Is the torque constant of a DC motor the same regardless of voltage?

[bbq_build]

Hi, we know that the speed varies inversely with the torque. The more the torque, the lower the speed. From torque-speed curve, we can calculate the slope. When the applied voltage increases, the straight line shifts upward while when the voltage decreases, the straight line shifts down. How about the torque vs. current line? We know that current varies proportionally with the torque. The more the torque, the more the current. Am I right that when the voltage increases, this line also shifts upward? The torque constant Kt is Torque/Current. Is this Kt the same regardless of the supply voltage?

 

[bbq_build]

Since Torque = Kt * Current, Kt should be the same even the input voltage is changed. Could anybody please confirm?

 

[bbq_build]

times

 

[jim hardy]

Since Torque = Kt * Current, Kt should be the same even the input voltage is changed. Could anybody please confirm?

A question well stated is half answered .

What about flux, Φ ?

Here's the two empirical formulas i was taught

Counter EMF = K X Φ X RPM
Torque = same K X Φ X I_armature X 7.04 , the 7.04 being for Torque in foot-pounds or some other number for Newton-Meters
Φ being flux from field if you neglect armature reaction(which we do when first beginning our studies) .
So i grew up with only one K and a constant for units (7.04 for foot pounds) not separate K's for t and e,, plus a flux term Φ.

So i don't know how you've accounted for flux in whatever motor problem it is you're attacking .
That's why it is better to state the formula you're using,

Nowadays i see textbook formulas with no flux term at all so i guess they lump it in with the K's. Read very carefully to be sure of what your author did.

If your motor has a permanent magnet for its field then Φ is obviously unaffected by applied voltage and your Kt will likewise be unaffected.
If it has instead a wound field in parallel with the armature(shunt field) then obviously Φ and Kt will both be in proportion to applied voltage .

If it's got a wound field in series with the armature(series field) then Φ is harder to nail down.
IIf it has both a series and a parallel field (compound) then Φ is even harder yet to nail down but not impossible.

So my question is two part (and i hope this is well stated) :

"... Does your Kt term include flux?
If not,
.. What kind of field does this DC motor have: shunt ,series, compound, or permanent magnet?"
Nail that down and your question is half answered. Maybe more.

old jim

 

[bbq_build]

A question well stated is half answered .

What about flux, Φ ?

..."... Does your Kt term include flux?
If not,
.. What kind of field does this DC motor have: shunt ,series, compound, or permanent magnet?"
Nail that down and your question is half answered. Maybe more.

old jim

Thanks old jim.

Can I measure the flux, Φ without special equipment ?
As for finding the kind of field, do I have to disassemble the motor and see if the internal is similar to one of those mentioned in:

https://www.electrical4u.com/types-of-dc-motor-separately-excited-shunt-series-compound-dc-motor/

 

[jim hardy]

Can I measure the flux, Φ without special equipment ?
As for finding the kind of field, do I have to disassemble the motor and see if the internal is similar to one of those mentioned in:

https://www.electrical4u.com/types-of-dc-motor-separately-excited-shunt-series-compound-dc-motor/

Great link !

Best way is to look,
but if a screwdriver is strongly attracted to the frame it's a pretty safe bet there's a field magnet inside.

You can get the product KΦ by spinning at known RPM (electric drill perhaps?) and measuring how much voltage it makes.
since open circuit voltage E = KΦRPM , KΦ = E/RPM which is probably same as your author's Ke .
If you can measure speed and plot volts vs RPM you'll have repeated a school lab exercise.

If it's permanent magnet motor you're all set. Kt = Ke X a constant for torque units, 7.04 for ft-lbs.

Have Fun ! Post pictures ?

 

[bbq_build]

I tried different screwdrivers but I cannot open the motor. Know what kind of screw driver I need to do that? The only photos I can get right now are posted as following.

https://bbqbbq.smugmug.com/Motor/i-ZVPc6T4/A
https://bbqbbq.smugmug.com/Motor/i-zXb2B87/A
https://bbqbbq.smugmug.com/Motor/i-KJgwKCR/A
https://bbqbbq.smugmug.com/Motor/i-KJgwKCR/A
https://bbqbbq.smugmug.com/Motor/i-zXb2B87/A
https://bbqbbq.smugmug.com/Motor/i-ZVPc6T4/A

 

[jim hardy]

Know what kind of screw driver I need to do that?

wow looks like a square drive might turn it
they're used for wood screws, your lumberyard will have bits for ¼ inch hex driver

Does that inside part with the winding rotate, or is it part of the frame as it looks ?

Sure looks like a field winding. The wires look big enough to be a series field but i have no idea of scale. Can you estimate a gauge?
What's the rated current of your motor ? What's it made for?
Series motors have great starting torque so are used for hard to start loads like engine starters or winches or locomotive traction motors..

You're getting close !
Can you dream up a way to measure its torque - drill a hole in a wood scrap and affix to shaft, then with a fish scale measure torque?
A plot of locked rotor torque vs current will be linear for permanent magnet, and a square relation for series wound.

 

[bbq_build]

When I rotated the motor shaft by hand, the inside part with the winding rotated as well. It was not a smooth turn. I felt like there were six pauses as I turned the shaft to its original orientation. To characterize the motor, is this way of measuring the torque ok?

 

[jim hardy]

When I rotated the motor shaft by hand, the inside part with the winding rotated as well.

That's Good, means they're not the field .

Here's what i think I'm seeing, perhaps you can feel if the "magnet" attracts a small screwdriver tip.

To characterize the motor, is this way of measuring the torque ok?

Indeed that's the exact principle.
Vise Grips - handy as Duct Tape , aren't they ?

Can you control current through it somehow? Maybe a car headlight in series?
Plot torque versus three different currents and you'll know if it's linear.
From your description and picture I suspect it's a permanent magnet motor and you'll find it pretty linear.

It's Platitude time again ---
We learn better by doing than by reading about doing.
One experiment is worth a thousand expert opinions.

Go man , go!

 

[jim hardy]

I felt like there were six pauses as I turned the shaft to its original orientation.

That too sounds like a permanent magnet motor.
Though we can get fooled by stepper motors. Only two wires on this one i trust?

I'm anticipating your success - have fun !

 

[CWatters]

Ideal DC permanent magnet motors accelerate until the back EMF equals the supply voltage so they are more like constant speed devices rather than constant torque. Real world motors however have losses due to the brush and winding resistance that effectively reduces the supply voltage and hence the rpm becomes dependant on the load torque.

This effect is more pronounced for ferrite motors than rare Earth motors of the same specification. That's because ferrite magnet motors need more turns to create the motor constant k that determine the motor speed (K is in rpm/volt). More turns means more resistance.

 

[bbq_build]

That too sounds like a permanent magnet motor.
Though we can get fooled by stepper motors. Only two wires on this one i trust?

I'm anticipating your success - have fun !

It is not a stepper motor but a DC motor for sure. Yes, only two wires coming out from the motor. I asked the company to confirm if it is permanent magnet. We will see.

Supposing that it is, all I need to do is to measure the torque (using the method in the YouTube video I linked above) and the current. Then Torque/current to obtain the Torque Constant Kt. Am I correct?

Could you please check my setup for measuring the current?
https://bbqbbq.smugmug.com/Motor/i-zKbVVGf/A

 

[jim hardy]

Then Torque/current to obtain the Torque Constant Kt. Am I correct?

Right.

Could you please check my setup for measuring the current?

Looks right . Be Aware though it'll surprise you with its high starting current so you want a source of less than motor's rated voltage.

if it's a one amp motor it might draw ten amps with normal voltage and its rotor locked by the torque measuring device.
Furthermore at that high amperage, internal flux will get pushed out of its intended shape by something called "armature reaction" so you'll get a low number for Kt.

Armature Reaction is just the magnetic field from armature current interfering with the one from the field magnet. It pushes the field off-axis. It's a necessary evil in DC motors but at normal current levels it's not much of an issue.​

Some way to limit current to about normal running amps will help you both not blow the fuse in your ammeter AND get a good Kt measurement .

Maybe a car headlight wired in series, or use a supply that's only capable of whatever your motor should draw.
I have used a car battery charger set for the 2 amp position .

Practical as you are you'll come up with something.

old jim

 

[bbq_build]

Thanks old jim. From what I gathered from the company, at the recommended voltage of 7V, the no load current is 1.2A and the lock current is 19A. The motor has a gearhead pre-installed. Not sure if the data were obtained with or without the gearhead. Probably no because it is "no load" current? My amp meter can take a max of 20A. Do you think the setup should be fine as is without extra component? At what voltage do you recommend me to do the experiment under? Note that I plan to drive the motor at about 12V in my application. (I have tried it before. Motor could survive without problem.) I don't have a car so no headlight nor car battery charger. What other component do you recommend?

Due to the way I measure the torque using the method presented in the youtube video, I should have the gearhead connected to the motor shaft. In this case, even there is nothing connected to the shaft coming out from the gearhead, the current is not a no load current due to the gears. So, if I model the motor using the method presented in the following video, all the parameters (resistance, inductance, back EMF constant, Torque constant, Voltage caused by back EMF, speed, inertia of the motor system) should be measured with the gearhead installed. I will not need to multiply the simulated torque result by the gear ratio. I also do not need to divide the speed by the gear ratio as all the parameters are measured with the gearhead on. In other words, I model the motor with the gearhead on all the time rather than model the motor without the gearhead and then try to adjust the resulting simulated torque and speed. Am I right?

 

[jim hardy]

What other component do you recommend?

I'd try a six volt lantern battery. You can get one with a lantern for around four bucks at Walmart.

Due to the way I measure the torque using the method presented in the youtube video, I should have the gearhead connected to the motor shaft. In this case, even there is nothing connected to the shaft coming out from the gearhead, the current is not a no load current due to the gears.

I like graphical solutions for i can visualize what I'm doing.

To determine Kt by torque measurement your rotor will be locked. So the gears won't matter, just they'll multiply torque.
That's why you want limited current so as to neither strain the gears nor distort field flux.
If you can get several pints to plot you'll see hopefully a pretty straight line.
At stall there's no counter-EMF . That means if you measure voltage across motor while it's stalled you can calculate its internal resistance by ohm's law.Your Unloaded test is for determining Ke and you need to measure RPM.

If you run both tests with the gearhead attached you will get K's that include the gear ratio.
You can estimate gearhead.friction by measuring running current with and without gearbox attached, Δamps X Kt .

That same Δamps X internal resistance is how much more of the applied voltage would make more RPM were the motor unloaded.
So on your RPM versus volts plot, extend the line on out that many more volts. That's your endpoint for RPM vs Volts plot, slope is Ke.

Manufacturer doubtless has curves for this motor but without the gearhead. You can adjust speed and torque for the gear ratio.

Please Double-Check my thinking - I've been sort of fuzzy headed of late . See my recent blooper over in Mechanical Engineering Dwell Meter thread !old jim

 

[jim hardy]

In other words, I model the motor with the gearhead on all the time rather than model the motor without the gearhead and then try to adjust the resulting simulated torque and speed. Am I right?

That is what i would do.

See my suggestion for estimating gearhead friction in post immediately previous, apply that correction to your "No Load " RPM. measurement

Motors are fun because they're so logical. Imagine the excitement those 19th century guys must've shared, you can sense it in their writings.
This is a fascinating book. My copy is the 1901 edition.

https://archive.org/details/dynamoelectricma00thomrich
And No, it wasn't my textbook in school . I'm not THAT old !

old jim

 

[bbq_build]

Thanks old Jim. I read several books mentioning that Kt = Ke. Isn't it true all the time? I thought by measuring the torque and the current, I can then get the Kt by dividing the measured torque by current. Then, since Back EMF constant = Torque constant, by measuring the angular speed of the motor (with the gearhead on), I could get the Back EMF voltage by multiplying Kt by the angular speed. Am I right that this paragraph is true only if the motor is under load but it is still turning.

By "to determine Kt by torque measurement your rotor will be locked", what do you actually mean? Applying a torque so that the motor does not turn (no angular speed) at all? If so, I have to fix the motor to the desk or keep holding the motor down tightly to measure the torque using the method in Post#11.

 

[jim hardy]

I read several books mentioning that Kt = Ke. Isn't it true all the time?

Not in any course i ever took.
Every author is free to define his terms and derive whatever he wants. Can you post an excerpt?

I thought by measuring the torque and the current, I can then get the Kt by dividing the measured torque by current.

That's quite so.

Then, since Back EMF constant = Torque constant,

You'll have to show me how that can be so. I don't accept it.

by measuring the angular speed of the motor (with the gearhead on), I could get the Back EMF voltage by multiplying Kt by the angular speed.

Sounds wrong. You'd multiply speed by Ke not by Kt.
Check what units your author uses. I am accustomed to Volts Amps RPM and foot pounds . Your K's will be different from mine if you use radians per second or Newton-Meters.

LATE EDIT - Turned out this is the heart of the issue at question, units.
In Si units of Newton Meters for torque and radians per second for speed ,Kt and Ke are numerically equal and when one goes back to freshman physics it's apparent why.
If one mixes systems using Foot Pounds for torque and RPM for speed, then Kt includes factors to adjust for those units and that's my 7.04. I went through school "when slide rules roamed the earth" and SI was still catching on. Even though Volts and Amps are already SI I regarded it just another fad, never dreaming that Mechanicals would capitulate !.
old jim

By "to determine Kt by torque measurement your rotor will be locked", what do you actually mean? Applying a torque so that the motor does not turn (no angular speed) at all?

Well of course ! You showed a picture with a vise-grip plier holding the shaft. How could it possibly turn ?

Go to your own picture.

To determine Kt you lock the rotor so there's zero counter-emf V_B, apply current and measure torque. Kt = Torque per ampere.
If you had a proper dynamometer you could do that while the motor is turning and indeed we did that in my machinery class, but you haven't indicated that you have a dynamometer.

Next you force armature current to zero by open circuiting the motor and measuring its open circuit voltage, that is completely unloaded, .at known speed. That forces zero volts across R and L giving you V_B and RPM, their quotient is Ke.
But since you don't have a method to spin the motor open circuited you'll have to sneak up on V_B.
So try this:
Run it unloaded and measure RPM and current.
Since you measured R earlier by volts across motor and amps through it when you had the rotor locked for torque test,
you can figure what is drop across R when running unloaded at less current..
V_B is what's left when you subtract that drop across R from applied voltage. Do it with as little load as possible so drop is small compared to V_B
Ke is V_B/RPM



Think in steps. We all want to leap straight to an answer. That's why Fortran is so good for young developing brains, it makes us think in single steps toward a result.
How many one line Fortran programs have you seen?

old jim

 

[jim hardy]

Hmmm i see Baluncore gave the same advice yesterday.

 

[bbq_build]

I have seen Kt = Ke in several places. Maybe I have overlooked some implicit assumptions?

Enclosed are some links for your reference:

https://bbqbbq.smugmug.com/Motor/i-MxmVDQk/A
https://bbqbbq.smugmug.com/Motor/i-TNXwnwx/A
https://bbqbbq.smugmug.com/Motor/i-m4hgz5t/A

http://www.motioncontroltips.com/faq-difference-between-torque-back-emf-motor-constant/
http://ctms.engin.umich.edu/CTMS/index.php?example=MotorSpeed&section=SystemModeling
http://www.swarthmore.edu/NatSci/mzucker1/e28_f2014/homework4.pdf
http://www.eng.utoledo.edu/~wevans/Lab6_EET4350.pdf
http://academic.csuohio.edu/richter_h/courses/mce380/DCmotor_estim.pdf

 

[David Lewis]

The current drawn by a freely spinning motor is the no-load current (I_o).
A more accurate formula for available torque is: T = (I -I_o)Kt
Kt is expressed in Newton-metres per amp (N-m/A)
Ke is in volts per radian per second = volt-seconds per radian (V-s/rad)
If you do the math, N-m/A and V-s/rad work out to be the same unit.
For any given motor, Kt = Ke

 

[jim hardy]

I have seen Kt = Ke in several places. Maybe I have overlooked some implicit assumptions?

No it looks more as if my 7.04 is the ratio of Ke to Kt I'd get because I work in RPM and foot-pounds.
and
1.0 is the ratio you'd get because you work in radians per second and Newton-meters

1RPM = 0.10472 rad/sec

1 ft-lb = 1.3558 N-m

and i notice that my 7.04 X 0.10472 X 1.3558 = 0.9995 which is close enough to your 1.0 for my slide rule accuracy of three digits.

Okay, the old guy learned a new trick ! Thanks ! Deja Vu here - did i go through this around 1975 ? If so it slipped away..

That's downright handy to know.
It means you can cross check your measurements of K by comparing the stalled torque & current result with a voltage & speed measurement.
And do I ever like cross checks !old jim

 

[bbq_build]

Thanks David and Jim for checking. Let's get the ideas clear.

1. If I use Nm and rad/sec, then the Motor Constant Kt = Back EMF Constant Ke all the time under ALL situation. Am I right?

2. So, "I thought by measuring the torque and the current, I can then get the Kt by dividing the measured torque by current. Then, since Back EMF constant = Torque constant, by measuring the angular speed of the motor (with the gearhead on), I could get the Back EMF voltage by multiplying Kt by the angular speed." is correct. Am I right?

3. Am I correct that the above paragraph is correct only if the motor is under load but it is still turning?

4. If the motor is under load but the torque is so high that the motor does not turn, then there is no back EMF. In this special case, Back EMF (i.e. Ke) = 0. However, as Kt = Torque/current, Kt is NOT equal to Ke. So, Point 1 above is correct Only Except for this situation. Am I correct?

 

[CWatters]

To determine Kt you lock the rotor so there's zero counter-emf VB, apply current and measure torque. Kt = Torque per ampere.

Just take care you don't burn out the motor doing that.

 

[jim hardy]

Thanks David and Jim for checking. Let's get the ideas clear.

1. If I use Nm and rad/sec, then the Motor Constant Kt = Back EMF Constant Ke all the time under ALL situation. Am I right?

True. Physics says so.
The practical troubles you might encounter are things like not knowing what is voltage drop across the brushes. I think you hinted at difficulty measuring ohms in your other thread in mechanical engineering forum. That sliding contact with commutator makes the reading fluctuate. In large motors it's common to assume 2 volts for brush drop.

2. So, "I thought by measuring the torque and the current, I can then get the Kt by dividing the measured torque by current. Then, since Back EMF constant = Torque constant, by measuring the angular speed of the motor (with the gearhead on), I could get the Back EMF voltage by multiplying Kt by the angular speed." is correct. Am I right?

Right.

3. Am I correct that the above paragraph is correct only if the motor is under load but it is still turning?

Strictly speaking that paragraph is correct even for motor not turning, ie zero angular speed, because Kt X zero angular speed will give you zero volts, and that's correct back EMF for zero speed. Why you would calculate that i don't know, I'm just making the words precise.

4. If the motor is under load but the torque is so high that the motor does not turn, then there is no back EMF. In this special case,

Correct.

Back EMF (i.e. Ke) = 0.

NO ! Back EMF is product Ke X speed and SPEED term not the Ke term that's zero.

However, as Kt = Torque/current, Kt is NOT equal to Ke.

see statement immediately above.

So, Point 1 above is correct Only Except for this situation. Am I correct?

No, stalled condition is not an exception.
You seem to be confusing Ke with product Ke X angular speed(your term, i like ω)

Any help?
Seems to me you might just call it one motor constant K and cross check it by measurements of speed with it as close to unloaded as you can get, and torque when stalled.
In lab we actually spun the motor with nothing connected to its terminals but a voltmeter, and measured how much voltage it makes . That guarantees zero current so all the voltage IS Back EMF.

Bravo on your stepwise thinking !
...
.

old jim

 

[David Lewis]

1. If I use Nm and rad/sec, then the Motor Constant Kt = Back EMF Constant Ke all the time under ALL situation. Am I right?

Yes, but remember it's an idealized model. We usually ignore iron losses, temperature coefficients, friction, armature reaction, etc. because, for a well engineered motor operating within its normal design range, it's close enough.

Moreover, it doesn't matter what units you use because motor formulae tell you what is actually going on in reality. Units of measure are just a convenience for the human mind. They have no effect on physical phenomena.

 

[bbq_build]

True. Physics says so.
The practical troubles you might encounter are things like not knowing what is voltage drop across the brushes. I think you hinted at difficulty measuring ohms in your other thread in mechanical engineering forum. That sliding contact with commutator makes the reading fluctuate. In large motors it's common to assume 2 volts for brush drop.
old jim

Thanks old jim for the verifications. So, we could use this method as a double check to verify the result obtained using the method suggested the thread "Is this a valid way to measure the Back EMF of a DC motor?" in the mechanical engineering forum?

I am a bit confused about measuring the resistance of the motor. In that thread, it was suggested that I "Measure resistance of motor when still, R.". In this thread, you mentioned that "if you measure voltage across motor while it's stalled you can calculate its internal resistance by ohm's law." Are these two ways to measure the internal resistance of the motor and the values should be very similar?

I should be able to test the resistance of the motor when it is still using a better multimeter in a day. I will report the result.

 

[bbq_build]

Yes, but remember it's an idealized model. We usually ignore iron losses, temperature coefficients, friction, armature reaction, etc. because, for a well engineered motor operating within its normal design range, it's close enough.

Moreover, it doesn't matter what units you use because motor formulae tell you what is actually going on in reality. Units of measure are just a convenience for the human mind. They have no effect on physical phenomena.

Thanks David.

 

[jim hardy]

Thanks old jim for the verifications. So, we could use this method as a double check to verify the result obtained using the method suggested the thread "Is this a valid way to measure the Back EMF of a DC motor?" in the mechanical engineering forum?

I am a bit confused about measuring the resistance of the motor. In that thread, it was suggested that I "Measure resistance of motor when still, R.". In this thread, you mentioned that "if you measure voltage across motor while it's stalled you can calculate its internal resistance by ohm's law." Are these two ways to measure the internal resistance of the motor and the values should be very similar?

I should be able to test the resistance of the motor when it is still using a better multimeter in a day. I will report the result.

The sliding contact between bottom of carbon brush and copper commutator makes it hard to get a steady reading with a multimeter.
In operation there's arcing so you get voltage variations. In big machines it's common to assume two volts of brush drop but obviously on 1.5 volt motors that run on a flashlight battery that's not plausible.Try it both ways, with your ohmmeter and by the voltage drop at stall method i suggested.
Then reach into the motor and measure right on two commutator segments opposite each other. That'll be the actual armature resistance and probably less than an ohm. I don't know what to expect for resistance of your brushes, but if you're curious pull and measure them.

.........................................

I've woke up two mornings now marveling at the revelation about Kt and Ke can be equal.
Kt depends on area of armature coils because that determines their magnetic dipole moment
http://hyperphysics.phy-astr.gsu.edu/hbase/magnetic/magmom.html

Ke depends on area of armature coils because that determines how much flux they encircle, and time derivative of that flux is induced voltage.
http://hyperphysics.phy-astr.gsu.edu/hbase/electric/farlaw.html

So at the heart of both K terms we find
area of armature coils and flux density
but for different reasons.
For Kt area determines lengths of elements making a torque couple
for Ke area determines number of Webers encircled.

By judicious choice of units, namely SI throughout (Volts and Amps and Webers/m² already are SI),
we can arrive at one constant for the motor
just call it K
which can be multiplied by current to predict torque
and can be multiplied by dθ/dt = ω to predict voltage

That's just amazing !

Thanks @bbqbuild and @David Lewis

old jim

 

[David Lewis]

There is a third constant which is 1/Ke:
Motor speed = some constant * voltage
I don't know the correct name. I call it the speed constant (Ks).

 

[bbq_build]

Hello all, I did some measurement of the resistance using a better meter. Here is a photo of the motor:

Is it permanent magnet? The company told me that there is no information on this.
https://bbqbbq.smugmug.com/Motor/i-kCbqzvx/A

1. There are two wires coming out from the DC motor. When I connected the end of the two wires to the probes, I got 0.1 Ohm.
2. When I connected the probes to the two terminals that the two wires are connecting to the two metals coming out of the motor, the values and unit kept changing like cray. Sometimes KOhm, sometimes Ohm. Sometimes O.LMOhm.

So, what is the internal resistance of the motor?

 

[jim hardy]

1. There are two wires coming out from the DC motor. When I connected the end of the two wires to the probes, I got 0.1 Ohm.
2. When I connected the probes to the two terminals that the two wires are connecting to the two metals coming out of the motor, the values and unit kept changing like cray. Sometimes KOhm, sometimes Ohm. Sometimes O.LMOhm.

My guess is you're shaking the brushes when you push on the metal tab that holds them and losing that tenuous sliding contact.

Try to read between commutator segments
then from ach commutator segment to the wire of the brush touching that segment. Wiggling the shaft might help the reading settle down. Take the lowest stable reading..

So, what is the internal resistance of the motor?

From the readings you report , looks like less than 0.1 ohm.

How about if you search for a datasheet on that motor , using its model number ? Surely it has one.

 

[Tom.G]

at the recommended voltage of 7V, the no load current is 1.2A and the lock current is 19A.

Post 17 yields a motor resistance of 7Volts/19Amps = 0.368 Ohms.

 

[CharlesS]

On a DC motor as the voltage increases the amperage required to turn the motor decreases. Torque and RPM also increase with increased voltage. DC motors are used in variable speed applications, where as the voltage determines the RPM of the motor. As the voltage increases and the amperage required to turn the motor the operating temperature of the windings decreases and the motor will run cooler.

 

[bbq_build]

Post 17 yields a motor resistance of 7Volts/19Amps = 0.368 Ohms.

This calculated value is quit different from the measured 0.1 Ohms. Know what is going on?

Manufacturer does not want to provide full specifications nor tell me if it is a permanent magnet. The company guy provided a few values through a connection. He said that these are all he got from the manufacturer. He has no idea what they mean and he cannot get further information from the manufacturer as they do not usually provide even these information to the public. Some values they provided do not seem to match (e.g. Points 3 and 4).At the recommended input voltage of 7V. Don't know if they measured these values with the gearhead on or not. Since I am driving the motor at a higher voltage, may be I can use only a few of these values?
-------------------------------------------------------------------------------------------------------------------------------------
Poles: 3P
No load speed: 54rpm
No load current: 1.2A
Lock current: 19A (at this characteristics)
Torque Constant :3.550(M.N.M/Amp.)
E.M.F Constant :3.550(MV/RAD/SEC.)
Dynamic Resistance :0.123(Ohms)
Motor Regulation :82.740(RPM./M.N.M.) <----- What is motor regulation?
Moment of Inertia :no data
Elect/mech. Time Constant :no data

Motor characteristics curves: https://bbqbbq.smugmug.com/Motor/i-DD9TXqp/A

1. Is Dynamic Resistance the resistance of the windings? 0.123 Ohms seems to be close to the measured 0.1 Ohms. Since resistance is constant regardless of input voltage, am I right that to simulate the motor, I just let R = 0.123 Ohms or 0.1 Ohms?
2. Since the torque constant is constant regardless of the input voltage, if I drive the motor at 12V, will both the torque constant Kt and emf constant Ke be 3.55 as well?
3. V = IR but 7V does not equal to 19A*0.123 Ohms nor 1.2A*0.123 Ohms. I am confused. Anybody knows what is going on?
4. Based on the torque vs current curve, 0 kgcm at 1.2A and 135 kgcm at 17.5A. After unit conversion, 0 Nm at 1.2A and 13.24Nm at 17.5A. Since Torque = Kt * I, Kt = 13.24Nm/17.5 = 0.76 but the listed torque constant is 3.55. Am I missing something?

 

[CWatters]

Got a link to the motor on the supplier/manufacturer web site?

Any text on the motor backplate? Sometimes there are numbers moulded into it.

It looks like a cheap permanent magnet motor perhaps one of the Mabuchi 380, 400 or 500 family? However many manufacturers make motors in these general sizes so that doesn't help at all.

Motor Regulation :82.740(RPM./M.N.M.) <----- What is motor regulation?

That's the slope of the rpm vs torque curve. eg the change in rpm for a given change in load torque (measured in milli Newton meters mNm)

 

[bbq_build]

Yes, I could reach the commutator segments. The measured resistance was 0.1 Ohms.
I also used paper to separate the commutator from the two golden color square-like terminals. The measured resistance was 0.2 and moved down to 0.1 Ohms.

 

[jim hardy]

ost 17 yields a motor resistance of 7Volts/19Amps = 0.368 Ohms.

Well it sure does - i thought i was being attentive but breezed right past it.

From what I gathered from the company, at the recommended voltage of 7V, the no load current is 1.2A and the lock current is 19A.

So there must be some sort of datasheet from which that came ? Just whoever bbq_build is communicating with doesn't have it.

Aha maybe he does after all
from curve linked in post 38,

Motor characteristics curves: https://bbqbbq.smugmug.com/Motor/i-DD9TXqp/A

annotated ...

I never before encountered the term "Dynamic Resistance" applied to a motor.

Quick search turns up this
https://electronics.stackexchange.c...f-a-brushed-dc-motor-for-use-in-speed-control
which suggests it includes resistance of the brushes and that tenuous sliding contact i mentioned.

I suspect your voltmeter is incable of resolving to better than 0.1 ohm, that's why i suggested measuring volts and amps with locked rotor.
Note the guys at stack exchange do that at about 1/4 current , doubtless to avoid warming the armature coils.

What do you get for Kt and Ke using that speed-torque curve you linked?
How close do your measurements agree with their curve ?

 

[bbq_build]

What do you get for Kt and Ke using that speed-torque curve you linked?
How close do your measurements agree with their curve ?

Based on the speed-torque curve data, I calculated Kt. The answer is in Post#38. Over there, I mentioned that the calculated value is very different from the value given by the manufacturer. Please check that post for details.

 

[David Lewis]

The slope of the torque versus speed curve = (Ke)²/R

 

[David Lewis]

It was suggested that I "Measure resistance of motor when still, R.". In this thread, you mentioned that "if you measure voltage across motor while it's stalled you can calculate its internal resistance by Ohm's Law." Are these two ways to measure the internal resistance of the motor and the values should be very similar?

The simple ohmmeter measurement is not accurate. The locked shaft method is a little better. You need to have at least 0.25 times the normal operating current flowing through the motor. (For a short enough time that the armature doesn't heat up excessively.) Step the shaft through several different positions and take the average.

 

[jim hardy]

Based on the speed-torque curve data, I calculated Kt. The answer is in Post#38. Over there, I mentioned that the calculated value is very different from the value given by the manufacturer. Please check that post for details.

I saw that.

Have you yet locked the shaft with vise-grip plier and measured volts&amps at stall ?
That was my question, to verify the curve you were given is the curve for the motor you have.
I think in tiny steps. That'd be my next one.

 

[bbq_build]

I saw that.
Have you yet locked the shaft with vise-grip plier and measured volts&amps at stall ? .

I set the power supply output to 4V. Then, measured the voltage across the motor and at the same time measured the current (using a multi-meter in series with the motor). I did 10 runs. The averaged voltage 5.77 Volt and the averaged current is 0.554A. At each trial, after turning on the motor, I locked the shaft with a vise-grip piler. Wait for a few seconds and did the measurements. How could the measured voltage at each trial be larger than the supplied voltage? If I calculated the resistance this way, it will be: R = V/I = 10.4 Ohms which is different from the 0.1 Ohms I reported previously and the 0.123 Ohms provided.

 

[bbq_build]

I am very sorry. I made a mistake in Post #46. It appears that the averaged voltage was 0.554V while the averaged current was 5.77A. (The reverse seems to make more sense though.) As a result, since R = V/I, R = 0.553V/5.834A = 0.09 Ohms which is close to my measured 0.1 Ohms and the 0.123 Ohms given by the company. What do you think? How come the voltage across the motor dropped by about 10 times to 0.554V? The increased current seems to make sense as more torque (stall the motor) requires more current in an attempt to spin the motor.

 

[David Lewis]

How come the voltage across the motor dropped by about 10 times to 0.554V?

Possibly your 4V power supply has a high internal impedance.

Note also, the effective armature resistance varies depending on the operating point of the motor. This is the net resistance value you want to plug into your performance calculations.

 

[bbq_build]

Possibly your 4V power supply has a high internal impedance.

Note also, the effective armature resistance varies depending on the operating point of the motor. This is the net resistance value you want to plug into your performance calculations.

Thanks David. I did one more set of tests. 10 runs at 5 volt from the power supply. The averaged measured voltage was 0.638V while the averaged measured current was 7.35A. The calculated resistance became 0.638/7.35 = 0.0868 Ohms.

When I tried input voltage at 6 Volt from the power supply, I smelled smoke and stopped immediately.

Which net resistance value do you mean?

 

[David Lewis]

The armature resistance varies under dynamic conditions; you have armature reaction, a small (we hope) amount of arcing at the commutator, heating of the coils, and so forth.

The most accurate method is to run the motor near the design operating point and use:
Armature resistance = (power supply voltage - back EMF)/(motor current)

 

[bbq_build]

The armature resistance varies under dynamic conditions; you have armature reaction, a small (we hope) amount of arcing at the commutator, heating of the coils, and so forth.

The most accurate method is to run the motor near the design operating point and use:
Armature resistance = (power supply voltage - back EMF)/(motor current)

Thanks David. In this equation, I would need to have the back EMF first.

I think now we are trying to calculate the back EMF using the resistance. So, there are many underlying factors that affect the results. Based on the experiments, should I choose 0.0868, 0.09, 0.1 or 0.123 Ohms as the resistance to put in the Simulink model?

 

[David Lewis]

None of the above. Normally you measure Ke first by back driving the motor,
and using a tachometer and voltmeter.
After you know Ke, calculating back EMF is easy.