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Old Feb 23, 2004, 05:15 PM
Bertrand MICHELS
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Belgium
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efficient 4mm pager motor ?

I found that the 4mmx11 pager 13 ohms motor is MUCH more efficient that the 4mmx8 10 ohms version ( but still in the low 25% efficiency ! !), and Unfortunately, it is also 50% HEAVIER that the 4mmx8 version.

Does someone know better 4mm motors that the usual DIDEL versions. ?

SMOOVY used to manufacture high-end 3mm and 5mm motors for industrial use. I believe that they do not do this anymore.

DOES someone knows High-tech 4mm sourcing, of higher quality (and price of course !) that the PAGER type ?
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Old Feb 23, 2004, 10:12 PM
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bmaa
I do have a 13 ohm 4mm x 11mm cored pager. It came from Electronics Goldmine I think. Not sure if it is more efficient than the coreless motors. It is a surplus pager motor.
I am interested in your IR RX how did you get it down to .3g with sensor? I have done wuite a bit of lightening and can only get mine down to .6g with wires,plugs, and sensor. I am also interested in your source for gears. I seen the picture of your model in the other thread. Very nice work and weight.

Billy
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Old Feb 24, 2004, 03:21 AM
Bertrand MICHELS
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Belgium
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motor comes from DIDEL, the gears and motor mount comes from a FALCOM servo (there are 0,25 mod).

The I/R has been menufactured by a Friend. I will ask mim additional infos for you. He espect to reduce even further the weight to 0,2 by sanding the components (a lot of plastic for casing the IC and the I/R receiver)
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Old Feb 24, 2004, 10:17 AM
Sticky Shepherd
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Smoovy motors are brushless not coreless
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Old Feb 24, 2004, 01:37 PM
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bmaa,

it is also possible that the 10 Ohm pager was mismatcheed to the prop or the gearbox in your tests. You will usually find the most output power at 50% of the stall curent at a given voltage, the best efficiency is near 30-35% of the stall current. To make things more complicated, one motor might get damaged at a voltage that another motor withstands easily.

Of course I don't know your background, but if you want to look deeper into the theory then my article in the Inside Story might help. Look here: <http://www.rcgroups.com/links/index.php?id=4068>.

Regards, Jochen
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Old Feb 24, 2004, 05:15 PM
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BMAA- did you dyno test the motors? Comparing the efficiencies of tiny motors is a tricky business to do without errors. I do direct measurements measuring torque, RPM, input power, Jochen has a different technique.
-Matt
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Old Mar 03, 2004, 05:42 PM
Bertrand MICHELS
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I do not mesure torke, but I always make sure that the motor run between 35% to 50% of the stall curent. Idealy at 50% under full trotle (max power) and 35% at mid-trotle (max efficency)
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Old Mar 03, 2004, 06:17 PM
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I have a crazy question.
What is stall current?

And how much of a load do you have to put on a motor to do no load current test for RPM without burning up the motor?

Billy
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Old Mar 03, 2004, 06:51 PM
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Stall current is the current that flows through the motor if you were to grab the motor shaft with pliers and hold it from moving. Don't do it, unless you want to take a chance of damaging the motor. It's one way of telling if the motor has developed internal shorts in the coils from breakdown of the insulation. That will cause the stall current to be higher.

This current should be equal to the applied voltage divided by the motor resistance.

If you let the motor turn freely, less current flows.

And a no load current test is just that...no load. It's a measure of how much current flows when the motor is allowed to turn freely.

If the no load current for one motor is significantly higher than that of a comperable motor, it has probably sustained some damage.
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Old Mar 03, 2004, 07:05 PM
in persuit of low wing loading
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Billy,
I think you can find the answer in Jochen's motor theory article in the Feb 2003 Inside Story column. Sorry, don't have time to find it right now.

Gordon
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Old Mar 04, 2004, 05:44 AM
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Jochen's equations uses Iddle curent I guess that is no load current. Using Joachin's equations you do not have to know stall currrent just Idle current and then take measurements of rpm, currrent , and voltage with 2 different props that will be a small load and big load on the motor.

I think I have seen some posts that mention stall current so how do you find it without burning up a motor?

With pager motors how much of a load do you have to put on a motor to keep it from burning up when measuring Idle current? I was thinking maybe a ply disk or something might work. I have burnt up a motor or 2 running them with no load.

I know a guy that is pretty smart with pumps and fans and stuff. His dad made him wind a fan motor when he was a kid and told him if he could make it run in a window he would give him a hefty sum of money for it. Well the kid thought he had done good with the motor since it ran so goodion the floor and he put it in the window and it burnt up.

Later on he came up with this equation for pumps
efficiency%= Qrp/Qnp
Qrp = flow at rated presure
Qnp = flow at no pressure

He derived this using some other equations probably similar to what Jochen has done.

So I was thinking there might be some simple way like this to calculate efficiency
with motors and props.
Since we can not easily measure flow and pressure with air then I guess Jochen's equations are the simplest thing we can do.

Thanks Gordon and Nic

Billy
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Old Mar 04, 2004, 07:38 AM
in persuit of low wing loading
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Billy,
Why do you need to know stall current? If you want to possibly sacrifice a motor Nick has already told you how to find it out. I've done it, and it's no big deal. Unless you plan to use the motor for an actuator I'm not sure this information is crucial.

Gordon
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Old Mar 04, 2004, 08:51 AM
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Gordon
I'm not sure I need to know stall curent I had seen it mentioned in some other posts and thought maybe it might be some information you needed for calculations but I guess since it is not needed using Jochen's equations I dont need to know stall current. I guess what I should be wondering how come folks keep mentioning it since you dont need to know it to figure out how to run your motors at best efficiency. I don't have enough good motors to sacrifice anyways. I did have plenty of 3 ohm motors but have used most of them for parts and stuff.

Billy
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Old Mar 04, 2004, 12:54 PM
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Billy,

If you are burning up motors by running them with no load, the reason was probably that you were running at too high of a voltage for that motor. Too much voltage causes too much current to flow, which heats up the windings.

With pager motors, it doesn't take more than a very brief moment at too high of a voltage and the motor will be ruined. One type of failure I have heard described is that the windings get warm enough to melt the tape holding them and the tape drags on the motor causing it to take more current to run than it should.

Any (DC) motor, when voltage is FIRST applied, just before it starts moving, has the "stall" current running through it. As the motor starts to turn, the current drops, until it reaches a speed where all of the forces inside balance out. (resistance losses, mechanical friction, air friction, etc, balanced by the motor torque) Assuming there is no load, this is the idle current.

If you put a load on the motor, the motor will slow down and the current will rise a bit. As you put a bigger and bigger load on the motor it will keep slowing, the current will keep rising, until the motor comes to a stop and the current will rise to the stall current.

One way of safely estimating the stall current, is to measure the resistance of the motor. If you divide the voltage by this resistance, (simple Ohm's law) you will get an estimation of the stall current. Unless you take the motor apart, you will be measuring through the commutator, so the measurement may not be accurate. You might have to turn the shaft of the motor a few times to get a good reading.

I hope that's not too basic for you. As long as you don't exceed the motor's voltage (or it's RPM) rating, there's no problem with running a dc motor without a load. (anyone know of any exceptions to that?) At least, not with the motors we use.

If you point out where stall current is mentioned, some one should be able to explain why they were talking about it.
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Old Mar 04, 2004, 01:14 PM
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Nick it all makes sense now. Thanks a bunch.
So I can run pager motors with no load to get the no load current as long as it is only for a breif period? I guess the tape had melted on the motors I burnt up with no load. I must have ran them too long with too high of voltage. I dont remember what I was doing but I know they never ran the same after that. It was quite a while back and I have not ran a motor without a load since.

Billy
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