Any one know how to get RM correctly.

[swatson144]

RM being the resistance of 1 phase. In a brushed motor this is not measured but calculated by placing a load resistor in series with the battery. Stalling the motor, measure voltage across the motor, and current from the battery. R=E/I or RM=volts divided by amps. Apparently this is done because it's very hard to measure the tiny resistances of windings with a DVM.

My motors don't quite jib with the motor calculators. IO is prtty simple to get. KV is also simple and either way of measuring don't make that much of a change. I'm thinking my RM values suck is the most likely reason. My meter is a Fluke 77 which should still be a good un.

Seems the direct resistance readings would be more a measurment of the leads resistance than the 2.2 feet of windings.

I know the motor sims aren't perfect but they probably don't suck as bad as the RM readings.

Here is what it says for one of my motors

Dia Inches Pitch Inches I Amps V Volts RPM Thrust Ounces Pitch Speed MPH Power In Watts Power Out Watts Efficiency %
12 6 12.80 9.6 5015 26.1 28.5 123.5 83.7 67.7

actual

12 6 gws ep 15.6 10.2 159.1 5190 34 77.6 48.8 29.5

monkey with RM until close

12 6 16.04 9.4 5659 33.2 32.2 150.1 120.2 80.1

It is also closer to what I got for the smallest prop tested. Sso I surmise that my RM is actually closer to .1 vice .2. I'm sure there are small inaccuracies in the other figures as well, Not having a lab and lab equipment to do the measurements in. Though I'd guess the next biggest is prop constants.

Just wondered if anyone had a more accurate way of measuring RM. SO I can jail the worst offender and be more accurate.

I'll be static testing the 8p for those watching the other threads today.

Steve

[Felix]

Apparently this is done because it's very hard to measure the tiny resistances of windings with a DVM.

Steve

Hello Steve, I don't have any answers for you but I do have questions. I just did a rough calculation and was wondering why you couldn't just measure the resistance with your fluke directy? For example if the windings are 2.2 ft long and you are connected in wye the direct measurement would be yield 2 RM across any two motor leads. Just to see what that would be approx. 26 gauge is ~ 43.6 ohms per thousand, so .0436 x 2.2 x 2 = .1892 ohms as the direct measurement which means RM would be about .0946. Seems like the fluke ought to deal with a direct measurement of .1892 with reasonable accuracy.

Am I misunderstanding what you are saying ? Or looking at this incorrectly ?

Thanks,

Felix

[swatson144]

well direct measurement is what I have been attempting to do. But the accuracy seems to me to be sub optimal. I've actually carefully measured my last 10t triple wind of 28g and continue to get .3! this is a 5mm thick motor! Motor worked fine at 22.2oz at 15660 rpm with a 6x3.5 hp prop. not even warm. tried it with a 7x5 and it was letting out too much smoke to get a good tach reading :cry: ran it down to 1.5 amp to cool and save the magnets. will wrap it again and have a look. I didn't even get to test it on 2s.

I'll have to look at what the accuracy is of the fluke in ohms measrurement but it seems it can't measure fractional ohms fer doo doo

[swatson144]

Thanks Felix.

He PM'd me that after looking up on flukes site the resolution of a modern DVM was .1 ohm. My 15yr old 77 probably ain't no better!

Also for the fact that since I'll be rewinding a motor soon :oops: I'll use his idea and measure the wire and calc the resistance and plug that in and see how it works, comparing motor sim readings based on that RM compared to actual measurements. It could be that simple?

[LBMiller5]

Our motors have very low resistances in the range of about 0.05 to 0.20 ohms. Since most of the digital meters only measure to the nearest .1 ohms they cannot be used to measure the resistance directly. However using our friend Ohms Law, we can measure other values and calculate the motor resistance quite easily.

The easiest way I know of is to use a current meter and a volt meter at the same time. By doing it this way you can calculate the current through the winding and the voltage drop across the winding at the same point in time and get an accurate reading.

Here is how you would do it.

First you need a voltage supply. If you have a power supply you can use it, in fact a power supply is best since the voltage does not change during the test. You can also use a Ni-Cad or Li-Po battery pack as the power supply as well.

Depending on the size of the wire you have wound the stator with, you need to select a current limiting resistor so you do not exceed the maximum current of the wire or the battery pack if you are using one.

Most of the motors we wind have a value for RM in the range of .1 to .2 ohms or something close to this value. For the purposes here I will limit the current through the stator windings to around 2-3 amps. This will give a fairly accurate reading without overheating the windings.

The only part you will need to buy is a current limiting resistor. I would recommend a ceramic power resistor. These are available at Radio Shack, or Fry's electronics, or any other large Electronics Supply Store.

If you are planning on using a 2-Cell Li-Po or 6-7 cell Ni-Cad or Ni-MH battery, your input voltage will be in the 7.5 to 8.5 volt range. For this test I would suggest a resistor of around 3 ohms. You could use a 2.7 ohm, a 3.0 ohm or a 3.3 ohm as any would work. The resistor should be a minimum of 10 Watts, and a 25 watt would be best. At Radio Shack they only carry a few values of power resistors, so if that is your only source, buy three 1 ohm 10 watt resistors and solder the three of them together end to end. this will give you the total of a 3 ohm 30 watt resistor.

If you are planning on using a 12 volt power supply, or a 3-cell Li-Po battery pack, or a 10-cell Ni-Cad or Ni-MH battery, then you will need a different value resistor. For this you will want to use something around 5-6 ohms. You could use a 4.7 ohm 25 watt, or a 5.6 ohm 25 watt. You could also take 2 radio shack 10 ohm 10 watt resistors and solder them together in parallel. To do this, take the 2 resistors and put them side by side. Twist the wires together at each end and solder them together. This will give you a 5 ohm 20 watt resistor.

Now that you have your resistor you are ready to test. If you only have 1 meter you can still do the test, you just have to use a different procedure. The most accurate way is to use 2 meters, one to measure current, and the other to measure voltage. I will describe this method first, and then I will explain the alternate procedure if you only have 1 meter.

OK, Assuming you have 2 meters this is how you would set up the test circuit.

http://innov8tivedesigns.com/rcgroups/MotorTest.jpg

You connect the positive terminal of the battery or power supply to the positive terminal of the current meter. Make sure you set the meter to the 10 amp scale or plug the wire into the 10 amp socket depending on your meter type. Next run the negative terminal of the current meter to one end of your current limiting resistor. Now you connect the other end of the resistor to one of the wires on the motor under test. Finally you connect the other end of the motor wire to the negative terminal of the power supply or battery. To read the voltage drop across the motor winding, connect your volt meter to the motor as shown above. You can turn the power on and off with the switch on the power supply, or if you are using a battery, you can unplug one of the current meter leads to shut the circuit off.

Now that you have the test circuit set up all you have to do is read the values on the 2 meters and do some simple math to get the RM value for the motor. Since the current is the same everywhere in a single closed loop circuit, the current read by the current meter is the exact same current that flows through the motor windings. Each of these examples will use hypothetical values, so each one will be different and should not be compared against one another.

For the first example lets say that we are using a 2-cell Li-Po battery for the power source, and a 3.0 ohm resistor in the circuit. We set up the circuit, plug in the battery, and read the following values:

Current = 2.35 amps

Voltage across motor = .306 volts

To get the resistance of the motor you take the voltage and divide it by the current. In this case it would be .306 divided by 2.35 which equals .1302 ohms.


For another example, lets assume we are using a 12 volt power supply for the test and a 4.7 ohm resistor. After turning on the power supply we might read the following values:

Current = 2.61 amps

Voltage across motor = .487 volts

Again, to get the resistance we divide the voltage across the motor by the current to get the resistance. In this case it would be .487 divided by 2.61 which equals .1866 ohms.


Now, for those of you that only have 1 meter, and have no way of getting another one, you can also measure the resistance of your motor. It takes a couple extra steps, and is not quite as accurate, but it will get you pretty close to the correct value.

I am assuming that you have a digital meter for these tests, since 99% of the meters sold today are digital. The lowest resistance on most of the digital meters is the 200 ohm scale, and you can only measure to the nearest 1/10th of an ohm. Because of this, if you are measuring a resistor in the 3 to 5 ohm range, the best accuracy you can possibly hope for is about +/- 3%. You can avoid this limitation if you buy a precision resistor that is rated at 1% or 2% tolerance on the resistance value. If you use one of these you can just use the value stamped on the part as the resistance, since it is more accurate than you can measure. If you are using a resistor that has a resistance tolerance of 5% or 10%, then you can measure the value with your ohm meter since it will be more accurate.

To get the closest measurement of your resistor measure the resistance across the part and write this number down. When measuring such small resistance values, make sure that you have good contact between the meter probes and the resistor leads. Press them together real hard to make sure you have a good connection.

After you get a resistance reading for the resistor, touch the two meter leads together tightly and write down the value you get. This number is the resistance of the test leads themselves, and must be subtracted from the first reading of the resistor to get the most accurate answer.

For example, lets say we have a 3.3 ohm 25 watt 10% tolerance resistor. When we measure it with the ohm meter we get a value of 3.7 ohms. Then when we short the test leads together we get a reading of 0.3 ohms. Subtracting these 2 numbers we get 3.7 - 0.3 or 3.4 ohms of resistance. So we can use the value of 3.4 ohms for the math later on.

Now we hook up the circuit just as before, except this time we leave out the current meter, and hook the positive lead of the power source directly to the current limiting resistor. Now we take the volt meter and take 2 voltage readings. First, we measure the voltage across the resistor, then we measure the voltage across the motor. With these 2 voltage readings, plus the resistance value we measured previously, we can calculate the motor resistance.

Lets say that in this example we are using a 2-cell Li-Po battery again. When we hook the resistor and motor together and plug in the battery we get the following readings:

Voltage across resistor = 7.14 volts

Voltage across motor = 0.29 volts

With these 2 values we can calculate the resistance of the motor. First we calculate the current through the circuit by taking the voltage drop across the resistor and dividing that number by the restance value we got earlier. For this example we would take 7.14 volts and divide that by 3.4 ohms and get 2.10 amps.

Now, in order to calculate the motor resistance, we take the voltage drop we measured across the motor and divide it by the calculated current value. For this example it would be 0.29 volts divided by 2.10 amps which equals .138 ohms.

So there you have it. A simple, fairly accurate way of measuring your motor resistance with a standard digital voltmeter.

Hopefully that will ge you going in the right direction.

Lucien

[swatson144]

It was that plain all the time...Makes me feel like a very simple man, special even :oops: . The motor must be stalled for the test. Hmm aplying straight dc to the brushless motor it would be stalled! I kept thinking through the speed controller for some reason. Talk about forrest and trees. Thanks Lucien.

Having been led back to the way of the light it occurs to me to use my battery cycler set on 3 amps for a limit (heck I can start at 1/2A and jog it up as I go to see how much difference I get with currents as the fluke has a 300mv scale just to see where it is accurate with minimum motor heat) with the motor in series with the battery pack. using 2 meters this would even test the accuracy of the discharge amp meter in the Triton. Just set it to discharge PB and run the test on the car battery I use for 12v supply as that should provide a stable voltage :D

I'll solder up a test harness this very morning! I'd been putting off testing the accuracy of my watt meter, the triton, and my home made shunt any way and I can get all that behind me while i am building the harness.

[swatson144]

I still haven't completed my constant current rig for testing RM. :oops:

http://www.rcgroups.com/forums/showthread.php?t=233250#post2344085

But I found mumtats. a spreadsheet that calculates RM (and other values) by inputing several performance database tests, among other things. Such as what voltage would give best performance for a motor. It's very effeciency oriented which is great. high PIN + high effeciency = great performance.

One of the problems I encountered was with a motor that behaved way different in the simulators than I tested. As mentioned above I decided it was RM that was off (the others are too simple to measure). I had tested my theorum by changing the RM from a known motor until it most closely matched my measured performance in the sims and arrived at .3 for a on of my gbx doubles. Plugging my test results into mumtats it arrived at .314!

I'll not be using it to calculate for official postings in the database. When I get my constant current test rig finished I'll use it to double check though.

I thought I'd mention it for the reason that most of the motors in the db have no constants and this is a very good way to get them to see how a listed motor might perform in your usage.

Be sure to read the article which will explain how to use it (kinda). Max power needs tinkering to get 50w per 22.7mm stator.