If you want to see the 1nm 100 rule thing I can post calcs showing that later.

[cnc2]

Hi Mike, thanks for taking the time !

In my mailbox I saw that you replied on this thread without reading the email, I opened the link to the thread, checked out my other emails then, got out of my mailbox to take a look at this thread...but I didn't find your post, I could swear I saw your post in the email, but I had no time left to recheck...So, here's your previous post:

Here is the message that has just been posted:
***************
cnc2,

Thanks for double checking my! I appreciate it when someone will check me - much better to get the right answer if I mess up than give the wrong one, so nope, I am happy, not p'ed! Your 1.5 & 3v were down in the mud and may be giving misleading info.

Let's check your rule of thumb idea another way too, but with the real values.... 1nm is a torque, and it needs a speed associated with it to get a power so lets use the 1500rpm I think your motor will go. so HP=T*N/7121 where T is nm, N is rpm. so 1nm*1500rpm/7121= .33HP... .21hp*746w/hp=157watts.... I like your eff of .82, prob close at speed, so 157/.82 means you need to put in 191watts for 1nm working it backwards... so .5nm would calculate this way to half that - 96 watts.....hmmmm looks like I may be off by a factor of 2 like you say! Only problem is if the Kb is off, so is the Kt and so if they go up so it turns out rated 1nm, the max speed will come down by 1/2 too to 750rpm or so and so the hp & watts will be the same again! Only way to get the power down to match the Kb/Kt numbers would be to have it rated 1/2 the current... something is not jiving. Guess you will have to run it at higher voltages to get it up out of the mud so we can get better numbers. All we need is a good speed reading at a decent voltage (12?) to clean this current rating part up.

The torque rating of 1/2nm was based on similar physical size motors. Since torque goes up by the sq of the dia and up l linearly by length of the actual motor portion (dont count the bearings and brush length), it was easy to see this size motor should be around .5nm comparing it to other similar size brush motors. There is no magic here. I assumed this motor had better than ceramic magnets, so I already assumed it would have higher output torque than a low cost same size motor.

Keep in mind also when considering stopping the shaft that this is the THERMAL OR CONTINUOUS torque rating; as you know motors will output easily 5-10x more torque intermittently if they get enough current, so when you try to stop it by hand, you will be getting much more torque depending on your power supply capability. so you really need an ampmeter in series with the motor lead to verify the current pulled as you load it by hand for the feel tetst - which is probably accurate too!

So get that bike tach hooked up and run that puppy with 12v and let's see what that calculates out to!
***************

Well, I have no car battery, so I'm using the battery charger:

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.

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

At 24V the tach reads 1800rpm.

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.

At 24V I try to stop it, but I only slow it down from 1800rpm to around 1100rpm & it draws up to 1.7A-------In the other direction I had a better hold on it & could slow it down to 840rpm & the ammeter read around 2.4A (I'm not sure it wasn't a peak, because i looked at the meter & released the shaft just before my fingers start burning) Well could slow it down to 945rpm & it read 2.6A then at 814 it read around 2.7A.....I'll have to find some rubber band which will have more grip than a piece of cloth without risking to burn my fingers

Edit: after testing with a piece of rubber, I could slow it down to 900rpm and it read around 2.45A, the rubber melted a bit(but not my fingers) but it was a relatively accurate read. After all this testing, the motor case is a tiny bit warm. when the charger is set to 24V the volt meter measures 20.3V with no load. there's something weird with my voltmeter, when I try to measure the voltage when the motor is running unloaded, the reading goes completely wrong, it doesn't care about polarity anymore & it reads 24V 35V 50 58 78V...crazy sh*t.

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 ? (I think the poles are OK because when I invert the ammeter's poles it shows me a minus sign) so, is it correct to put the ammeter after the motor ? (I use it very rarely)

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

That's it for today, I think you have enough numbers to play with

Thanks !
cnc2.

[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.

[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